A day is the period of time during which the Earth completes one rotation around its axis. A solar day is the length of time which elapses between the Sun reaching its highest point in the sky two consecutive times. In 1960, the second was redefined in terms of the orbital motion of the Earth in year 1900, was designated the SI base unit of time; the unit of measurement "day", was symbolized d. In 1967, the second and so the day were redefined by atomic electron transition. A civil day is 86,400 seconds, plus or minus a possible leap second in Coordinated Universal Time, plus or minus an hour in those locations that change from or to daylight saving time. Day can be defined as each of the twenty-four-hour periods, reckoned from one midnight to the next, into which a week, month, or year is divided, corresponding to a rotation of the earth on its axis; however its use depends on its context, for example when people say'day and night','day' will have a different meaning. It will mean the interval of light between two successive nights.
However, in order to be clear when using'day' in that sense, "daytime" should be used to distinguish it from "day" referring to a 24-hour period. The word day may refer to a day of the week or to a calendar date, as in answer to the question, "On which day?" The life patterns of humans and many other species are related to Earth's solar day and the day-night cycle. Several definitions of this universal human concept are used according to context and convenience. Besides the day of 24 hours, the word day is used for several different spans of time based on the rotation of the Earth around its axis. An important one is the solar day, defined as the time it takes for the Sun to return to its culmination point; because celestial orbits are not circular, thus objects travel at different speeds at various positions in their orbit, a solar day is not the same length of time throughout the orbital year. Because the Earth orbits the Sun elliptically as the Earth spins on an inclined axis, this period can be up to 7.9 seconds more than 24 hours.
In recent decades, the average length of a solar day on Earth has been about 86 400.002 seconds and there are about 365.2422 solar days in one mean tropical year. Ancient custom has a new day start at either the setting of the Sun on the local horizon; the exact moment of, the interval between, two sunrises or sunsets depends on the geographical position, the time of year. A more constant day can be defined by the Sun passing through the local meridian, which happens at local noon or midnight; the exact moment is dependent on the geographical longitude, to a lesser extent on the time of the year. The length of such a day is nearly constant; this is the time as indicated by modern sundials. A further improvement defines a fictitious mean Sun that moves with constant speed along the celestial equator. A day, understood as the span of time it takes for the Earth to make one entire rotation with respect to the celestial background or a distant star, is called a stellar day; this period of rotation is about 4 minutes less than 24 hours and there are about 366.2422 stellar days in one mean tropical year.
Other planets and moons have solar days of different lengths from Earth's. A day, in the sense of daytime, distinguished from night time, is defined as the period during which sunlight directly reaches the ground, assuming that there are no local obstacles; the length of daytime averages more than half of the 24-hour day. Two effects make daytime on average longer than nights; the Sun has an apparent size of about 32 minutes of arc. Additionally, the atmosphere refracts sunlight in such a way that some of it reaches the ground when the Sun is below the horizon by about 34 minutes of arc. So the first light reaches the ground when the centre of the Sun is still below the horizon by about 50 minutes of arc. Thus, daytime is on average around 7 minutes longer than 12 hours; the term comes from the Old English dæg, with its cognates such as dagur in Icelandic, Tag in German, dag in Norwegian, Danish and Dutch. All of them from the Indo-European root dyau which explains the similarity with Latin dies though the word is known to come from the Germanic branch.
As of October 17, 2015, day is the 205th most common word in US English, the 210th most common in UK English. A day, symbol d, defined as 86 400 seconds, is not an SI unit, but is accepted for use with SI; the Second is the base unit of time in SI units. In 1967–68, during the 13th CGPM, the International Bureau of Weights and Measures redefined a second as … the duration of 9 192 631 770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium 133 atom; this makes the SI-based day last 794 243 384 928 000 of those periods. Due to tidal effects, the
The apparent magnitude of an astronomical object is a number, a measure of its brightness as seen by an observer on Earth. The magnitude scale is logarithmic. A difference of 1 in magnitude corresponds to a change in brightness by a factor of 5√100, or about 2.512. The brighter an object appears, the lower its magnitude value, with the brightest astronomical objects having negative apparent magnitudes: for example Sirius at −1.46. The measurement of apparent magnitudes or brightnesses of celestial objects is known as photometry. Apparent magnitudes are used to quantify the brightness of sources at ultraviolet and infrared wavelengths. An apparent magnitude is measured in a specific passband corresponding to some photometric system such as the UBV system. In standard astronomical notation, an apparent magnitude in the V filter band would be denoted either as mV or simply as V, as in "mV = 15" or "V = 15" to describe a 15th-magnitude object; the scale used to indicate magnitude originates in the Hellenistic practice of dividing stars visible to the naked eye into six magnitudes.
