Alpha Centauri is the closest star system and closest planetary system to the Solar System at 4.37 light-years from the Sun. It is a triple star system, consisting of three stars: α Centauri A, α Centauri B, α Centauri C. Alpha Centauri A and B are Sun-like stars, together they form the binary star Alpha Centauri AB. To the naked eye, the two main components appear to be a single star with an apparent magnitude of −0.27, forming the brightest star in the southern constellation of Centaurus and the third-brightest in the night sky, outshone only by Sirius and Canopus. Alpha Centauri A has 1.1 times the mass and 1.519 times the luminosity of the Sun, while Alpha Centauri B is smaller and cooler, at 0.907 times the Sun's mass and 0.445 times its luminosity. The pair orbit about a common centre with an orbital period of 79.91 years. Their elliptical orbit is eccentric, so that the distance between A and B varies from 35.6 astronomical units, or about the distance between Pluto and the Sun, to that between Saturn and the Sun.
Alpha Centauri C, or Proxima Centauri, is a faint red dwarf. Though not visible to the naked eye, Proxima Centauri is the closest star to the Sun at a distance of 4.24 light-years closer than Alpha Centauri AB. The distance between Proxima Centauri and Alpha Centauri AB is about 13,000 astronomical units, equivalent to about 430 times the radius of Neptune's orbit. Proxima Centauri b is an Earth-sized exoplanet in the habitable zone of Proxima Centauri. Α Centauri is the system's designation given by Johann Bayer in 1603. It once bore the name Rigil Kentaurus, a Latinisation of the Arabic name رجل القنطورس Rijl al-Qanṭūris, meaning'Foot of the Centaur'. Alpha Centauri C was discovered in 1915 by Robert T. A. Innes, who suggested that it be named Proxima Centaurus amended to Proxima Centauri; the name is from Latin, meaning'nearest of Centaurus'. In 2016, the Working Group on Star Names of the International Astronomical Union, having decided to attribute proper names to individual stars rather than entire multiple systems, approved the names Rigil Kentaurus for Alpha Centauri A and Proxima Centauri for Alpha Centauri C.
In 10 August 2018, IAU approved the name Toliman for Alpha Centauri B. Alpha Centauri is a triple star system, with its two main stars, Alpha Centauri A and Alpha Centauri B, being a binary component; the AB designation, or older A×B, denotes the mass centre of a main binary system relative to companion star in a multiple star system. AB-C refers to the component of Proxima Centauri in relation to the central binary, being the distance between the centre of mass and the outlying companion; because the distance between Proxima and either of Alpha Centauri A or B is similar, the AB binary system is sometimes treated as a single gravitational object. The A and B components of Alpha Centauri have an orbital period of 79.91 years. Their orbit is moderately eccentric, e = 0.5179. Viewed from Earth, the apparent orbit of A and B means that their separation and position angle are in continuous change throughout their projected orbit. Observed stellar positions in 2019 are separated by 4.92 arcsec through the PA of 337.1°, increasing to 5.49 arcsec through 345.3° in 2020.
The closest recent approach was in February 2016, at 4.0 arcsec through the PA of 300°. The observed maximum separation of these stars is about 22 arcsec, while the minimum distance is 1.7 arcsec. The widest separation occurred during February 1976, the next will be in January 2056; the most recent, true orbit, closest approach or periastron was in August 1955, the next will be in May 2035. The furthest orbital separation or apastron last occurred in May 1995, the next will be in 2075; the apparent distance between Alpha Centauri A and B is decreasing, at least until 2019. Alpha Centauri C is about 13,000 astronomical units away from Alpha Centauri AB; this is equivalent to 0.21 ly or 1.9 trillion km—about 5% the distance between Alpha Centauri AB and the Sun. For a long time, estimates of Proxima's small orbital speed around AB were insufficiently accurate to determine whether Proxima Centauri is bound to the Alpha Centauri system or an unrelated star that happens to be passing by at a low speed.
