1.
Saturn
–
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with a radius about nine times that of Earth. Although it has only one-eighth the average density of Earth, with its larger volume Saturn is just over 95 times more massive, Saturn is named after the Roman god of agriculture, its astronomical symbol represents the gods sickle. Saturns interior is composed of a core of iron–nickel and rock. This core is surrounded by a layer of metallic hydrogen, an intermediate layer of liquid hydrogen and liquid helium. Saturn has a yellow hue due to ammonia crystals in its upper atmosphere. Saturns magnetic field strength is around one-twentieth of Jupiters, the outer atmosphere is generally bland and lacking in contrast, although long-lived features can appear. Wind speeds on Saturn can reach 1,800 km/h, higher than on Jupiter, sixty-two moons are known to orbit Saturn, of which fifty-three are officially named. This does not include the hundreds of moonlets comprising the rings, Saturn is a gas giant because it is predominantly composed of hydrogen and helium. It lacks a definite surface, though it may have a solid core, Saturns rotation causes it to have the shape of an oblate spheroid, that is, it is flattened at the poles and bulges at its equator. Its equatorial and polar radii differ by almost 10%,60,268 km versus 54,364 km, Jupiter, Uranus, and Neptune, the other giant planets in the Solar System, are also oblate but to a lesser extent. Saturn is the planet of the Solar System that is less dense than water—about 30% less. Although Saturns core is considerably denser than water, the specific density of the planet is 0.69 g/cm3 due to the atmosphere. Jupiter has 318 times the Earths mass, while Saturn is 95 times the mass of the Earth, together, Jupiter and Saturn hold 92% of the total planetary mass in the Solar System. On 8 January 2015, NASA reported determining the center of the planet Saturn, the temperature, pressure, and density inside Saturn all rise steadily toward the core, which causes hydrogen to transition into a metal in the deeper layers. Standard planetary models suggest that the interior of Saturn is similar to that of Jupiter, having a rocky core surrounded by hydrogen. This core is similar in composition to the Earth, but more dense, in 2004, they estimated that the core must be 9–22 times the mass of the Earth, which corresponds to a diameter of about 25,000 km. This is surrounded by a liquid metallic hydrogen layer, followed by a liquid layer of helium-saturated molecular hydrogen that gradually transitions to a gas with increasing altitude
2.
Titan (moon)
–
Titan is the largest moon of Saturn. It is the only known to have a dense atmosphere. Titan is the sixth ellipsoidal moon from Saturn, frequently described as a planet-like moon, Titan is 50% larger than Earths Moon, and it is 80% more massive. It is the second-largest moon in the Solar System, after Jupiters moon Ganymede, and is larger than the smallest planet, Mercury, discovered in 1655 by the Dutch astronomer Christiaan Huygens, Titan was the first known moon of Saturn, and the sixth known planetary satellite. Titan orbits Saturn at 20 Saturn radii, from Titans surface, Saturn subtends an arc of 5.09 degrees and would appear 11.4 times larger in the sky than the Moon from Earth. Titan is primarily composed of ice and rocky material. The geologically young surface is smooth, with few impact craters, although mountains. The atmosphere of Titan is largely nitrogen, minor components lead to the formation of methane and ethane clouds and nitrogen-rich organic smog. The climate—including wind and rain—creates surface features similar to those of Earth, such as dunes, rivers, lakes, seas, and deltas, Huygens was inspired by Galileos discovery of Jupiters four largest moons in 1610 and his improvements in telescope technology. Christiaan, with the help of his brother Constantijn Huygens, Jr. began building telescopes around 1650 and it was the sixth moon to be discovered. He named it Saturni Luna, publishing in the 1655 tract De Saturni Luna Observatio Nova, after Giovanni Domenico Cassini published his discoveries of four more moons of Saturn between 1673 and 1686, astronomers fell into the habit of referring to these and Titan as Saturn I through V. Other early epithets for Titan include Saturns ordinary satellite, Titan is officially numbered Saturn VI because after the 1789 discoveries the numbering scheme was frozen to avoid causing any more confusion. Numerous small moons have been discovered closer to Saturn since then and he suggested the names of the mythological Titans, brothers and sisters of Cronus, the Greek Saturn. In Greek mythology, the Titans were a race of powerful deities, descendants of Gaia and Uranus, Titan orbits Saturn once every 15 days and 22 hours. Because of this, there is a point on its surface. Longitudes on Titan are measured westward, starting from the passing through this point. Its orbital eccentricity is 0.0288, and the plane is inclined 0.348 degrees relative to the Saturnian equator. Viewed from Earth, Titan reaches a distance of about 20 Saturn radii from Saturn
3.
Mercury (planet)
–
Mercury is the smallest and innermost planet in the Solar System. Its orbital period around the Sun of 88 days is the shortest of all the planets in the Solar System and it is named after the Roman deity Mercury, the messenger to the gods. Like Venus, Mercury orbits the Sun within Earths orbit as a planet, so it can only be seen visually in the morning or the evening sky. Also, like Venus and the Moon, the displays the complete range of phases as it moves around its orbit relative to Earth. Seen from Earth, this cycle of phases reoccurs approximately every 116 days, although Mercury can appear as a bright star-like object when viewed from Earth, its proximity to the Sun often makes it more difficult to see than Venus. Mercury is tidally or gravitationally locked with the Sun in a 3,2 resonance, as seen relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun. As seen from the Sun, in a frame of reference that rotates with the orbital motion, an observer on Mercury would therefore see only one day every two years. Mercurys axis has the smallest tilt of any of the Solar Systems planets, at aphelion, Mercury is about 1.5 times as far from the Sun as it is at perihelion. Mercurys surface appears heavily cratered and is similar in appearance to the Moons, the polar regions are constantly below 180 K. The planet has no natural satellites. Mercury is one of four planets in the Solar System. It is the smallest planet in the Solar System, with a radius of 2,439.7 kilometres. Mercury is also smaller—albeit more massive—than the largest natural satellites in the Solar System, Ganymede, Mercury consists of approximately 70% metallic and 30% silicate material. Mercurys density is the second highest in the Solar System at 5.427 g/cm3, Mercurys density can be used to infer details of its inner structure. Although Earths high density results appreciably from gravitational compression, particularly at the core, Mercury is much smaller, therefore, for it to have such a high density, its core must be large and rich in iron. Geologists estimate that Mercurys core occupies about 55% of its volume, Research published in 2007 suggests that Mercury has a molten core. Surrounding the core is a 500–700 km mantle consisting of silicates, based on data from the Mariner 10 mission and Earth-based observation, Mercurys crust is estimated to be 35 km thick. One distinctive feature of Mercurys surface is the presence of narrow ridges
4.
Natural satellite
–
A natural satellite or moon is, in the most common usage, an astronomical body that orbits a planet or minor planet. In the Solar System there are six planetary satellite systems containing 178 known natural satellites, four IAU-listed dwarf planets are also known to have natural satellites, Pluto, Haumea, Makemake, and Eris. As of January 2012, over 200 minor-planet moons have been discovered, the Earth–Moon system is unique in that the ratio of the mass of the Moon to the mass of Earth is much greater than that of any other natural-satellite–planet ratio in the Solar System. At 3,474 km across, Earths Moon is 0.27 times the diameter of Earth, the first known natural satellite was the Moon, but it was considered a planet until Copernicus introduction of heliocentrism in 1543. Until the discovery of the Galilean satellites in 1610, however, galileo chose to refer to his discoveries as Planetæ, but later discoverers chose other terms to distinguish them from the objects they orbited. The first to use of the satellite to describe orbiting bodies was the German astronomer Johannes Kepler in his pamphlet Narratio de Observatis a se quatuor Iouis satellitibus erronibus in 1610. He derived the term from the Latin word satelles, meaning guard, attendant, or companion, the term satellite thus became the normal one for referring to an object orbiting a planet, as it avoided the ambiguity of moon. In 1957, however, the launching of the artificial object Sputnik created a need for new terminology, to further avoid ambiguity, the convention is to capitalize the word Moon when referring to Earths natural satellite, but not when referring to other natural satellites. A few recent authors define moon as a satellite of a planet or minor planet, there is no established lower limit on what is considered a moon. Small asteroid moons, such as Dactyl, have also been called moonlets, the upper limit is also vague. Two orbiting bodies are described as a double body rather than primary. Asteroids such as 90 Antiope are considered double asteroids, but they have not forced a clear definition of what constitutes a moon, some authors consider the Pluto–Charon system to be a double planet. In contrast, irregular satellites are thought to be captured asteroids possibly further fragmented by collisions, most of the major natural satellites of the Solar System have regular orbits, while most of the small natural satellites have irregular orbits. The Moon and possibly Charon are exceptions among large bodies in that they are thought to have originated by the collision of two large proto-planetary objects. The material that would have placed in orbit around the central body is predicted to have reaccreted to form one or more orbiting natural satellites. As opposed to planetary-sized bodies, asteroid moons are thought to form by this process. Triton is another exception, although large and in a close, circular orbit, its motion is retrograde, most regular moons in the Solar System are tidally locked to their respective primaries, meaning that the same side of the natural satellite always faces its planet. The only known exception is Saturns natural satellite Hyperion, which rotates chaotically because of the influence of Titan
5.
Rings of Saturn
–
The rings of Saturn are the most extensive planetary ring system of any planet in the Solar System. They consist of small particles, ranging from μm to m in size. The ring particles are made almost entirely of ice, with a trace component of rocky material. Although reflection from the rings increases Saturns brightness, they are not visible from Earth with unaided vision. In 1610, the year after Galileo Galilei turned a telescope to the sky, he became the first person to observe Saturns rings, in 1655, Christiaan Huygens was the first person to describe them as a disk surrounding Saturn. Although many people think of Saturns rings as being made up of a series of tiny ringlets and it is more correct to think of the rings as an annular disk with concentric local maxima and minima in density and brightness. On the scale of the clumps within the rings there is empty space. Stabilizing resonances, on the hand, are responsible for the longevity of several rings, such as the Titan Ringlet. Well beyond the rings is the Phoebe ring, which is tilted at an angle of 27 degrees to the other rings and, like Phoebe. Galileo Galilei was the first to observe the rings of Saturn in 1610 using his telescope, but was unable to identify them as such. He wrote to the Duke of Tuscany that The planet Saturn is not alone and they are arranged in a line parallel to the zodiac, and the middle one is about three times the size of the lateral ones. He also described the rings as Saturns ears, in 1612 the Earth passed through the plane of the rings and they became invisible. Mystified, Galileo remarked I do not know what to say in a case so surprising, so unlooked for and he mused, Has Saturn swallowed his children. — referring to the myth of the Titan Saturn devouring his offspring to forestall the prophecy of them overthrowing him and he was further confused when the rings again became visible in 1613. Early astronomers used anagrams as a form of commitment scheme to lay claim to new discoveries before their results were ready for publication, Galileo used smaismrmilmepoetaleumibunenugttauiras for Altissimum planetam tergeminum observavi for discovering the rings of Saturn. In 1655, Christiaan Huygens became the first person to suggest that Saturn was surrounded by a ring, robert Hooke was another early observer of the rings of Saturn, and noted the casting of shadows on the rings. In 1675, Giovanni Domenico Cassini determined that Saturns ring was composed of multiple smaller rings with gaps between them, the largest of these gaps was later named the Cassini Division and this division is a 4,800 km-wide region between the A Ring and B Ring. In 1787, Pierre-Simon Laplace suggested that the rings were composed of a number of solid ringlets
6.