The brightest stars in the night sky were said to be of first magnitude, whereas the faintest were of sixth magnitude, the limit of human visual perception. Each grade of magnitude was considered twice the brightness of the following grade, although that ratio was subjective as no photodetectors existed; this rather crude scale for the brightness of stars was popularized by Ptolemy in his Almagest and is believed to have originated with Hipparchus. In 1856, Norman Robert Pogson formalized the system by defining a first magnitude star as a star, 100 times as bright as a sixth-magnitude star, thereby establishing the logarithmic scale still in use today; this implies that a star of magnitude m is about 2.512 times as bright as a star of magnitude m + 1. This figure, the fifth root of 100, became known as Pogson's Ratio; the zero point of Pogson's scale was defined by assigning Polaris a magnitude of 2. Astronomers discovered that Polaris is variable, so they switched to Vega as the standard reference star, assigning the brightness of Vega as the definition of zero magnitude at any specified wavelength.
Apart from small corrections, the brightness of Vega still serves as the definition of zero magnitude for visible and near infrared wavelengths, where its spectral energy distribution approximates that of a black body for a temperature of 11000 K. However, with the advent of infrared astronomy it was revealed that Vega's radiation includes an Infrared excess due to a circumstellar disk consisting of dust at warm temperatures. At shorter wavelengths, there is negligible emission from dust at these temperatures. However, in order to properly extend the magnitude scale further into the infrared, this peculiarity of Vega should not affect the definition of the magnitude scale. Therefore, the magnitude scale was extrapolated to all wavelengths on the basis of the black-body radiation curve for an ideal stellar surface at 11000 K uncontaminated by circumstellar radiation. On this basis the spectral irradiance for the zero magnitude point, as a function of wavelength, can be computed. Small deviations are specified between systems using measurement apparatuses developed independently so that data obtained by different astronomers can be properly compared, but of greater practical importance is the definition of magnitude not at a single wavelength but applying to the response of standard spectral filters used in photometry over various wavelength bands.
With the modern magnitude systems, brightness over a wide range is specified according to the logarithmic definition detailed below, using this zero reference. In practice such apparent magnitudes do not exceed 30; the brightness of Vega is exceeded by four stars in the night sky at visible wavelengths as well as the bright planets Venus and Jupiter, these must be described by negative magnitudes. For example, the brightest star of the celestial sphere, has an apparent magnitude of −1.4 in the visible. Negative magnitudes for other bright astronomical objects can be found in the table below. Astronomers have developed other photometric zeropoint systems as alternatives to the Vega system; the most used is the AB magnitude system, in which photometric zeropoints are based on a hypothetical reference spectrum having constant flux per unit frequency interval, rather than using a stellar spectrum or blackbody curve as the reference. The AB magnitude zeropoint is defined such that an object's AB and Vega-based magnitudes will be equal in the V filter band.
As the amount of light received by a telescope is reduced by transmission through the Earth's atmosphere, any measurement of apparent magnitude is corrected for what it would have been as seen from above the atmosphere. The dimmer an object appears, the higher the numerical value given to its apparent magnitude, with a difference of 5 magnitudes corresponding to a brightness factor of 100. Therefore, the apparent magnitude m, in the spectral band x, would be given by m x = − 5 log 100 , more expressed in terms of common logarithms as m x
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 for which precise measurements have confirmed a shape consistent with hydrostatic equilibrium, after dwarf planet Ceres, it was discovered in 1672 by Giovanni Domenico Cassini. Rhea was discovered by Giovanni Domenico Cassini on 23 December 1672, it was the second moon of Saturn that Cassini discovered, the third moon discovered around Saturn overall. Rhea is named after the Titan Rhea of Greek mythology, the "mother of the gods", it is 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, to Saturn VIII with the discovery of Hyperion in 1848, before the numbering scheme was frozen to prevent further confusion. Rhea was not named until 1847, when John Herschel suggested in Results of Astronomical Observations made at the Cape of Good Hope that the names of the Titans and brothers of Cronos, be used.