Radial velocity measurements made in 2017 were precise enough to show that Proxima Centauri and Alpha Centauri AB are gravitationally bound. The orbital period of Proxima Centauri is 547000+6600−4000 years, with an eccentricity of 0.50 ± 0.08, more eccentric than Mercury's. Proxima Centauri comes within 4300+1100−900 AU of AB at periastron, the apastron occurs at 13000+300−100 AU. Asteroseismic studies, chromospheric activity, stellar rotation are all consistent with the Alpha Centauri system being similar in age to, or older than, the Sun. Asteroseismic analyses that incorporate tight observational constraints on the stellar parameters for the Alpha Centauri stars have yielded age estimates of 4.85±0.5 Gyr, 5.0±0.5 Gyr, 5.2 ± 1.9 Gyr, 6.4 Gyr, 6.52±0.3 Gyr. Age estimates for the stars based on chromospheric activity yield 4.4 ± 2.1 Gyr, whereas gyrochronology yields 5.0±0.3 Gyr. Stellar evolution theory implies both stars are older than the Sun at 5 to 6 billion years, as derived by their mass and spectral characteristics.
From the orbital elements, th
The Andromeda Galaxy known as Messier 31, M31, or NGC 224, is a spiral galaxy 780 kiloparsecs from Earth, the nearest major galaxy to the Milky Way. Its name stems from the area of the Earth's sky; the virial mass of the Andromeda Galaxy is of the same order of magnitude as that of the Milky Way, at a trillion solar masses. The mass of either galaxy is difficult to estimate with any accuracy, but it was long thought that the Andromeda Galaxy is more massive than the Milky Way by a margin of some 25% to 50%; this has been called into question by a 2018 study which cited a lower estimate on the mass of the Andromeda Galaxy, combined with preliminary reports on a 2019 study estimating a higher mass of the Milky Way. The Andromeda Galaxy has a diameter of about 220,000 light-years, making it the largest member of the Local Group at least in terms of extension, if not mass; the number of stars contained in the Andromeda Galaxy is estimated at one trillion, or twice the number estimated for the Milky Way.
The Milky Way and Andromeda galaxies are expected to collide in ~4.5 billion years, merging to form a giant elliptical galaxy or a large disc galaxy. With an apparent magnitude of 3.4, the Andromeda Galaxy is among the brightest of the Messier objects making it visible to the naked eye from Earth on moonless nights when viewed from areas with moderate light pollution. Around the year 964, the Persian astronomer Abd al-Rahman al-Sufi described the Andromeda Galaxy, in his Book of Fixed Stars as a "nebulous smear". Star charts of that period labeled it as the Little Cloud. In 1612, the German astronomer Simon Marius gave an early description of the Andromeda Galaxy based on telescopic observations; the German philosopher Immanuel Kant in 1755 in his work Universal Natural History and Theory of the Heavens conjectured that the blurry spot was an island universe. In 1764, Charles Messier cataloged Andromeda as object M31 and incorrectly credited Marius as the discoverer despite it being visible to the naked eye.
In 1785, the astronomer William Herschel noted a faint reddish hue in the core region of Andromeda. He believed Andromeda to be the nearest of all the "great nebulae", based on the color and magnitude of the nebula, he incorrectly guessed that it is no more than 2,000 times the distance of Sirius. In 1850, William Parsons, 3rd Earl of Rosse and made the first drawing of Andromeda's spiral structure. In 1864, William Huggins noted; the spectra of Andromeda displays a continuum of frequencies, superimposed with dark absorption lines that help identify the chemical composition of an object. Andromeda's spectrum is similar to the spectra of individual stars, from this, it was deduced that Andromeda has a stellar nature. In 1885, a supernova was seen in the first and so far only one observed in that galaxy. At the time Andromeda was considered to be a nearby object, so the cause was thought to be a much less luminous and unrelated event called a nova, was named accordingly. In 1887, Isaac Roberts took the first photographs of Andromeda, still thought to be a nebula within our galaxy.
Roberts mistook Andromeda and similar spiral nebulae as solar systems being formed. In 1912, Vesto Slipher used spectroscopy to measure the radial velocity of Andromeda with respect to our Solar System—the largest velocity yet measured, at 300 kilometres per second. In 1917, Heber Curtis observed a nova within Andromeda. Searching the photographic record, 11 more novae were discovered. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred elsewhere in the sky; as a result, he was able to come up with a distance estimate of 500,000 light-years. He became a proponent of the so-called "island universes" hypothesis, which held that spiral nebulae were independent galaxies. In 1920, the Great Debate between Harlow Shapley and Curtis took place concerning the nature of the Milky Way, spiral nebulae, the dimensions of the Universe. To support his claim of the Great Andromeda Nebula being, in fact, an external galaxy, Curtis noted the appearance of dark lanes within Andromeda which resembled the dust clouds in our own galaxy, as well as historical observations of Andromeda Galaxy's significant Doppler shift.