Rhea (moon)
–
Rhea is the second-largest moon of Saturn and the ninth-largest moon in the Solar System. It is the second smallest body in the Solar System, after the asteroid and dwarf planet Ceres and it was discovered in 1672 by Giovanni Domenico Cassini. Rhea was discovered by Giovanni Domenico Cassini on 23 December 1672 and it was the third moon discovered around Saturn and the second by him. Rhea is named after the Titan Rhea of Greek mythology, the mother of the gods and it is also designated Saturn V. Cassini named the four moons he discovered Sidera Lodoicea to honor King Louis XIV. Astronomers fell into the habit of referring to them and Titan as Saturn I through Saturn V, once Mimas and Enceladus were discovered, in 1789, the numbering scheme was extended to Saturn VII. Rhea is an icy body with a density of about 1.236 g/cm3 and this low density indicates that it is made of ~25% rock and ~75% water ice. Although Rhea is the ninth-largest moon, it is only the tenth-most-massive moon, earlier it was assumed that Rhea had a rocky core in the center. However measurements taken during a flyby by the Cassini orbiter in 2005 cast this into doubt. In a paper published in 2007 it was claimed that the axial dimensionless moment of inertia coefficient was 0.4, such a value indicated that Rhea had an almost homogeneous interior while the existence of a rocky core would imply a moment of inertia of about 0.34. In the same year another paper claimed the moment of inertia was about 0.37 implying that Rhea was partially differentiated. A year later yet another paper claimed that the moon may not be in equilibrium meaning that the moment of inertia can not be determined from the gravity data alone. In 2008 an author of the first paper tried to reconcile these three disparate results.4, again implying a homogeneous interior, the triaxial shape of Rhea is consistent with a homogeneous body in hydrostatic equilibrium rotating at Rheas angular velocity. Rheas features resemble those of Dione, with leading and trailing hemispheres, suggesting similar composition. The temperature on Rhea is 99 K in direct sunlight and between 73 K and 53 K in the shade, Rhea has two very large impact basins on its anti-Cronian hemisphere, which are about 400 and 500 km across. The more northerly and less degraded of the two, called Tirawa, is comparable to the basin Odysseus on Tethys. There is a 48 km-diameter impact crater at 112°W that is prominent because of a system of bright rays. This crater, called Inktomi, is nicknamed The Splat, no evidence of any endogenic activity has been discovered. This suggests that a major resurfacing event occurred some time during its formation, the leading hemisphere is heavily cratered and uniformly bright
7.
Hydrostatic equilibrium
–
In fluid mechanics, a fluid is said to be in hydrostatic equilibrium or hydrostatic balance when it is at rest, or when the flow velocity at each point is constant over time. This occurs when external forces such as gravity are balanced by a pressure gradient force, hydrostatic equilibrium is the current distinguishing criterion between dwarf planets and small Solar System bodies, and has other roles in astrophysics and planetary geology. This qualification typically means that the object is symmetrically rounded into a spheroid or ellipsoid shape, there are 31 observationally confirmed such objects, sometimes called planemos, in the Solar System, seven more that are virtually certain, and a hundred or so more that are likely. Newtons laws of state that a volume of a fluid that is not in motion or that is in a state of constant velocity must have zero net force on it. This means the sum of the forces in a given direction must be opposed by a sum of forces in the opposite direction. This force balance is called a hydrostatic equilibrium, the fluid can be split into a large number of cuboid volume elements, by considering a single element, the action of the fluid can be derived. If the density is ρ, the volume is V and g the standard gravity, then, the volume of this cuboid is equal to the area of the top or bottom, times the height — the formula for finding the volume of a cube. This sum equals zero if the velocity is constant. Dividing by A,0 = P b o t t o m − P t o p − ρ ⋅ g ⋅ h, or, P t o p − P b o t t o m = − ρ ⋅ g ⋅ h. Ptop − Pbottom is a change in pressure, and h is the height of the volume change in the distance above the ground. By saying these changes are small, the equation can be written in differential form. Density changes with pressure, and gravity changes with height, so the equation would be, d P = − ρ ⋅ g ⋅ d h. Note finally that this last equation can be derived by solving the three-dimensional Navier–Stokes equations for the situation where u = v = ∂ p ∂ x = ∂ p ∂ y =0. Then the only equation is the z -equation, which now reads ∂ p ∂ z + ρ g =0. Thus, hydrostatic balance can be regarded as a simple equilibrium solution of the Navier–Stokes equations. M is a foliation of spheres weighted by the mass density ρ, the hydrostatic equilibrium pertains to hydrostatics and the principles of equilibrium of fluids. A hydrostatic balance is a balance for weighing substances in water. Hydrostatic balance allows the discovery of their specific gravities, in any given layer of a star, there is a hydrostatic equilibrium between the outward thermal pressure from below and the weight of the material above pressing inward
8.
Atmosphere of Titan
–
The atmosphere of Titan is the layer of gases surrounding Titan, the largest moon of Saturn. It is the thick atmosphere of a natural satellite in the Solar System. The high surface pressure meant that methane could only form a small fraction of Titans atmosphere, in 1980, Voyager 1 made the first detailed observations of Titans atmosphere, revealing that its surface pressure was higher than Earths, at 1.5 bars. Observations from the Voyager space probes have shown that the Titanean atmosphere is denser than Earths, Titans atmosphere is about 1.19 times as massive as Earths overall, or about 7.3 times more massive on a per surface area basis. It supports opaque haze layers block most visible light from the Sun and other sources. The atmosphere is so thick and the gravity so low that humans could fly through it by flapping wings attached to their arms, the atmosphere of Titan is opaque at many wavelengths and a complete reflectance spectrum of the surface is impossible to acquire from the outside. It was not until the arrival of Cassini–Huygens in 2004 that the first direct images of Titans surface were obtained, the orange color as seen from space must be produced by other more complex chemicals in small quantities, possibly tholins, tar-like organic precipitates. The hydrocarbons are thought to form in Titans upper atmosphere in reactions resulting from the breakup of methane by the Suns ultraviolet light and this may ionize and carry away some molecules from the top of the atmosphere. Their structure is not currently known, but they are thought to be tholins, energy from the Sun should have converted all traces of methane in Titans atmosphere into more complex hydrocarbons within 50 million years — a short time compared to the age of the Solar System. This suggests that methane must be replenished by a reservoir on or within Titan itself. Many astronomers have suggested that the origin for the methane in Titans atmosphere is from within Titan itself. A possible biological origin for the methane has not been discounted, there is also a pattern of air circulation found flowing in the direction of Titans rotation, from west to east. Observations by Cassini of the made in 2004 also suggest that Titan is a super rotator, like Venus. Titans ionosphere is also more complex than Earths, with the ionosphere at an altitude of 1,200 km. This splits Titans atmosphere to some extent into two separate radio-resonating chambers, the source of natural extremely-low-frequency waves on Titan, as detected by Cassini–Huygens, is unclear as there does not appear to be extensive lightning activity. Titans internal magnetic field is negligible, and perhaps even nonexistent and its orbital distance of 20.3 Saturn radii does place it within Saturns magnetosphere occasionally. However, the difference between Saturns rotational period and Titans orbital period causes a speed of about 100 km/s between the Saturns magnetized plasma and Titan. That can actually intensify reactions causing atmospheric loss, instead of guarding the atmosphere from the solar wind, on April 3,2013, NASA reported that complex organic chemicals could arise on Titan based on studies simulating the atmosphere of Titan
9.
Lakes of Titan
–
The lakes of Titan, Saturns largest moon, are bodies of liquid ethane and methane that have been detected by the Cassini–Huygens space probe, and had been suspected long before. The large ones are known as maria and the ones as lacūs. The possibility that there were seas on Titan was first suggested based on data from the Voyager 1 and 2 space probes, launched in August, the data showed Titan to have a thick atmosphere of approximately the correct temperature and composition to support them. The Cassini mission affirmed the former hypothesis, although not immediately, the possibility remained that liquid ethane and methane might be found on Titans polar regions, where they were expected to be abundant and stable. In Titans south polar region, a dark feature named Ontario Lacus was the first suspected lake identified. A possible shoreline was also identified near the pole via radar imagery, following a flyby on July 22,2006, in which the Cassini spacecrafts radar imaged the northern latitudes, a number of large, smooth patches were seen dotting the surface near the pole. Based on the observations, scientists announced definitive evidence of lakes filled with methane on Saturns moon Titan in January 2007, the Cassini–Huygens team concluded that the imaged features are almost certainly the long-sought hydrocarbon lakes, the first stable bodies of surface liquid found off Earth. Some appear to have associated with liquid and lie in topographical depressions. Channels in some regions have created surprisingly little erosion, suggesting erosion on Titan is extremely slow, or some other recent phenomena may have wiped out older riverbeds and landforms. Overall, the Cassini radar observations have shown that lakes cover only a few percent of the surface and are concentrated near the poles, making Titan much drier than Earth. The high relative humidity of methane in Titans lower atmosphere could be maintained by evaporation from lakes covering only 0. 002–0. 02% of the whole surface, a flyby of Titans southern polar regions in October 2007 revealed similar, though far smaller, lakelike features. During a close Cassini flyby in December 2007 the visual and mapping instrument observed a lake, Ontario Lacus and this instrument identifies chemically different materials based on the way they absorb and reflect infrared light. This makes Titan the only object in the Solar system other than Earth known to have stable ambient-temperature liquid on its surface. Radar measurements made in July 2009 and January 2010 indicate that Ontario Lacus is extremely shallow, with a depth of 0. 4–3.2 m. It may thus resemble a terrestrial mudflat, in contrast, the northern hemispheres Ligeia Mare has depths of 170 m. The desert sand dunes along the equator, while devoid of open liquid and it has been estimated that the visible lakes and seas of Titan contain about 300 times the volume of Earths proven oil reserves. In June 2008, Cassinis Visible and Infrared Mapping Spectrometer confirmed the presence of liquid ethane beyond doubt in a lake in Titans southern hemisphere, the exact blend of hydrocarbons in the lakes is unknown. Benzene is expected to fall like snow and quickly dissolve into the lakes, the excess benzene would then build up in a mud-like sludge on the shores and on the lake floors before eventually being eroded by ethane rain, forming a complex cave-riddled landscape
10.
Enceladus
–
Enceladus N-cell-AH-dus, is the sixth-largest moon of Saturn. It is about 500 kilometers in diameter, about a tenth of that of Saturns largest moon, Enceladus is mostly covered by fresh, clean ice, making it one of the most reflective bodies of the solar system. Consequently, its temperature at noon only reaches −198 °C. Enceladus was discovered on August 28,1789, by William Herschel, in 2005, the Cassini spacecraft started multiple close flybys of Enceladus, revealing its surface and environment in greater detail. In particular, Cassini discovered water-rich plumes venting from the polar region. Over 100 geysers have been identified, some of the water vapor falls back as snow, the rest escapes, and supplies most of the material making up Saturns E ring. According to NASA scientists, the plumes are similar in composition to comets, in 2014, NASA reported that Cassini found evidence for a large south polar subsurface ocean of liquid water with a thickness of around 10 km. These geyser observations, along with the finding of escaping internal heat and very few craters in the south polar region. Like many other satellites in the systems of the giant planets. Its resonance with Dione excites its orbital eccentricity, which is damped by tidal forces, tidally heating its interior, and possibly driving the geological activity. Enceladus was discovered by Fredrick William Herschel on August 28,1789, during the first use of his new 1.2 m telescope and its faint apparent magnitude and its proximity to the much brighter Saturn and Saturns rings make Enceladus difficult to observe from Earth with smaller telescopes. Like many satellites of Saturn discovered prior to the Space Age, Enceladus was first observed during a Saturnian equinox, at such times, the reduction in glare from the rings makes the moons easier to observe. Prior to the Voyager missions the view of Enceladus improved little from the dot first observed by Herschel, only its orbital characteristics were known, with estimations of its mass, density and albedo. Enceladus is named after the giant Enceladus of Greek mythology and he chose these names because Saturn, known in Greek mythology as Cronus, was the leader of the Titans. Features on Enceladus are named by the International Astronomical Union after characters and places from Burtons translation of The Book of One Thousand, impact craters are named after characters, whereas other feature types, such as fossae, dorsa, planitia, and sulci, are named after places. The IAU has officially named 85 features on Enceladus, most recently Samaria Rupes, Enceladus is one of the major inner satellites of Saturn along with Dione, Tethys, and Mimas. It orbits at 238,000 km from Saturns center and 180,000 km from its cloud tops and it orbits Saturn every 32.9 hours, fast enough for its motion to be observed over a single night of observation. Enceladus is currently in a 2,1 mean-motion orbital resonance with Dione and this resonance maintains Enceladuss orbital eccentricity, which is known as a forced eccentricity
11.