Rhea is an icy body with a density of about 1.236 g/cm3. This low density indicates. Although Rhea is the ninth-largest moon, it is only the tenth-most-massive moon. Before the Cassini-Huygens mission, it was assumed. However, measurements taken during a close 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 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. Rhea being either or differentiated would be consistent with the observations of the Cassini probe. A year yet another paper claimed that the moon may not be in hydrostatic 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.
He concluded that there is a systematic error in the Cassini radio Doppler data used in the analysis, but after restricting the analysis to a subset of data obtained closest to the moon, he arrived at his old result that Rhea was in hydrostatic equilibrium and had the moment inertia of about 0.4, again implying a homogeneous interior. The triaxial shape of Rhea is consistent with a homogeneous body in hydrostatic equilibrium rotating at Rhea's angular velocity. Models suggest that Rhea could be capable of sustaining an internal liquid-water ocean through heating by radioactive decay. Rhea's features resemble those of Dione, with dissimilar leading and trailing hemispheres, suggesting similar composition and histories; the temperature on Rhea is between 73 K and 53 K in the shade. Rhea has a rather typical cratered surface, with the exceptions of a few large Dione-type chasmata or fractures on the trailing hemisphere and a faint "line" of material at Rhea's equator that may have been deposited by material deorbiting from its rings.
Rhea has two 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, prominent because of an extended system of bright rays; this crater, called Inktomi, is nicknamed "The Splat", may be one of the youngest craters on the inner moons of Saturn. No evidence of any endogenic activity has been discovered, its surface can be divided into two geologically different areas based on crater density. This suggests; the leading hemisphere is cratered and uniformly bright. As on Callisto, the craters lack the high relief features seen on the Mercury, it has been theorized. On the trailing hemisphere there is a network of bright swaths on a dark background and few visible craters, it had been thought that these bright areas might be material ejected from ice volcanoes early in Rhea's history when its interior was still liquid.
However, observations of Dione, which has an darker trailing hemisphere and similar but more prominent bright streaks, show that the streaks are ice cliffs resulting from extensive fracturing of the moon's surface. The extensive dark areas are thought to be deposited tholins, which are a mix of complex organic compounds generated on the ice by pyrolysis and radiolysis of simple compounds containing carbon and hydrogen; the January 17, 2006 distant flyby by the Cassini spacecraft yielded images of the wispy hemisphere at better resolution and a lower Sun angle than previous observations. Image
S/2009 S 1
S/2009 S 1 is a "propeller moonlet" of Saturn orbiting at a distance of 117,000 km, in the outer part of the B Ring, with an approximate diameter of 300 m. The moonlet was discovered by the Cassini Imaging Team during the Cronian equinox event on 26 July 2009, when it cast a shadow 36 km long onto the B Ring. S/2009 S 1 protrudes 150 m north of the ring; the image was taken 296,000 km from Saturn. S/2009 S 1 was first identified by the Cassini Imaging Team on 26 July 2009, it was discovered during 2009's equinox by an 36 kilometres long shadow that it cast on the planet Saturn's B ring. Moons of Saturn
Prometheus is an inner satellite of Saturn. It was discovered in 1980 from photos taken by the Voyager 1 probe, was provisionally designated S/1980 S 27. In late 1985 it was named after Prometheus, a Titan in Greek mythology, it is designated Saturn XVI. Pronunciation for Prometheus is, prə-MEE-thee-əs. Prometheus is elongated, measuring 136 km × 79 km × 59 km, it has several ridges and valleys and a number of impact craters of about 20 km diameter are visible, but it is less cratered than nearby Pandora and Janus. From its low density and high albedo, it is that Prometheus is a porous icy body. There is a lot of uncertainty in these values, so this remains to be confirmed. Prometheus is a shepherd satellite for the inner edge of Saturn's narrow F Ring. Pandora orbits just outside the F Ring, has traditionally been viewed as an outer shepherd of the ring. Images from the Cassini probe show that the Promethean gravitational field creates kinks and knots in the F Ring as it'steals' material from it.