In 1922 Ernst Öpik presented a method to estimate the distance of Andromeda using the measured velocities of its stars. His result placed the Andromeda Nebula far outside our galaxy at a distance of about 450,000 parsecs. Edwin Hubble settled the debate in 1925 when he identified extragalactic Cepheid variable stars for the first time on astronomical photos of Andromeda; these were made using the 2.5-metre Hooker telescope, they enabled the distance of Great Andromeda Nebula to be determined. His measurement demonstrated conclusively that this feature was not a cluster of stars and gas within our own galaxy, but an separate galaxy located a significant distance from the Milky Way. In 1943, Walter Baade was the first person to resolve stars in the central region of the Andromeda Galaxy. Baade identified two distinct populations of stars based on their metallicity, naming the young, high-velocity stars in the disk Type I and the older, red stars in the bulge Type II; this nomenclature was subsequently adopted for stars within the Milky Way, elsewhere.
Baade discovered that there were two types of Cepheid variables, which resulted in a doubling of the distance estimate to Andromeda, as well as the remainder o
Ceres (dwarf planet)
Ceres is the largest object in the asteroid belt that lies between the orbits of Mars and Jupiter closer to Mars's orbit. With a diameter of 945 km, Ceres is the largest of the minor planets and the only dwarf planet inside Neptune's orbit, it is the 33rd-largest known body in the Solar System. Ceres comprises rock and ice, contains one-third of the mass of the entire asteroid belt. Ceres is the only object in the asteroid belt known to be rounded by its own gravity, although detailed analysis was required to exclude Vesta. From Earth, the apparent magnitude of Ceres ranges from 6.7 to 9.3, peaking once at opposition every 15 to 16 months, its synodic period. Thus at its brightest, it is too dim to be seen by the naked eye, except under dark skies. Ceres was the first asteroid to be discovered, it was considered a planet, but was reclassified as an asteroid in the 1850s after many other objects in similar orbits were discovered. Ceres appears to be differentiated into a rocky core and an icy mantle, may have a remnant internal ocean of liquid water under the layer of ice.
The surface is various hydrated minerals such as carbonates and clay. In January 2014, emissions of water vapor were detected from several regions of Ceres; this was unexpected because large bodies in the asteroid belt do not emit vapor, a hallmark of comets. The robotic NASA spacecraft Dawn entered orbit around Ceres on 6 March 2015. Pictures with a resolution unattained were taken during imaging sessions starting in January 2015 as Dawn approached Ceres, showing a cratered surface. Two distinct bright spots inside a crater were seen in a 19 February 2015 image, leading to speculation about a possible cryovolcanic origin or outgassing. On 3 March 2015, a NASA spokesperson said the spots are consistent with reflective materials containing ice or salts, but that cryovolcanism is unlikely. However, on 2 September 2016, scientists from the Dawn team claimed in a Science paper that a massive cryovolcano called Ahuna Mons is the strongest evidence yet for the existence of these mysterious formations.
On 11 May 2015, NASA released a higher-resolution image showing that, instead of one or two spots, there are several. On 9 December 2015, NASA scientists reported that the bright spots on Ceres may be related to a type of salt a form of brine containing magnesium sulfate hexahydrite. In June 2016, near-infrared spectra of these bright areas were found to be consistent with a large amount of sodium carbonate, implying that recent geologic activity was involved in the creation of the bright spots. In July 2018, NASA released a comparison of physical features found on Ceres with similar ones present on Earth. From June to October, 2018, Dawn orbited Ceres from as close as 35 km and as far away as 4,000 km; the Dawn mission ended on 1 November 2018. In October 2015, NASA released a true-color portrait of Ceres made by Dawn. In February 2017, organics were detected on Ceres in Ernutet crater. Johann Elert Bode, in 1772, first suggested that an undiscovered planet could exist between the orbits of Mars and Jupiter.