Orbital inclination
–
Orbital inclination measures the tilt of an objects orbit around a celestial body. It is expressed as the angle between a plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Earth directly above the equator, the plane of the orbit is the same as the Earths equatorial plane. The general case is that the orbit is tilted, it spends half an orbit over the northern hemisphere. If the orbit swung between 20° north latitude and 20° south latitude, then its orbital inclination would be 20°, the inclination is one of the six orbital elements describing the shape and orientation of a celestial orbit. It is the angle between the plane and the plane of reference, normally stated in degrees. For a satellite orbiting a planet, the plane of reference is usually the plane containing the planets equator, for planets in the Solar System, the plane of reference is usually the ecliptic, the plane in which the Earth orbits the Sun. This reference plane is most practical for Earth-based observers, therefore, Earths inclination is, by definition, zero. Inclination could instead be measured with respect to another plane, such as the Suns equator or the invariable plane, the inclination of orbits of natural or artificial satellites is measured relative to the equatorial plane of the body they orbit, if they orbit sufficiently closely. The equatorial plane is the perpendicular to the axis of rotation of the central body. An inclination of 30° could also be described using an angle of 150°, the convention is that the normal orbit is prograde, an orbit in the same direction as the planet rotates. Inclinations greater than 90° describe retrograde orbits, thus, An inclination of 0° means the orbiting body has a prograde orbit in the planets equatorial plane. An inclination greater than 0° and less than 90° also describe prograde orbits, an inclination of 63. 4° is often called a critical inclination, when describing artificial satellites orbiting the Earth, because they have zero apogee drift. An inclination of exactly 90° is an orbit, in which the spacecraft passes over the north and south poles of the planet. An inclination greater than 90° and less than 180° is a retrograde orbit, an inclination of exactly 180° is a retrograde equatorial orbit. For gas giants, the orbits of moons tend to be aligned with the giant planets equator, the inclination of exoplanets or members of multiple stars is the angle of the plane of the orbit relative to the plane perpendicular to the line-of-sight from Earth to the object. An inclination of 0° is an orbit, meaning the plane of its orbit is parallel to the sky. An inclination of 90° is an orbit, meaning the plane of its orbit is perpendicular to the sky
12.
Co-orbital configuration
–
There are several classes of co-orbital objects, depending on their point of libration. The most common and best-known class is the trojan, which librates around one of the two stable Lagrangian points, L4 and L5, 60° ahead of and behind the larger body respectively. Another class is the orbit, in which objects librate around 180° from the larger body. Objects librating around 0° are called quasi-satellites, an exchange orbit occurs when two co-orbital objects are of similar masses and thus exert a non-negligible influence on each other. The objects can exchange semi-major axes or eccentricities when they approach each other, orbital parameters that are used to describe the relation of co-orbital objects are the longitude of the periapsis difference and the mean longitude difference. The longitude of the periapsis is the sum of the longitude and the mean anomaly and the mean longitude is the sum of the longitude of the ascending node. Trojan objects orbit 60° ahead of or behind a massive object. The best known example are the asteroids that orbit ahead of or behind Jupiter around the Sun, trojan objects do not orbit exactly at one of either Lagrangian points, but do remain relatively close to it, appearing to slowly orbit it. In technical terms, they librate around =, the point around which they librate is the same, irrespective of their mass or orbital eccentricity. There are several thousand known trojan minor planets orbiting the Sun, most of these orbit near Jupiters Lagrangian points, the traditional Jupiter trojans. As of 2015 there are also 13 Neptune trojans,7 Mars trojans,1 Uranus trojan and 1 Earth trojan known to exist, the Saturnian system contains two sets of trojan moons. Both Tethys and Dione have two moons, Telesto and Calypso in Tethyss L4 and L5 respectively, and Helene and Polydeuces in Diones L4. Polydeuces is noticeable for its wide libration, it wanders as far as ±30° from its Lagrangian point and ±2% from its orbital radius. A pair of co-orbital exoplanets was proposed to be orbiting the star Kepler-223, the possibility of a trojan planet to Kepler-91b was studied but the conclusion was that the transit-signal was a false-positive. One possibility for the zone is a trojan planet of a giant planet close to its star. Objects in a horseshoe orbit librate around 180° from the primary and their orbits encompass both equilateral Lagrangian points, i. e. L4 and L5. The Saturnian moons Janus and Epimetheus share their orbits, the difference in semi-major axes being less than eithers mean diameter and this means the moon with the smaller semi-major axis will slowly catch up with the other. As it does this, the moons gravitationally tug at each other, increasing the semi-major axis of the moon that has caught up and this reverses their relative positions proportionally to their masses and causes this process to begin anew with the moons roles reversed
13.
Shepherd moon
–
A shepherd moon is a small natural satellite that clears a gap in planetary-ring material or keeps particles within a ring contained. The name is a result of the fact they limit the herd of the particles as a shepherd. Due to their effect, they pick up particles and deflect them from their original orbits through orbital resonance. This causes gaps in the system, such as the particularly striking Cassini Division, as well as other characteristic bands. Several of Jupiters small innermost moons, namely Metis and Adrastea, are within Jupiters ring system and are also within Jupiters Roche limit. It is possible that these rings are composed of material that is being pulled off these two bodies by Jupiters tidal forces, possibly facilitated by impacts of ring material on their surfaces, the complex ring system of Saturn has several such satellites. These include Prometheus, Daphnis, Pan, Janus, and Epimetheus, uranus also has shepherd moons on its ε ring, Cordelia and Ophelia. They are interior and exterior shepherds, respectively, both moons are well within Uranuss synchronous orbit radius, and their orbits are therefore slowly decaying due to tidal deceleration. Neptunes rings are unusual in that they first appeared to be composed of incomplete arcs in Earth-based observations. It is thought that the influence of the shepherd moon Galatea. Rings around some centaurs have been identified, chariklos rings are remarkably well-defined and are suspected to either be very young or kept in place by a shepherd moon similar in mass to the rings. Chiron is also thought to have similar in form to those of Chariklo. Kirkwood gap Arnold Hanslmeier, Einführung in Astronomie und Astrophysik
14.
Hyperion (moon)
–
Hyperion, also known as Saturn VII, is a moon of Saturn discovered by William Cranch Bond, George Phillips Bond and William Lassell in 1848. It is distinguished by its shape, its chaotic rotation. It was the first non-round moon to be discovered, the moon is named after Hyperion, the Titan god of watchfulness and observation – the elder brother of Cronus, the Greek equivalent of Saturn – in Greek mythology. It is also designated Saturn VII, the adjectival form of the name is Hyperionian. William Lassell, who saw Hyperion two days after William Bond, had already endorsed Herschels naming scheme and suggested the name Hyperion in accordance with it and he also beat Bond to publication. Hyperion is one of the largest bodies known to be highly irregularly shaped in the Solar System, the only larger moon known to be irregular in shape is Neptunes moon Proteus. Hyperion has about 15% of the mass of Mimas, the least massive known ellipsoidal body, the largest crater on Hyperion is approximately 121.57 km in diameter and 10.2 km deep. A possible explanation for the shape is that Hyperion is a fragment of a larger body that was broken up by a large impact in the distant past. A proto-Hyperion could have been 350–1,000 km in diameter, over about 1,000 years, ejecta from a presumed Hyperion breakup would have impacted Titan at low speeds, building up volatiles in the atmosphere of Titan. Like most of Saturns moons, Hyperions low density indicates that it is composed largely of ice with only a small amount of rock. It is thought that Hyperion may be similar to a loosely accreted pile of rubble in its physical composition, however, unlike most of Saturns moons, Hyperion has a low albedo, indicating that it is covered by at least a thin layer of dark material. This may be material from Phoebe that got past Iapetus, Hyperion is redder than Phoebe and closely matches the color of the dark material on Iapetus. Hyperion has a porosity of about 0.46, Voyager 2 passed through the Saturn system, but photographed Hyperion only from a distance. It discerned individual craters and a ridge, but was not able to make out the texture of Hyperions surface. Early images from the Cassini orbiter suggested an unusual appearance, Hyperions surface is covered with deep, sharp-edged craters that give it the appearance of a giant sponge. Dark material fills the bottom of each crater, the reddish substance contains long chains of carbon and hydrogen and appears very similar to material found on other Saturnian satellites, most notably Iapetus. Scientists attribute Hyperions unusual, sponge-like appearance to the fact that it has a low density for such a large object. Its low density makes Hyperion quite porous, with a surface gravity
15.
Orbital resonance
–
Orbital resonances greatly enhance the mutual gravitational influence of the bodies, i. e. their ability to alter or constrain each others orbits. In most cases, this results in an interaction, in which the bodies exchange momentum. Under some circumstances, a resonant system can be stable and self-correcting, examples are the 1,2,4 resonance of Jupiters moons Ganymede, Europa and Io, and the 2,3 resonance between Pluto and Neptune. Unstable resonances with Saturns inner moons give rise to gaps in the rings of Saturn, thus the 2,3 ratio above means Pluto completes two orbits in the time it takes Neptune to complete three. In the case of resonance relationships between three or more bodies, either type of ratio may be used and the type of ratio will be specified. Since the discovery of Newtons law of gravitation in the 17th century. The stable orbits that arise in a two-body approximation ignore the influence of other bodies and it was Laplace who found the first answers explaining the remarkable dance of the Galilean moons. It is fair to say that this field of study has remained very active since then. Before Newton, there was consideration of ratios and proportions in orbital motions, in what was called the music of the spheres. In general, a resonance may involve one or any combination of the orbit parameters. Act on any scale from short term, commensurable with the orbit periods, to secular. Lead to either long-term stabilization of the orbits or be the cause of their destabilization, a mean-motion orbital resonance occurs when two bodies have periods of revolution that are a simple integer ratio of each other. Depending on the details, this can either stabilize or destabilize the orbit, stabilization may occur when the two bodies move in such a synchronised fashion that they never closely approach. For instance, The orbits of Pluto and the plutinos are stable, despite crossing that of the much larger Neptune, the resonance ensures that, when they approach perihelion and Neptunes orbit, Neptune is consistently distant. Other Neptune-crossing bodies that were not in resonance were ejected from that region by strong perturbations due to Neptune. There are also smaller but significant groups of resonant trans-Neptunian objects occupying the 1,1,3,5,4,7,1,2 and 2,5 resonances, among others, with respect to Neptune. In the asteroid belt beyond 3.5 AU from the Sun, orbital resonances can also destabilize one of the orbits. For small bodies, destabilization is actually far more likely, for instance, In the asteroid belt within 3.5 AU from the Sun, the major mean-motion resonances with Jupiter are locations of gaps in the asteroid distribution, the Kirkwood gaps
16.