The orbit of Prometheus appears to be chaotic, due to a series of four 121:118 mean-motion resonances with Pandora. The most appreciable changes in their orbits occur every 6.2 years, when the periapsis of Pandora lines up with the apoapsis of Prometheus, when they approach to within 1400 km. Prometheus is itself a significant perturber of Atlas, with which it is in a 53:54 mean-longitude resonance. Citations Sources Marsden, Brian G.. "Satellites of Saturn". IAU Circular. 3532. Retrieved 2011-12-29. Marsden, Brian G.. "Satellites of Saturn and Pluto". IAU Circular. 4157. Retrieved 2011-12-29. Renner, Stéfan F.. "Prometheus and Pandora: Masses and orbital positions during the Cassini tour". Icarus. 174: 230–240. Bibcode:2005Icar..174..230R. Doi:10.1016/j.icarus.2004.09.005. Spitale, J. N.. "The orbits of Saturn's small satellites derived from combined historic and Cassini imaging observations". The Astronomical Journal. 132: 692–710. Bibcode:2006AJ....132..692S. Doi:10.1086/505206. Thomas, P. C.. "Sizes and derived properties of the saturnian satellites after the Cassini nominal mission".
Icarus. 208: 395–401. Bibcode:2010Icar..208..395T. Doi:10.1016/j.icarus.2010.01.025. USGS/IAU. "Planet and Satellite Names and Discoverers". Gazetteer of Planetary Nomenclature. USGS Astrogeology. Retrieved 2011-12-29. Media related to Prometheus at Wikimedia Commons "Cassini–Huygens: Multimedia-Videos / Soft Collision". NASA. Archived from the original on 29 October 2007. Prometheus collides with the diffuse inner edge of Saturn's F ring... pulls a streamer of material from the ring and leaves behind a dark channel. Prometheus Profile at NASA's Solar System Exploration site The Planetary Society: Prometheus 3-D anaglyph view of Prometheus
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 185 known natural satellites. Four IAU-listed dwarf planets are known to have natural satellites: Pluto, Haumea and Eris; as of September 2018, there are 334 other minor planets known to have moons. 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, the 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 De revolutionibus orbium coelestium in 1543; until the discovery of the Galilean satellites in 1610, there was no opportunity for referring to such objects as a class. Galileo chose to refer to his discoveries as Planetæ, but discoverers chose other terms to distinguish them from the objects they orbited.
The first to use of the term 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", because the satellites accompanied their primary planet in their journey through the heavens; 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, the launching of the artificial object Sputnik created a need for new terminology. Sputnik was created by Soviet Union, it was the first satellite ever; the terms man-made satellite and artificial moon were quickly abandoned in favor of the simpler satellite, as a consequence, the term has become linked with artificial objects flown in space – including, sometimes those not in orbit around a planet. Because of this shift in meaning, the term moon, which had continued to be used in a generic sense in works of popular science and in fiction, has regained respectability and is now used interchangeably with natural satellite in scientific articles.
When it is necessary to avoid both the ambiguity of confusion with Earth's natural satellite the Moon and the natural satellites of the other planets on the one hand, artificial satellites on the other, the term natural satellite is used. To further avoid ambiguity, the convention is to capitalize the word Moon when referring to Earth's natural satellite, but not when referring to other natural satellites. Many authors define "satellite" or "natural satellite" as orbiting some planet or minor planet, synonymous with "moon" – by such a definition all natural satellites are moons, but Earth and other planets are not satellites. A few recent authors define "moon" as "a satellite of a planet or minor planet", "planet" as "a satellite of a star" – such authors consider Earth as a "natural satellite of the sun". There is no established lower limit on what is considered a "moon"; every natural celestial body with an identified orbit around a planet of the Solar System, some as small as a kilometer across, has been considered a moon, though objects a tenth that size within Saturn's rings, which have not been directly observed, have been called moonlets.