Mimas (moon)
–
Mimas is a moon of Saturn which was discovered in 1789 by William Herschel. It is named after Mimas, a son of Gaia in Greek mythology, with a diameter of 396 kilometres it is the smallest astronomical body that is known to be rounded in shape because of self-gravitation. Mimas was discovered by the astronomer William Herschel on 17 September 1789, Mimas is named after one of the Giants in Greek mythology, Mimas. He named them after Titans specifically because Saturn, was the leader of the Titans, the adjectival form of Mimas is Mimantean or Mimantian. The surface area of Mimas is slightly less than the area of Spain. The low density of Mimas,1.15 g/cm3, indicates that it is composed mostly of ice with only a small amount of rock. Due to the forces acting on it, Mimas is noticeably prolate. The ellipsoidal shape of Mimas is especially noticeable in some recent images from the Cassini probe, Mimass most distinctive feature is a giant impact crater 130 km across, named Herschel after the discoverer of Mimas. Herschels diameter is almost a third of Mimass own diameter, its walls are approximately 5 km high, parts of its floor measure 10 km deep, and its central peak rises 6 km above the crater floor. If there were a crater of an equivalent scale on Earth it would be over 4,000 km in diameter, wider than Australia, the Mimantean surface is saturated with smaller impact craters, but no others are anywhere near the size of Herschel. Although Mimas is heavily cratered, the cratering is not uniform, most of the surface is covered with craters larger than 40 km in diameter, but in the south polar region, there are generally no craters larger than 20 km in diameter. Three types of features are officially recognized on Mimas, craters, chasmata. A number of features in Saturns rings are related to resonances with Mimas, Mimas is responsible for clearing the material from the Cassini Division, the gap between Saturns two widest rings, the A Ring and B Ring. Particles in the Huygens Gap at the edge of the Cassini division are in a 2,1 resonance with Mimas. They orbit twice for each orbit of Mimas, the repeated pulls by Mimas on the Cassini division particles, always in the same direction in space, force them into new orbits outside the gap. The boundary between the C and B ring is in a 3,1 resonance with Mimas, recently, the G Ring was found to be in a 7,6 co-rotation eccentricity resonance with Mimas, the rings inner edge is about 15,000 km inside Mimass orbit. Mimas is also in a 2,1 mean-motion resonance with the larger moon Tethys, however, in 2017 it was concluded that an ocean would lead to surface tidal stresses comparable to or greater than those on tectonically active Europa. Thus, the lack of evidence for cracking or other tectonic activity on Mimas argues against the presence of such an ocean
17.
Titan (mythology)
–
In Greek mythology, the Titans and Titanesses were members of the second generation of divine beings, descending from the primordial deities and preceding the Olympian deities. Based on Mount Othrys, the Titans most famously included the first twelve children of the primordial Gaia and they were giant deities of incredible strength, who ruled during the legendary Golden Age, and also comprised the first pantheon of Greek deities. The first twelve Titans comprised the females Mnemosyne, Tethys, Theia, Phoebe, Rhea, and Themis and the males Oceanus, Hyperion, Coeus, Cronus, Crius, like Cronus overthrowing his father Uranus, the Titans were overthrown by Cronus children, in the Titanomachy. The Greeks may have borrowed this mytheme from the Ancient Near East, Greeks of the classical age knew of several poems about the war between the Olympians and Titans. The dominant one, and the one that has survived, was in the Theogony attributed to Hesiod. A lost epic, Titanomachia was mentioned in passing in an essay On Music that was attributed to Plutarch. The Titans also played a prominent role in the poems attributed to Orpheus, although only scraps of the Orphic narratives survive, they show interesting differences with the Hesiodic tradition. Sometimes the elders are supplanted, and sometimes the rebels lose and are either cast out of power entirely or incorporated into the pantheon, the Titanomachy lasted for ten years. The Titans were imprisoned in Tartarus after the war had ended, Tartarus is the deepest spot known in the Underworld, where the most evil beings would be cast into to be tortured for all eternity. Hesiod does not have the last word on the Titans, surviving fragments of poetry ascribed to Orpheus preserve some variations on the myth. In such text, Zeus does not simply set upon his father violently, instead, Rhea spreads out a banquet for Cronus so that he becomes drunk upon fermented honey. Rather than being consigned to Tartarus, Cronus is dragged—still drunk—to the cave of Nyx, another myth concerning the Titans that is not in Hesiod revolves around Dionysus. At some point in his reign, Zeus decides to give up the throne in favor of the infant Dionysus, who like the infant Zeus, is guarded by the Kouretes. The Titans decide to slay the child and claim the throne for themselves, they paint their faces white with gypsum, distract Dionysus with toys, then him and boil. Zeus, enraged, slays the Titans with his thunderbolt, Athena preserves the heart in a gypsum doll and this story is told by the poets Callimachus and Nonnus, who call this Dionysus Zagreus, and in a number of Orphic texts, which do not. Pindar, Plato, and Oppian refer offhandedly to the Titanic nature of humans, according to them, the body is the titanic part, while soul is the divine part of humans. Other early writers imply that humanity was born out of the malevolent blood shed by the Titans in their war against Zeus, Martin Litchfield West also asserts this in relation to shamanistic initiatory rites of early Greek religious practices. Beekes connects the word with τιτώ, burket, Walter, The Orientalizing Revolution, Near Eastern Influence on Greek Culture in the Early Archaic Age, Harvard University Press,1995
18.
Saturn (mythology)
–
Saturn is a god in ancient Roman religion, and a character in myth. Saturn is a complex figure because of his associations and long history. In later developments he came to be also a god of time and his reign was depicted as a Golden Age of plenty and peace. The Temple of Saturn in the Roman Forum housed the state treasury, in December, he was celebrated at what is perhaps the most famous of the Roman festivals, the Saturnalia, a time of feasting, role reversals, free speech, gift-giving and revelry. Saturn the planet and Saturday are both named after the god, the Roman soil preserved the remembrance of a very remote time during which Saturn and Janus reigned on the site of the city before its foundation, the Capitol was named mons Saturnius. The Romans identified Saturn with the Greek Cronus, whose myths were adapted for Latin literature, in particular, Cronuss role in the genealogy of the Greek gods was transferred to Saturn. As early as Livius Andronicus, Jupiter was called the son of Saturn, Saturn had two consorts who represented different aspects of the god. The name of his wife Ops, the Roman equivalent of Greek Rhea, means wealth, abundance, the association with Ops though is considered a later development, as this goddess was originally paired with Consus. Earlier was Saturns association with Lua, a goddess who received the weapons of enemies destroyed in war. Under Saturns rule, humans enjoyed the spontaneous bounty of the earth without labour in the Golden Age described by Hesiod, according to Varro, Saturns name was derived from satu, meaning sowing. Even though this etymology looks implausible on linguistic grounds nevertheless it does reflect a feature of the god. A more probable etymology connects the name with Etruscan god Satre and placenames such as Satria, an ancient town of Latium, and Saturae palus and this root may be related to Latin phytonym satureia. Another epithet, variably Sterculius, Stercutus, and Sterces, referred to his agricultural functions, agriculture was important to Roman identity, and Saturn was a part of archaic Roman religion and ethnic identity. His name appears in the ancient hymn of the Salian priests, the temple was consecrated in 497 BC but the area Saturni was built by king Tullus Hostilius as confirmed by archaeological studies conducted by E. Gjerstad. It housed the state treasury throughout Roman history, the position of Saturns festival in the Roman calendar led to his association with concepts of time, especially the temporal transition of the New Year. In the Greek tradition, Cronus was sometimes conflated with Chronus, Time, in late antiquity, Saturn is syncretized with a number of deities, and begins to be depicted as winged, as is Kairos, Timing, Right Time. The figure of Saturn is one of the most complex in Roman religion, G. Dumézil refrained from discussing Saturn in his work on Roman religion on the grounds of our insufficient knowledge. Brelich and G. Piccaluga as his basis, the main difficulty scholars find in studying Saturn is in assessing what is original of his figure and what is due to later hellenising influences
19.
Minor planet
–
A minor planet is an astronomical object in direct orbit around the Sun that is neither a planet nor exclusively classified as a comet. Minor planets can be dwarf planets, asteroids, trojans, centaurs, Kuiper belt objects, as of 2016, the orbits of 709,706 minor planets were archived at the Minor Planet Center,469,275 of which had received permanent numbers. The first minor planet to be discovered was Ceres in 1801, the term minor planet has been used since the 19th century to describe these objects. The term planetoid has also used, especially for larger objects such as those the International Astronomical Union has called dwarf planets since 2006. Historically, the asteroid, minor planet, and planetoid have been more or less synonymous. This terminology has become complicated by the discovery of numerous minor planets beyond the orbit of Jupiter. A Minor planet seen releasing gas may be classified as a comet. Before 2006, the IAU had officially used the term minor planet, during its 2006 meeting, the IAU reclassified minor planets and comets into dwarf planets and small Solar System bodies. Objects are called dwarf planets if their self-gravity is sufficient to achieve hydrostatic equilibrium, all other minor planets and comets are called small Solar System bodies. The IAU stated that the minor planet may still be used. However, for purposes of numbering and naming, the distinction between minor planet and comet is still used. Hundreds of thousands of planets have been discovered within the Solar System. The Minor Planet Center has documented over 167 million observations and 729,626 minor planets, of these,20,570 have official names. As of March 2017, the lowest-numbered unnamed minor planet is 1974 FV1, as of March 2017, the highest-numbered named minor planet is 458063 Gustavomuler. There are various broad minor-planet populations, Asteroids, traditionally, most have been bodies in the inner Solar System. Near-Earth asteroids, those whose orbits take them inside the orbit of Mars. Further subclassification of these, based on distance, is used, Apohele asteroids orbit inside of Earths perihelion distance. Aten asteroids, those that have semi-major axes of less than Earths, Apollo asteroids are those asteroids with a semimajor axis greater than Earths, while having a perihelion distance of 1.017 AU or less. Like Aten asteroids, Apollo asteroids are Earth-crossers, amor asteroids are those near-Earth asteroids that approach the orbit of Earth from beyond, but do not cross it
20.
Saturn's Inuit group of satellites
–
The Inuit group is a dynamical grouping of the prograde irregular satellites of Saturn which follow similar orbits. Their semi-major axes range between 11 and 18 Gm, their inclinations between 40° and 50°, and their eccentricities between 0.15 and 0.48, the diagram illustrates the Inuit group in relation to other irregular satellites of Saturn. The eccentricity of the orbits is represented by the segments with the inclination represented on Y axis. The five members of the group are, Kiviuq Ijiraq Paaliaq Siarnaq Tarqeq The International Astronomical Union uses names taken from Inuit mythology for these moons, the group appeared quite homogenous in early observations, the satellites displaying light-red colour and similar infrared spectra. Recent observations however, revealed that Ijiraq is distinctively redder than Paaliaq, Siarnaq, in addition, unlike the other three, Ijiraqs spectrum does not display a weak absorption near 0.7 μm. This feature is attributed to a possible water hydration, the spectral homogeneity is consistent with a common origin in the break-up of a single object but the dispersion of the orbital parameters requires further explanation. Recently reported secular resonances among the members could provide the explanation of the post-collisional dispersion
21.
Saturn's Norse group of satellites
–
The Norse group is a large group of retrograde irregular satellites of Saturn. Their semi-major axes range between 12 and 24 Gm, their inclinations between 136° and 175° and their eccentricities between 0.13 and 0.77. Unlike for the Inuit and Gallic groups, the parameters are widely dispersed. For example, the satellites with inclinations around 174 degrees alone are thought to comprise at least two subgroups, eight other moons form the Skathi subgroup. Their semi-major axes range between 15 and 20 Gm, their inclinations between 147° and 158°, narvi forms a separate subgroup with Bestla. The diagram illustrates the Norse group in relation to other satellites of Saturn. The eccentricity of the orbits is represented by the segments with the inclination represented on Y axis. The exception is Phoebe, the largest, which was discovered long before the others, S. Sheppards classification of Saturns irregular moons
22.
Saturn's Gallic group of satellites
–
The Gallic group is a dynamical grouping of the prograde irregular satellites of Saturn following similar orbits. Their semi-major axes range between 16 and 19 Gm, their inclinations between 35° and 40°, and their eccentricities around 0.53, the International Astronomical Union reserves names taken from Gallic mythology for these moons. Similar mean orbital elements led the discoverers to postulate a common origin for the group in a breakup of a larger body, the diagram illustrates the Gallic group in relation to other irregular satellites of Saturn. The eccentricity of the orbits is represented by the segments with the inclination represented on Y axis. Instead of the progenitor, it was postulated that Tarvos and Erriapus could be fragments of Albiorix, leaving a large. Such an impact would require a body with the diameter in excess of 1 km and relative velocity close to 5 km/s, numerous, very large craters observed on Phoebe, prove the existence of such collisions in the Saturnian systems past. Ephemeris from MPC Mean orbital parameters from JPL David Jewitt pages Scott Sheppard pages
23.
Phoebe (moon)
–
Phoebe is an irregular satellite of Saturn with a mean diameter of 213 km. It was the first satellite to be discovered photographically, Phoebe was the first target encountered upon the arrival of the Cassini spacecraft in the Saturn system in 2004, and is thus unusually well-studied for an irregular satellite of its size. Cassinis trajectory to Saturn and time of arrival were specifically chosen to permit this flyby, after the encounter and its insertion into orbit, Cassini did not go much beyond the orbit of Iapetus. Phoebe is roughly spherical and has a differentiated interior and it was spherical and hot early in its history and was battered out of roundness by repeated impacts. It is thought to be a captured planetesimal from the Kuiper belt and it was the first satellite to be discovered photographically. Phoebe was named after Phoebe, a Titan in Greek mythology that was associated with the Moon and it is also designated Saturn IX in some scientific literature. The IAU nomenclature standards have stated that features on Phoebe are to be named characters in the Greek myth of Jason. In 2005, the IAU officially named 24 craters, Phoebes orbit is retrograde, i. e. it orbits Saturn opposite to Saturns rotation. For more than 100 years, Phoebe was Saturns outermost known moon, Phoebe is almost 4 times more distant from Saturn than its nearest major neighbor, and is substantially larger than any of the other moons orbiting planets at comparable distances. All of Saturns regular moons except Iapetus orbit very nearly in the plane of Saturns equator, the outer irregular satellites follow moderately to highly eccentric orbits, and none are expected to rotate synchronously as all the inner moons of Saturn do. The Phoebe ring is one of the rings of Saturn and this ring is tilted 27 degrees from Saturns equatorial plane. It extends from at least 128 to 207 times the radius of Saturn, the ring is about 20 times as thick as the diameter of the planet. Inwardly migrating ring material would thus strike Iapetuss leading hemisphere, created its two-tone coloration, although very large, the ring is virtually invisible—it was discovered using NASAs infrared Spitzer Space Telescope. Phoebe is roughly spherical and has a diameter of 213±1.4 km, Phoebe rotates every nine hours 16 minutes and it completes a full orbit around Saturn in about 18 months. Its surface temperature is 75 K, most of Saturns inner moons have very bright surfaces, but Phoebes albedo is very low, as dark as lampblack. The Phoebean surface is heavily scarred, with craters up to 80 kilometres across. Phoebes dark coloring initially led to scientists surmising that it was a captured asteroid and these are chemically very primitive and are thought to be composed of original solids that condensed out of the solar nebula with little modification since then. In addition, quantities of carbon dioxide have been detected on the surface and it is estimated that Phoebe is about 50% rock, as opposed to the 35% or so that typifies Saturns inner moons
24.
Exposure (photography)
–
In photography, exposure is the amount of light per unit area reaching a photographic film or electronic image sensor, as determined by shutter speed, lens aperture and scene luminance. Exposure is measured in lux seconds, and can be computed from exposure value, in photographic jargon, an exposure generally refers to a single shutter cycle. For the same speed, the accumulated photometric exposure should be similar in both cases. A photograph may be described as underexposed when it has a loss of detail, that is. As the adjacent image shows, these terms are technical ones, there are three types of settings, manual, automatic and exposure compensation. Radiant exposure of a surface, denoted He and measured in J/m2, is given by H e = E e t, where Ee is the irradiance of the surface, measured in W/m2, t is the exposure duration, measured in s. Luminous exposure of a surface, denoted Hv and measured in lx⋅s, is given by H v = E v t, many photographic materials are also sensitive to invisible light, which can be a nuisance, or a benefit. The use of units is appropriate to characterize such sensitivity to invisible light. In sensitometric data, such as curves, the log exposure is conventionally expressed as log10. Photographers more familiar with base-2 logarithmic scales can convert using log2 ≈3.32 log10, correct exposure may be defined as an exposure that achieves the effect the photographer intended. A more technical approach recognises that a film has a physically limited useful exposure range. If, for any part of the photograph, the exposure is outside this range. In a very simple model, for example, out-of-range values would be recorded as black or white rather than the precisely graduated shades of colour and this ensures that no significant information is lost during capture. However, it is much easier to discard recorded information during post processing than to try to re-create unrecorded information. In a scene with strong or harsh lighting, the ratio between highlight and shadow luminance values may well be larger than the ratio between the maximum and minimum useful exposure values. A photograph may be described as underexposed when it has a loss of detail, that is. In manual mode, the photographer adjusts the lens aperture and/or shutter speed to achieve the desired exposure, manual exposure calculations may be based on some method of light metering with a working knowledge of exposure values, the APEX system and/or the Zone System. A camera in automatic exposure mode automatically calculates and adjusts exposure settings to match the subjects mid-tone to the mid-tone of the photograph, for most cameras this means using an on-board TTL exposure meter
25.
Astrophotography
–
Astrophotography is a specialized type of photography for recording photos of astronomical objects and large areas of the night sky. The first photograph of an object was taken in 1840. This is done by long time exposure since both film and digital cameras can accumulate and sum light photons over long periods of time. Astrophotography is a subdiscipline in amateur astronomy usually seeking aesthetically pleasing images rather than scientific data. Amateurs use a range of special equipment and techniques. With a few exceptions, astronomical photography employs long exposures since both film and digital imaging devices can accumulate and light photons over long periods of time, the amount of light hitting the film or detector is also increased by increasing the diameter of the primary optics being used. Since the Earth is constantly rotating, telescopes and equipment are rotated in the direction to follow the apparent motion of the stars overhead. This is accomplished by using either equatorial or computer-controlled altazimuth telescope mounts to keep celestial objects centered while the earth rotates, all telescope mount systems suffer from induced tracking error due to imperfect motor drives and mechanical sag of the telescope. Tracking errors are corrected by keeping a selected aiming point, usually a bright guide star, sometimes the object to be imaged is moving, so the telescope has to be kept constantly centered on that object. Guiding was formerly done manually throughout the exposure with an observer standing at the telescope making corrections to keep a cross hair on the guide star, since the advent of computer-controlled systems this is accomplished by an automated systems in professional and even amateur equipment. These all require specialized equipment such as telescopes designed for imaging, for wide field of view. Astronomical CCD cameras may use cryogenic cooling to reduce thermal noise, specialized filters are also used to record images in specific wavelengths. The development of astrophotography as a tool was pioneered in the mid-19th century for the most part by experimenters and amateur astronomers. Because of the long exposures needed to capture relatively faint astronomical objects. Early photographic processes also had limitations, the daguerreotype process was far too slow to record anything but the brightest objects, and the wet plate collodion process limited exposures to the time the plate could stay wet. The first-known attempt at astronomical photography was by Louis Jacques Mandé Daguerre, inventor of the process which bears his name. Tracking errors in guiding the telescope during the long exposure meant the photograph came out as an indistinct fuzzy spot, the Sun may have been first photographed in an 1845 daguerreotype by the French physicists Léon Foucault and Hippolyte Fizeau. No photographic alteration was caused by the light of the corona condensed by a lens for two minutes, during totality, on a sheet of paper prepared with bromide of silver, the Suns solar corona was first successfully imaged during the Solar eclipse of July 28,1851
26.
Optical telescope
–
The larger the objective, the more light the telescope collects and the finer detail it resolves. People use telescopes and binoculars for activities such as astronomy, ornithology, pilotage and reconnaissance. The telescope is more a discovery of optical craftsmen than an invention of a scientist and it is in the Netherlands in 1608 where the first recorded optical telescopes appeared. The invention is credited to the spectacle makers Hans Lippershey and Zacharias Janssen in Middelburg, galileo greatly improved on these designs the following year, and is generally credited as the first to use a telescope for astronomy. Galileos telescope used Hans Lippersheys design of an objective lens and a concave eye lens. Johannes Kepler proposed an improvement on the design used a convex eyepiece. For reflecting telescopes, which use a mirror in place of the objective lens. The theoretical basis for curved mirrors behaving similar to lenses was probably established by Alhazen, the late 20th century has seen the development of adaptive optics and space telescopes to overcome the problems of astronomical seeing. The basic scheme is that the primary light-gathering element the objective and this image may be recorded or viewed through an eyepiece, which acts like a magnifying glass. The eye then sees an inverted magnified virtual image of the object, most telescope designs produce an inverted image at the focal plane, these are referred to as inverting telescopes. In fact, the image is both turned upside down and reversed left to right, so that altogether it is rotated by 180 degrees from the object orientation, in astronomical telescopes the rotated view is normally not corrected, since it does not affect how the telescope is used. However, a diagonal is often used to place the eyepiece in a more convenient viewing location, and in that case the image is erect. In terrestrial telescopes such as spotting scopes, monoculars and binoculars, There are telescope designs that do not present an inverted image such as the Galilean refractor and the Gregorian reflector. These are referred to as erecting telescopes, many types of telescope fold or divert the optical path with secondary or tertiary mirrors. These may be part of the optical design, or may simply be used to place the eyepiece or detector at a more convenient position. Telescope designs may also use specially designed additional lenses or mirrors to improve image quality over a field of view. Design specifications relate to the characteristics of the telescope and how it performs optically, several properties of the specifications may change with the equipment or accessories used with the telescope, such as Barlow lenses, star diagonals and eyepieces. These interchangeable accessories dont alter the specifications of the telescope, however they alter the way the telescopes properties function, typically magnification, angular resolution and FOV
27.
Christiaan Huygens
–
Christiaan Huygens, FRS was a prominent Dutch mathematician and scientist. He is known particularly as an astronomer, physicist, probabilist and horologist, Huygens was a leading scientist of his time. His work included early telescopic studies of the rings of Saturn and the discovery of its moon Titan and he published major studies of mechanics and optics, and pioneered work on games of chance. Christiaan Huygens was born on 14 April 1629 in The Hague, into a rich and influential Dutch family, Christiaan was named after his paternal grandfather. His mother was Suzanna van Baerle and she died in 1637, shortly after the birth of Huygens sister. The couple had five children, Constantijn, Christiaan, Lodewijk, Philips, Constantijn Huygens was a diplomat and advisor to the House of Orange, and also a poet and musician. His friends included Galileo Galilei, Marin Mersenne and René Descartes, Huygens was educated at home until turning sixteen years old. He liked to play with miniatures of mills and other machines and his father gave him a liberal education, he studied languages and music, history and geography, mathematics, logic and rhetoric, but also dancing, fencing and horse riding. In 1644 Huygens had as his mathematical tutor Jan Jansz de Jonge Stampioen, Descartes was impressed by his skills in geometry. His father sent Huygens to study law and mathematics at the University of Leiden, Frans van Schooten was an academic at Leiden from 1646, and also a private tutor to Huygens and his elder brother, replacing Stampioen on the advice of Descartes. Van Schooten brought his mathematical education up to date, in introducing him to the work of Fermat on differential geometry. Constantijn Huygens was closely involved in the new College, which lasted only to 1669, Christiaan Huygens lived at the home of the jurist Johann Henryk Dauber, and had mathematics classes with the English lecturer John Pell. He completed his studies in August 1649 and he then had a stint as a diplomat on a mission with Henry, Duke of Nassau. It took him to Bentheim, then Flensburg and he took off for Denmark, visited Copenhagen and Helsingør, and hoped to cross the Øresund to visit Descartes in Stockholm. While his father Constantijn had wished his son Christiaan to be a diplomat, in political terms, the First Stadtholderless Period that began in 1650 meant that the House of Orange was not in power, removing Constantijns influence. Further, he realised that his son had no interest in such a career, Huygens generally wrote in French or Latin. While still a student at Leiden he began a correspondence with the intelligencer Mersenne. Mersenne wrote to Constantijn on his sons talent for mathematics, the letters show the early interests of Huygens in mathematics
28.