Small asteroid moons, such as Dactyl, have been called moonlets. The upper limit is vague. Two orbiting bodies are sometimes described as a double planet rather than satellite. 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. The most common dividing line on what is considered a moon rests upon whether the barycentre is below the surface of the larger body, though this is somewhat arbitrary, because it depends on distance as well as relative mass; the natural satellites orbiting close to the planet on prograde, uninclined circular orbits are thought to have been formed out of the same collapsing region of the protoplanetary disk that created its primary. In contrast, irregular satellites are thought to be captured asteroids 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 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 been 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; the capture of an asteroid from a heliocentric orbit is not always permanent. According to simulations, temporary satellites should be a common phenomenon; the only observed example is 2006 RH120, a temporary satellite of Earth for nine months in 2006 and 2007. Most regular moons (natural satellites following close and prograde orbits with small orb
Dione is a moon of Saturn. It was discovered by Italian astronomer Giovanni Domenico Cassini in 1684, it is named after the Titaness Dione of Greek mythology. It is 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; the satellites of Saturn were not named until 1847, when William Herschel's son John Herschel published Results of Astronomical Observations made at the Cape of Good Hope, suggesting that the names of the Titans be used. Dione orbits Saturn with a semimajor axis about 2% less than that of the Moon. However, reflecting Saturn's greater mass, Dione's orbital period is one tenth that of the Moon. Dione is in a 1:2 mean-motion orbital resonance with moon Enceladus, completing one orbit of Saturn for every two orbits completed by Enceladus; this resonance maintains Enceladus's orbital eccentricity, providing a source of heat for Enceladus's extensive geological activity, which shows up most in its cryovolcanic geyser-like jets.
The resonance maintains a smaller eccentricity in Dione's orbit, tidally heating it as well. Dione has two co-orbital, or trojan, moons and Polydeuces, they are located within Dione's Lagrangian points L4 and L5, 60 degrees ahead of and behind Dione respectively. At 1122 km in diameter, Dione is the 15th largest moon in the Solar System, is more massive than all known moons smaller than itself combined. About two thirds of Dione's mass is water ice, the remaining is a dense core silicate rock. Data gathered by Cassini indicates. Downward bending of the surface associated with the 1.5 km high ridge Janiculum Dorsa can most 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 km thick internal liquid water global ocean. Neither moon has a shape close to hydrostatic equilibrium. Dione's ice shell is thought to vary in thickness by less than 5%, with the thinnest areas at the poles, where tidal heating of the crust is greatest.
Though somewhat smaller and denser, Dione is otherwise similar to Rhea. They both have similar albedo features and varied terrain, both have dissimilar leading and trailing hemispheres. Dione's leading hemisphere is cratered and is uniformly bright, its trailing hemisphere, contains an unusual and distinctive surface feature: a network of bright ice cliffs. Scientists recognise Dionean geological features of the following types: Chasmata Dorsa Fossae Craters Catenae When the Voyager space probe photographed Dione in 1980, it showed what appeared to be wispy features covering its trailing hemisphere; the origin of these features was mysterious, because all, known was that the material has a high albedo and is thin enough that it does not obscure the surface features underneath. One hypothesis was that shortly after its formation Dione was geologically active, some process such as cryovolcanism resurfaced much of its surface, with the streaks forming from eruptions along cracks in Dione's surface that fell back to the surface as snow or ash.
After the internal activity and resurfacing ceased, cratering continued on the leading hemisphere and wiped out the streak patterns there. This hypothesis was proven wrong by the Cassini probe flyby of December 13, 2004, which produced close-up images; 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 closer flyby of Dione at 500 km on October 11, 2005, captured oblique images of the cliffs, showing that some of them are several hundred metres high. Dione features linear ` virgae' that are up to hundreds of km less than 5 km wide; these lines are only apparent at lower latitudes. They are brighter than everything around them and appear to overlay other features such as ridges and craters, indicating they are young, it has been proposed that these lines are of exogenic origin, as the result of the emplacement of material across the surface by low‐velocity impacts of material sourced from Saturn's rings, co‐orbital moons, or approaching comets.
Dione's icy surface includes cratered terrain, moderately cratered plains cratered plains, areas of tectonic fractures. The cratered terrain has numerous craters greater than 100 kilometres in diameter; the plains areas tend to have craters less than 30 kilometres in diameter. Some of the plains are more cratered than others. Much of the cratered terrain is located on the trailing hemisphere, with the less cratered plains areas present on the leading hemisphere; this is the opposite of. This suggests that during the period of heavy bombardment, Dione was tidally locked to Saturn in the opposite orientation; because Dione is small, an impa