Objective (optics)
–
In optical engineering, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be a lens or mirror, or combinations of several optical elements. They are used in microscopes, telescopes, cameras, slide projectors, CD players, objectives are also called object lenses, object glasses, or objective glasses. The objective lens of a microscope is the one at the bottom near the sample, at its simplest, it is a very high-powered magnifying glass, with very short focal length. This is brought close to the specimen being examined so that the light from the specimen comes to a focus inside the microscope tube. The objective itself is usually a cylinder containing one or more lenses that are made of glass. Microscope objectives are characterized by two parameters, magnification and numerical aperture, the magnification typically ranges from 4× to 100×. Numerical aperture for microscope lenses typically ranges from 0.10 to 1.25, corresponding to focal lengths of about 40 mm to 2 mm, respectively. A typical microscope has three or four lenses with different magnifications, screwed into a circular nosepiece which may be rotated to select the required lens. These lenses are often color coded for easier use, the least powerful lens is called the scanning objective lens, and is typically a 4× objective. The second lens is referred to as the objective lens and is typically a 10× lens. The most powerful out of the three is referred to as the large objective lens and is typically 40–100×. Some microscopes use an oil-immersion or water-immersion lens, which can have greater than 100. These objectives are designed for use with refractive index matching oil or water. These lenses give greater resolution at high magnification, numerical apertures as high as 1.6 can be achieved with oil immersion. Camera lenses need to cover a large plane so are made up of a number of optical lens elements to correct optical aberrations. Image projectors use objective lenses that simply reverse the function of a lens, with lenses designed to cover a large image plane. In a telescope the objective is the lens at the front end of a refractor or the primary mirror of a reflecting or catadioptric telescope
29.
Refracting telescope
–
A refracting telescope is a type of optical telescope that uses a lens as its objective to form an image. The refracting telescope design was used in spy glasses and astronomical telescopes but is also used for long focus camera lenses. A refractors magnification is calculated by dividing the length of the objective lens by that of the eyepiece. Refractors were the earliest type of optical telescope, Galileo Galilei, happening to be in Venice in about the month of May 1609, heard of the invention and constructed a version of his own. Galileo then communicated the details of his invention to the public, all refracting telescopes use the same principles. The objective in a refracting telescope refracts or bends light and this refraction causes parallel light rays to converge at a focal point, while those not parallel converge upon a focal plane. The telescope converts a bundle of rays to make an angle α. The ratio β/α is called the angular magnification and it equals the ratio between the retinal image sizes obtained with and without the telescope. Refracting telescopes can come in different configurations to correct for image orientation. Because the image was formed by the bending of light, or refraction, the design Galileo Galilei used in 1609 is commonly called a Galilean telescope. It used a convergent objective lens and a divergent eyepiece lens, a Galilean telescope, because the design has no intermediary focus, results in a non-inverted and upright image. Galileo’s best telescope magnified objects about 30 times, because of flaws in its design, such as the shape of the lens and the narrow field of view, the images were blurry and distorted. Despite these flaws, the telescope was still enough for Galileo to explore the sky. The Galilean telescope could view the phases of Venus, and was able to see craters on the Moon, parallel rays of light from a distant object would be brought to a focus in the focal plane of the objective lens. The eyepiece lens intercepts these rays and renders them parallel once more, non-parallel rays of light from the object traveling at an angle α1 to the optical axis travel at a larger angle after they passed through the eyepiece. This leads to an increase in the apparent angular size and is responsible for the perceived magnification, the final image is a virtual image, located at infinity and is the same way up as the object. The Keplerian telescope, invented by Johannes Kepler in 1611, is an improvement on Galileos design and it uses a convex lens as the eyepiece instead of Galileos concave one. The advantage of this arrangement is that the rays of light emerging from the eyepiece are converging and this allows for a much wider field of view and greater eye relief, but the image for the viewer is inverted
30.
Tethys (moon)
–
Tethys is a mid-sized moon of Saturn about 1,060 km across. It was discovered by G. D. Cassini in 1684 and is named after the titan Tethys of Greek mythology. Tethys has a low density of 0.98 g/cm3, the lowest of all the moons in the Solar System. This is confirmed by the spectroscopy of its surface, which identified water ice as the dominant surface material, a small amount of an unidentified dark material is present as well. The surface of Tethys is very bright, being the second-brightest of the moons of Saturn after Enceladus, Tethys is heavily cratered and cut by a number of large faults/graben. The largest impact crater, Odysseus, is about 400 km in diameter, whereas the largest graben, Ithaca Chasma, is about 100 km wide and these two largest surface features may be related. A small part of the surface is covered by smooth plains that may be cryovolcanic in origin, like all other regular moons of Saturn, Tethys formed from the Saturnian sub-nebula—a disk of gas and dust that surrounded Saturn soon after its formation. Tethys has been approached by several space probes including Pioneer 11, Voyager 1, Voyager 2, Tethys was discovered by Giovanni Domenico Cassini in 1684 together with Dione, another moon of Saturn. He had also discovered two moons, Rhea and Iapetus earlier, in 1671–72, Cassini observed all of these moons using a large aerial telescope he set up on the grounds of the Paris Observatory. Cassini named the four new moons as Sidera Lodoicea to honour king Louis XIV of France, by the end of the seventeenth century, astronomers fell into the habit of referring to them and Titan as Saturn I through Saturn V. Once Mimas and Enceladus were discovered in 1789, the scheme was extended to Saturn VII by bumping the older five moons up two slots. The discovery of Hyperion in 1848 changed the numbers one last time, henceforth, the numbering scheme would remain fixed. The modern names of all seven satellites of Saturn come from John Herschel, in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope, he suggested the names of the Titans, sisters and brothers of Kronos, be used. Tethys is named after the titaness Tethys of Greek mythology and it is also designated Saturn III or S III Tethys. The correct adjectival form of the name is Tethyan, although other forms are also used. Tethys orbits Saturn at a distance of about 295000 km from the center of the planet and its orbital eccentricity is negligible, whereas its orbital inclination is about 1°. Tethys is locked in a resonance with Mimas, which, however, does not cause any noticeable orbital eccentricity. The Tethyan orbit lies deep inside the magnetosphere of Saturn, so the plasma co-rotating with the planet strikes the trailing hemisphere of the moon, Tethys is also subject to constant bombardment by the energetic particles present in the magnetosphere
31.
Dione (moon)
–
Dione is a moon of Saturn. It was discovered by Italian astronomer Giovanni Domenico Cassini in 1684 and it is named after the Titaness Dione of Greek mythology. It is also designated Saturn IV, Giovanni Domenico Cassini named the four moons he discovered Sidera Lodoicea to honor king Louis XIV. Cassini found Dione in 1684 using a large aerial telescope he set up on the grounds of the Paris Observatory, Dione orbits Saturn with a semimajor axis about 2% less than that of the Moon. However, reflecting Saturns greater mass, Diones orbital period is one tenth that of the Moon, Dione is currently in a 1,2 mean-motion orbital resonance with moon Enceladus, completing one orbit of Saturn for every two orbits completed by Enceladus. The resonance also maintains a smaller eccentricity in Diones orbit, tidally heating it as well, Dione has two co-orbital, or trojan, moons, Helene and Polydeuces. They are located within Diones Lagrangian points L4 and L5,60 degrees ahead of, at 1122 km in diameter, Dione is the 15th largest moon in the Solar System, and is more massive than all known moons smaller than itself combined. About two thirds of Diones mass is water ice, and the remaining is a dense core, data gathered by Cassini indicates that Dione has an internal liquid water ocean. Downward bending of the associated with the 1.5 km high ridge Janiculum Dorsa can most easily be explained by the presence of such an ocean. Gravity and shape data points to a 99 ±23 km thick ice shell crust on top of a 65 ±30 internal liquid water global ocean, neither moon has a shape close to hydrostatic equilibrium, the deviations are maintained by isostasy. Diones ice shell is thought to vary in thickness by less than 5%, with the thinnest areas at the poles, though somewhat smaller and denser, Dione is otherwise very similar to Rhea. They both have similar features and varied terrain, and both have dissimilar leading and trailing hemispheres. Diones leading hemisphere is heavily cratered and is uniformly bright and its trailing hemisphere, however, contains an unusual and distinctive surface feature, a network of bright ice cliffs. The origin of these features was mysterious, because all that was known was that the material has an albedo and is thin enough that it does not obscure the surface features underneath. Later, after the activity and resurfacing ceased, cratering continued primarily on the leading hemisphere. This hypothesis was proven wrong by the Cassini probe flyby of December 13,2004 and these revealed that the wisps were, in fact, not ice deposits at all, but rather bright ice cliffs created by tectonic fractures. Dione has been revealed as a world riven by enormous fractures on its trailing hemisphere. The Cassini orbiter performed a flyby of Dione at 500 km on October 11,2005
32.
Iapetus (moon)
–
Iapetus is best known for its dramatic two-tone coloration. Discoveries by the Cassini mission in 2007 revealed several other unusual features, Iapetus was discovered by Giovanni Domenico Cassini, an Italian astronomer, in October 1671. He had discovered it on the side of Saturn and tried viewing it on the eastern side some months later. This was also the case the year, when he was again able to observe it on the western side. Cassini finally observed Iapetus on the side in 1705 with the help of an improved telescope. Cassini correctly surmised that Iapetus has a bright hemisphere and a dark hemisphere, and this means that the bright hemisphere is visible from Earth when Iapetus is on the western side of Saturn, and that the dark hemisphere is visible when Iapetus is on the eastern side. The dark hemisphere was later named Cassini Regio in his honour, Iapetus is named after the Titan Iapetus from Greek mythology. When first discovered, Iapetus was among four Saturnian moons labelled the Sidera Lodoicea by their discoverer Giovanni Cassini after King Louis XIV, however, astronomers fell into the habit of referring to them using Roman numerals, with Iapetus being Saturn V. Once Mimas and Enceladus were discovered in 1789, the scheme was extended. And with the discovery of Hyperion in 1848, Iapetus became Saturn VIII, geological features on Iapetus are named after characters and places from the French epic poem The Song of Roland. Examples of names used include the craters Charlemagne and Baligant, the one exception is Cassini Regio, the dark region of Iapetus, named after the regions discoverer, Giovanni Cassini. The orbit of Iapetus is somewhat unusual, although it is Saturns third-largest moon, it orbits much farther from Saturn than the next closest major moon, Titan. It has also the most inclined orbital plane of the regular satellites, the cause of this is unknown. From Iapetus, Saturn would appear to be 1°56 in diameter, the low density of Iapetus indicates that it is mostly composed of ice, with only a small amount of rocky materials. These features often lead it to be characterized as walnut-shaped, Iapetus is heavily cratered, and Cassini images have revealed large impact basins, at least five of which are over 350 km wide. The largest, Turgis, has a diameter of 580 km, its rim is extremely steep, Iapetus is known to support long-runout landslides or sturzstroms, possibly supported by ice sliding. In the 17th century, Giovanni Cassini observed that he could see Iapetus only on the west side of Saturn and he correctly deduced that Iapetus is locked in synchronous rotation about Saturn and that one side of Iapetus is darker than the other, conclusions later confirmed by larger telescopes. The difference in colouring between the two Iapetian hemispheres is striking, the leading hemisphere and sides are dark with a slight reddish-brown coloring, while most of the trailing hemisphere and poles are bright
33.
Sidera Lodoicea
–
Cassini intended the name to honor King Louis XIV of France, who reigned from 1643 to 1715, and who was Cassinis benefactor as patron of the Paris Observatory, of which Cassini was the director. The name was modelled on Sidera Medicea, Medicean stars, the Latin name used by Galileo to name the four Galilean satellites of Jupiter, an Extract of the Journal Des Scavans. Giving an Account of Two New Satellites of Saturn, Discovered Lately by Mr. Cassini at the Royal Observatory at Paris, philosophical Transactions of the Royal Society of London
34.
Giovanni Domenico Cassini
–
Giovanni Domenico Cassini was an Italian mathematician, astronomer, astrologer and engineer. Cassini was born in Perinaldo, near Imperia, at time in the County of Nice. Cassini is known for his work in the fields of astronomy, Cassini discovered four satellites of the planet Saturn and noted the division of the rings of Saturn, the Cassini Division was named after him. Giovanni Domenico Cassini was also the first of his family to work on the project of creating a topographic map of France. The Cassini spaceprobe, launched in 1997, was named after him and became the fourth to visit Saturn, Cassini was the son of Jacopo Cassini, a Tuscan, and Giulia Crovesi. Cassini accepted a position at the observatory at Panzano, near Bologna, to work with Marquis Cornelio Malvasia, during Cassinis time at the Panzano Observatory, Cassini was able to complete his education under the scientists Giovanni Battista Riccioli and Francesco Maria Grimaldi. In 1650 the senate of Bologna appointed Cassini as the chair of astronomy at the University of Bologna. In San Petronio, Bologna, he created an important meridian, Cassini remained in Bologna working until Colbert recruited him to come to Paris to help set up the Paris Observatory. Cassini departed from Bologna on 25 February 1669, during this time, Cassinis method of determining longitude was used to measure the size of France accurately for the first time. The country turned out to be smaller than expected. On 14 July 1673 Cassini obtained the benefits of French citizenship, in 1674 he married Geneviève de Laistre, the daughter of the lieutenant general of the comté of Clermont. From this marriage Cassini had two sons, the younger, Jacques Cassini, succeeded him as astronomer and geodesist under the name of Cassini II, in 1711 Cassini went blind and he died on 14 September 1712 in Paris at the age of 87. He was famous for many things such as, The first ever to observe the divisions in the rings of Saturn. The Cassini Division The Cassini Laws The Cassini Oval Cassini was an astronomer at the Panzano Observatory and he was appointed professor of astronomy at the University of Bologna in 1650 and became, in 1671, director of the Paris Observatory. In addition he discovered the Cassini Division in the rings of Saturn and he shares with Robert Hooke credit for the discovery of the Great Red Spot on Jupiter. Around 1690, Cassini was the first to observe differential rotation within Jupiters atmosphere, in 1672 he sent his colleague Jean Richer to Cayenne, French Guiana, while he himself stayed in Paris. The two made simultaneous observations of Mars and, by computing the parallax, determined its distance from Earth, in 1659 he presented a model of the planetary system that was in accord with the hypothesis of Tycho Brahe. In 1661 he developed a method, inspired by Kepler’s work, of mapping successive phases of solar eclipses, Cassini also rejected Newtons theory of gravity, after measurements he conducted which wrongly suggested that the Earth was elongated at its poles
35.
William Herschel
–
Frederick William Herschel, KH, FRS was a British astronomer and composer of German origin, and brother of fellow astronomer Caroline Herschel, with whom he worked. Born in the Electorate of Hanover, Herschel followed his father into the Military Band of Hanover, Herschel constructed his first large telescope in 1774, after which he spent nine years carrying out sky surveys to investigate double stars. The resolving power of the Herschel telescopes revealed that the nebulae in the Messier catalogue were clusters of stars, Herschel published catalogues of nebulae in 1802 and in 1820. In the course of an observation on 13 March 1781, he realized that one celestial body he had observed was not a star and this was the first planet to be discovered since antiquity and Herschel became famous overnight. As a result of this discovery, George III appointed him Court Astronomer and he was elected as a Fellow of the Royal Society and grants were provided for the construction of new telescopes. Herschel pioneered the use of astronomical spectrophotometry as a tool, using prisms. Other work included a determination of the rotation period of Mars, the discovery that the Martian polar caps vary seasonally. In addition, Herschel discovered infrared radiation, Herschel was made a Knight of the Royal Guelphic Order in 1816. He was the first President of the Royal Astronomical Society when it was founded in 1820 and he died in August 1822, and his work was continued by his only son, John Herschel. Herschel was born in the Electorate of Hanover in Germany, then part of the Holy Roman Empire and his father was an oboist in the Hanover Military Band. In 1755 the Hanoverian Guards regiment, in whose band Wilhelm, at the time the crowns of Great Britain and Hanover were united under King George II. As the threat of war with France loomed, the Hanoverian Guards were recalled from England to defend Hanover, after they were defeated at the Battle of Hastenbeck, Herschels father Isaak sent his two sons to seek refuge in England in late 1757. Although his older brother Jakob had received his dismissal from the Hanoverian Guards, Wilhelm, nineteen years old at this time, was a quick student of the English language. In England he went by the English rendition of his name, in addition to the oboe, he played the violin and harpsichord and later the organ. He composed numerous works, including 24 symphonies and many concertos. Six of his symphonies were recorded in April 2002 by the London Mozart Players, Herschel moved to Sunderland in 1761 when Charles Avison immediately engaged him as first violin and soloist for his Newcastle orchestra, where he played for one season. In ‘Sunderland in the County of Durh, apprill 20th 1761’ he wrote his Symphony No.8 in C Minor. He was head of the Durham Militia band 1760–61 and visited the home of Sir Ralph Milbanke at Halnaby Hall in 1760, after Newcastle he moved to Leeds and Halifax where he was the first organist at St John the Baptist church
36.
William Cranch Bond
–
William Cranch Bond was an American astronomer, and the first director of Harvard College Observatory. William Cranch Bond was born in Falmouth, Maine on September 9,1789. When he was young, his father, William Bond, established himself as a clockmaker after a business venture, trained by his father and aided by his penchant for engineering. Bond built his first clock when he was fifteen years old and he eventually took over his father’s business, becoming an expert clockmaker himself. The William Bond clock shop remained in existence at 9 Park Street in Boston until the 1970s, in 1806, when he was seventeen years old, Bond saw a solar eclipse. Soon thereafter, he became an amateur astronomer. When he built his first house, Bond made its parlor an observatory, in 1815, Bond traveled to Europe, commissioned by Harvard University to gather information on European observatories. On 18 July 1819 at Kingsbridge in Devon, England, Bond married his first cousin, Selina Cranch, after Selinas death in 1831, Bond married her older sister, Mary Roope Cranch. He was elected a Fellow of the American Academy of Arts, in 1839, Bond was allowed to move his personal astronomical equipment to Harvard and serve as its Astronomical Observer to the University. Later, in 1843, a sun-grazing comet aroused enough public interest in astronomy that Harvard was able to raise $25,730 towards the construction of a state-of-the-art observatory. Bond designed the building and the chair, and Harvard bought a fifteen-inch German-built refracting telescope. The telescope was first put to use on June 24,1847, father and son were the first to observe the then innermost ring of Saturn, termed the Crepe ring, when they pointed Harvard’s telescope towards Saturn in 1850. Working with John Adams Whipple, the Bonds pioneered astrophotography, taking the first daguerreotype image of a star ever taken from America, in all, the three took between 200 and 300 photos of celestial objects. A number of objects have been named in Bonds honor. A few of them include, The crater W. Bond on the Moon is named after him, a region on Hyperion is called the Bond-Lassell Dorsum Asteroid Bondia is jointly named after him and his son. The Bond Gap within Saturns C Ring is jointly named after him, MNRAS91, Discovery of a new satellite of Saturn Harvards site on Bond and their observatory Gillespie, Charles Couston, editor-in-chief. Dictionary of Scientific Biography, vol 2
37.
George Phillips Bond
–
George Phillips Bond was an American astronomer. He was the son of William Cranch Bond, some sources give his year of birth as 1826. His early interest was in nature and birds, but after his elder brother William Cranch Bond Jr. died and he succeeded his father as director of Harvard College Observatory from 1859 until his death. His cousin was Edward Singleton Holden, first director of Lick Observatory, Bond also studied Saturn and the Orion Nebula. He and his father jointly discovered Saturns moon, Hyperion, in addition to his astronomical contributions, Bond also surveyed the White Mountains of New Hampshire. Won the Gold Medal of the Royal Astronomical Society in 1865, mount Bond, West Bond, and Bondcliff among the White Mountains are all named after him. The crater G. Bond on the Moon is named after him, the Bond albedo, which is important for describing a planetary bodys energy balance, is also named for him. A region on Hyperion is called the Bond-Lassell Dorsum Asteroid Bondia is jointly named after him, the Bond Gap within Saturns C Ring is jointly named after him and his father. Http, //messier. seds. org/xtra/Bios/gpbond. html MNRAS91, Discovery of a new satellite of Saturn Presentation of RAS gold medal Brief obituary notice The Bonds, Pioneers of American Astronomy
38.
William Lassell
–
William Lassell, FRS FRSE FRSL FRAS was an English merchant and astronomer. He is remembered for his improvements to the Reflecting telescope and his ensuing planetary discoveries, william Lassell was born in Bolton, Lancashire, a town west of Manchester. He was educated first in Bolton then at Rochdale Academy, after the death of his father, he was apprenticed from 1814 to 1821 to a merchant in Liverpool. He then made his fortune as a brewer, which enabled him to indulge his interest in astronomy. He built an observatory at his house Starfield in West Derby, there he had a 24-inch reflector telescope, for which he pioneered the use of an equatorial mount for easy tracking of objects as the Earth rotates. He ground and polished the mirror himself, using equipment he constructed, the observatory was later moved further out of Liverpool, to Bradstone. In 1846 Lassell discovered Triton, the largest moon of Neptune, in 1848 he independently co-discovered Hyperion, a moon of Saturn. In 1851 he discovered Ariel and Umbriel, two moons of Uranus, when Queen Victoria visited Liverpool in 1851, Lassell was the only local she specifically requested to meet. In 1855, he built a 48-inch telescope, which he installed in Malta because of the conditions that were better than in often-overcast England. On his return to the UK after several years in Malta he moved to Maidenhead, the 48-inch telescope was dismantled and was eventually scrapped. Lassell was a Fellow of the Royal Astronomical Society from 1839, won the Gold Medal of the Royal Astronomical Society in 1849 and he was elected a Fellow of the Royal Society in 1849 and won their Royal Medal in 1858. Lassel was also a Fellow of the Royal Society of Literature, Lassell died in Maidenhead in 1880. Upon his death, he left a fortune of £80,000 and his telescope was presented to the Royal Observatory in Greenwich. The crater Lassell on the Moon, a crater on Mars, the asteroid 2636 Lassell, Biography and other topics Short biography and pictures Biography and technical detail of telescopes
39.
Long-exposure photography
–
Long-exposure photography captures one element that conventional photography does not, time. The paths of bright moving objects become clearly visible, clouds form broad bands, head and tail lights of cars become bright streaks, stars form trails in the sky and water smooths over. Only bright objects will form visible trails, however, dark objects usually disappear, boats during daytime long exposures will disappear, but will form bright trails from their lights at night. Whereas there is no fixed definition of what constitutes long, the intent is to create a photo that shows the effect of passing time. A 30-minute photo of an object and surrounding cannot be distinguished from a short exposure, hence. Images with exposure times of several minutes also tend to make moving people or dark objects disappear, often adding a serene, when a scene includes both stationary and moving subjects, a slow shutter speed can cause interesting effects, such as light trails. Long exposures are easiest to accomplish in low-light conditions, but can be done in brighter light using neutral density filters or specially designed cameras, when using a dense neutral density filter your cameras auto focus will not be able to function. It is best to compose and focus without the filter, then once you are happy with the composition, switch to manual focus and put the neutral density filter back on. Long-exposure photography is used in a night-time setting, where the lack of light forces longer exposures. Increasing ISO sensitivity allows for shorter exposures, but substantially decreases image quality through reduced dynamic range and higher noise. By leaving the cameras shutter open for a period of time, more light is absorbed. If the camera is stationary for the period of time that the shutter is open. In this technique, a scene is very dark and the photographer or an assistant takes a light source—it can be small penlight—and moves it about in patterns. The light source can be turned off between strokes, often, stationary objects in the scene are illuminated by briefly turning on studio lights, by one or more flashes from a strobe light, or by increasing the aperture. Long exposures can blur moving water so it has mist-like qualities while keeping stationary objects like land, solargraphy is a technique in which a fixed pinhole camera is used to expose photographic paper for an extremely long amount of time. It is most often used to show the path of the Sun across the sky, one example of this is a single six-month exposure taken by photographer Justin Quinnell, showing sun-trails over Clifton Suspension Bridge between 19 December 2007 and 21 June 2008. Part of the Slow light,6 months over Bristol exhibition and this method of solargraphy uses a simple pinhole camera securely fixed in a position which wont be disturbed. Quinnel constructed his camera from an empty drink can with a 0. 25mm aperture, on February 3,2015 a pinhole camera used in a Georgia State University solargraphy art project was blown up by the Atlanta bomb squad
40.
William Henry Pickering
–
William Henry Pickering was an American astronomer. Pickering constructed and established several observatories or astronomical observation stations, notably including Percival Lowells Flagstaff Observatory and he led solar eclipse expeditions and studied craters on the Moon, and hypothesized that changes in the appearance of the crater Eratosthenes were due to lunar insects. He spent much of the part of his life at his private observatory in Jamaica. William Pickering was born on February 15,1858 in Boston and his older brother was Edward Charles Pickering, director of the Harvard College Observatory for three decades. He graduated from the Massachusetts Institute of Technology in 1879 then became an instructor in physics from 1880 to 1887 and he was elected a Fellow of the American Academy of Arts and Sciences in 1883 at age 25. He discovered Saturns ninth moon Phoebe in 1899 from plates taken in 1898 and he produced a photographic atlas of the Moon, The Moon, A Summary of the Existing Knowledge of our Satellite in 1903. He believed he discovered a moon in 1905 from plates taken in 1904. For this discovery he was awarded the Lalande Prize of the French Academy of Sciences in 1905, Themis was later shown to not exist. Following George Darwin, he speculated in 1907 that the moon was once a part of the earth and that it broke away where now the Pacific Ocean lies. He also proposed a version of continental drift before Alfred Wegener where America, Asia, Africa, and Europe once formed a single continent, when the planet was named, he interpreted its symbol as a monogram referring to himself and Lowell by the phrase Pickering-Lowell. He claimed to have found vegetation on the moon in 1921, in 1923 he retired from Harvard University. He died on January 16,1938 in Mandeville, Jamaica and he won the Prix Lalande in 1905 and the Prix Jules Janssen in 1909. The asteroid 784 Pickeringia, and the craters Pickering on the Moon and Pickering on Mars, are named after him. William H. Pickering Papers, 1892-1893 from the Smithsonian Institution Archives Obituaries JRASC32157 MNRAS99328 PASP50122
41.
Equinox
–
An equinox is the moment in which the plane of Earths equator passes through the center of the Sun, which occurs twice each year, around 20 March and 23 September. On an equinox, day and night are of equal duration all over the planet. They are not exactly equal, however, due to the size of the sun. To avoid this ambiguity, the word equilux is sometimes used to mean a day in which the durations of light, see Length of equinoctial day and night for further discussion. The word is derived from the Latin aequinoctium, aequus and nox, the equinoxes are the only times when the solar terminator is perpendicular to the equator. As a result, the northern and southern hemispheres are equally illuminated, the word comes from Latin equi or equal and nox meaning night. In other words, the equinoxes are the times when the subsolar point is on the equator. The subsolar point crosses the equator moving northward at the March equinox, the equinoxes, along with solstices, are directly related to the seasons of the year. In the southern hemisphere, the equinox occurs in September. When Julius Caesar established the Julian calendar in 45 BC, he set 25 March as the date of the spring equinox. Because the Julian year is longer than the tropical year. By 1500 AD, it had drifted backwards to 11 March and this drift induced Pope Gregory XIII to create a modern Gregorian calendar. However, the leap year intervals in his calendar were not smooth and this causes the equinox to oscillate by about 53 hours around its mean position. This in turn raised the possibility that it could fall on 22 March, the astronomers chose the appropriate number of days to omit so that the equinox would swing from 19 to 21 March but never fall on the 22nd. Vernal equinox and Autumnal equinox, these names are direct derivatives of Latin. The equivalent common language English terms spring equinox and autumn equinox are even more ambiguous, March equinox and September equinox, names referring to the months of the year they occur, with no ambiguity as to which hemisphere is the context. They are still not universal, however, as not all use a solar-based calendar where the equinoxes occur every year in the same month. Northward equinox and southward equinox, names referring to the apparent direction of motion of the Sun
42.
Janus (moon)
–
Janus is an inner satellite of Saturn. It is also known as Saturn X and it is named after the mythological Janus. Janus was identified by Audouin Dollfus on 15 December 1966 and given the temporary designation S/1966 S2, previously, Jean Texereau had photographed Janus on 29 October 1966 without realising it. On December 18, Richard Walker observed an object in the orbit as Janus. Voyager 1 confirmed this orbital configuration in 1980, Janus was observed on subsequent occasions and given different provisional designations. Although the name was proposed soon after the initial 1966 discovery, it was not officially named until 1983. The Oxford English Dictionary lists the adjectival form of the name as Janian. Januss orbit is co-orbital with that of Epimetheus, Januss mean orbital radius from Saturn is, as of 2006, only 50 km less than that of Epimetheus, a distance smaller than either moons mean radius. In accordance with Keplers laws of motion, the closer orbit is completed more quickly. Because of the difference it is completed in only about 30 seconds less. Each day, the moon is an additional 0. 25° farther around Saturn than the outer moon. As the inner moon catches up to the moon, their mutual gravitational attraction increases the inner moons momentum. This added momentum means that the moons distance from Saturn and orbital period are increased. The timing and magnitude of the exchange is such that the moons effectively swap orbits. At each encounter Januss orbital radius changes by ~20 km and Epimetheuss by ~80 km, the exchange takes place close to every four years, the last close approaches occurred in January 2006,2010 and 2014, and the next in 2018. This is the only such orbital configuration known in the Solar System, the orbital relationship between Janus and Epimetheus can be understood in terms of the circular restricted three-body problem, as a case in which the two moons are similar in size to each other. Janus is extensively cratered with several craters larger than 30 km, Januss surface appears to be older than Prometheuss but younger than Pandoras. Janus has a low density and relatively high albedo, meaning that it is likely very icy
43.
Epimetheus (moon)
–
Epimetheus is an inner satellite of Saturn. It is also known as Saturn XI and it is named after the mythological Epimetheus, brother of Prometheus. Epimetheus occupies essentially the same orbit as the moon Janus, astronomers assumed that there was only one body in that orbit, and accordingly had difficulty determining their orbital characteristics. Observations were photographic and spaced apart in time, so that while the presence of two objects was not obvious, the observations were difficult to reconcile with a reasonable orbit. Audouin Dollfus observed a moon on 15 December 1966, which he proposed to be named Janus, on 18 December, Richard Walker made a similar observation which is now credited as the discovery of Epimetheus. However, at the time, it was believed there was only one moon, unofficially known as Janus. Twelve years later, in October 1978, Stephen M. Larson and this was confirmed in 1980 by Voyager 1, and so Larson and Fountain officially share the discovery of Epimetheus with Walker. Epimetheus received its name in 1983, the name Janus was approved by the IAU at the same time, although the name had been used informally since Dollfus proposed it shortly after the 1966 discovery. Epimetheuss orbit is co-orbital with that of Janus, Januss mean orbital radius from Saturn is, as of 2006, only 50 km less than that of Epimetheus, a distance smaller than either moons mean radius. In accordance with Keplers laws of motion, the closer orbit is completed more quickly. Because of the difference it is completed in only about 30 seconds less. Each day, the moon is an additional 0. 25° farther around Saturn than the outer moon. As the inner moon catches up to the moon, their mutual gravitational attraction increases the inner moons momentum. This added momentum means that the moons distance from Saturn and orbital period are increased. The timing and magnitude of the exchange is such that the moons effectively swap orbits. At each encounter Januss orbital radius changes by ~20 km and Epimetheuss by ~80 km, the exchange takes place close to every four years, the last close approaches occurred in January 2006,2010 and 2014, and the next in 2018. This is the only such orbital configuration known in the Solar System, the orbital relationship between Janus and Epimetheus can be understood in terms of the circular restricted three-body problem, as a case in which the two moons are similar in size to each other. There are several Epimethean craters larger than 30 km in diameter, the extensive cratering indicates that Epimetheus must be quite old
44.
Voyager program
–
The Voyager program is a continuing American scientific program that employs two robotic probes, Voyager 1 and Voyager 2, to study the outer Solar System. They were launched in 1977 to take advantage of an alignment of Jupiter, Saturn, Uranus, and Neptune. Their mission has been extended three times, and both continue to collect and relay useful scientific data. Neither Uranus nor Neptune has been visited by any other than Voyager 2. On August 25,2012, data from Voyager 1 indicated that it had become the first human-made object to enter space, traveling further than anyone, or anything. As of 2013, Voyager 1 was moving with a velocity of 17 kilometers per second relative to the Sun, Data and photographs collected by the Voyagers cameras, magnetometers, and other instruments revealed previously unknown details about each of the giant planets and their moons. Close-up images from the spacecraft charted Jupiter’s complex cloud forms, winds, saturn’s rings were found to have enigmatic braids, kinks, and spokes and to be accompanied by myriad ringlets. At Uranus Voyager 2 discovered a magnetic field around the planet and 10 additional moons. Its flyby of Neptune uncovered three complete rings and six hitherto unknown moons as well as a magnetic field and complex. Voyager 2 is still the only spacecraft to have visited the ice giants, the two Voyager space probes were originally conceived as part of the Mariner program, and they were thus initially named Mariner 11 and Mariner 12. The Voyager Program was similar to the Planetary Grand Tour planned during the late 1960s, the Grand Tour would take advantage of an alignment of the outer planets discovered by Gary Flandro, an aerospace engineer at the Jet Propulsion Laboratory. This alignment, which occurs once every 175 years, would occur in the late 1970s and make it possible to use gravitational assists to explore Jupiter, Saturn, Uranus, Neptune, and Pluto. The Planetary Grand Tour was to several pairs of probes to fly by all the outer planets along various trajectories, including Jupiter-Saturn-Pluto. Limited funding ended the Grand Tour program, but elements were incorporated into the Voyager Program, Voyager 2 was the first to launch. Its trajectory was designed to allow flybys of Jupiter, Saturn, Uranus and this encounter sent Voyager 1 out of the plane of the ecliptic, ending its planetary science mission. Had Voyager 1 been unable to perform the Titan flyby, the trajectory of Voyager 2 could have altered to explore Titan, forgoing any visit to Uranus. Voyager 1 was not launched on a trajectory that would have allowed it to continue to Uranus and Neptune, but could have continued from Saturn to Pluto without exploring Titan. The New Horizons probe, which had a higher velocity than Voyager 1, is traveling more slowly due to the extra speed Voyager 1 gained from its flybys of Jupiter
.jpg)
.jpg){kind=small}
. Bibcode:2008AJ....135.1083T. doi:10.1088/0004-6256/135/3/1083.