Kepler had noticed the gap between Mars and Jupiter in 1596. Bode based his idea on the Titius–Bode law, a now-discredited hypothesis, first proposed in 1766. Bode observed that there was a regular pattern in the semi-major axes of the orbits of known planets, that the pattern was marred only by the large gap between Mars and Jupiter; the pattern predicted that the missing planet ought to have an orbit with a semi-major axis near 2.8 astronomical units. William Herschel's discovery of Uranus in 1781 near the predicted distance for the next body beyond Saturn increased faith in the law of Titius and Bode, in 1800, a group headed by Franz Xaver von Zach, editor of the Monatliche Correspondenz, sent requests to twenty-four experienced astronomers, asking that they combine their efforts and begin a methodical search for the expected planet. Although they did not discover Ceres, they found several large asteroids. One of the astronomers selected for the search was Giuseppe Piazzi, a Catholic priest at the Academy of Palermo, Sicily.
Before receiving his invitation to join the group, Piazzi discovered Ceres on 1 January 1801. He was searching for "the 87th of the Catalogue of the Zodiacal stars of Mr la Caille", but found that "it was preceded by another". Instead of a star, Piazzi had found a moving star-like object. Piazzi observed Ceres a total of 24 times, the final time on 11 February 1801, when illness interrupted his observations, he announced his discovery on 24 January 1801 in letters to only two fellow astronomers, his compatriot Barnaba Oriani of Milan and Johann Elert Bode of Berlin. He reported it as a comet but "since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet". In April, Piazzi sent his complete observations to Oriani, Jérôme Lalande in Paris; the information was published in the September 1801 issue of the Monatliche Correspondenz. By this time, the apparent position of Ceres had changed, was too close to the Sun's glare for other astronomers to confirm Piazzi's observations.
Toward the end of the year, Ceres should have been vis
The Eagle Nebula is a young open cluster of stars in the constellation Serpens, discovered by Jean-Philippe de Chéseaux in 1745–46. Both the "Eagle" and the "Star Queen" refer to visual impressions of the dark silhouette near the center of the nebula, an area made famous as the "Pillars of Creation" imaged by the Hubble Space Telescope; the nebula contains several active star-forming gas and dust regions, including the aforementioned Pillars of Creation. The Eagle Nebula is part of a diffuse emission nebula, or H II region, catalogued as IC 4703; this region of active current star formation is about 7000 light-years distant. A spire of gas that can be seen coming off the nebula in the northeastern part is 9.5 light-years or about 90 trillion kilometers long. The cluster associated with the nebula has 8100 stars, which are concentrated in a gap in the molecular cloud to the north-west of the Pillars; the brightest star has an apparent magnitude of +8.24 visible with good binoculars. It is a binary star formed of an O3.5V star plus an O7.5V companion.
This star has a mass of 80 solar masses, a luminosity up to 1 million times that of the Sun. The cluster's age has been estimated to be 1–2 million years; the descriptive names reflect impressions of the shape of the central pillar rising from the southeast into the central luminous area. The name "Star Queen Nebula" was introduced by Robert Burnham, Jr. reflecting his characterization of the central pillar as the Star Queen shown in silhouette. Images taken by Jeff Hester and Paul Scowen using the Hubble Space Telescope in 1995 improved scientific understanding of processes inside the nebula. One of these photographs became famous as the "Pillars of Creation", depicting a large region of star formation; the small dark areas in the photograph are believed to be protostars. The pillar structure of the region resembles that of a much larger star formation region in the Soul Nebula of Cassiopeia, imaged with the Spitzer Space Telescope in 2005 and characterized as "Pillars of Star Creation". or "Pillars of Star Formation".
These columns – which resemble stalagmites protruding from the floor of a cavern – are composed of interstellar hydrogen gas and dust, which act as incubators for new stars. Inside the columns and on their surface astronomers have found knots or globules of denser gas, called EGGs. Stars are being formed inside some of these EGGs. X-ray images from the Chandra observatory compared with Hubble's "Pillars" image have shown that X-ray sources do not coincide with the pillars, but instead randomly dot the area. Any protostars in the pillars' EGGs are not yet hot enough to emit X-rays. Evidence from the Spitzer Telescope suggested that the pillars in M16 may have been destroyed by a supernova explosion. Hot gas observed by Spitzer in 2007 suggested that the area was disturbed by a supernova that exploded some 8,000 to 9,000 years ago. Due to the distance of the nebula, the light from the supernova would have reached Earth between 1,000 and 2,000 years ago; the more moving shock wave from the supernova would have taken a few thousand years to move through the nebula and would have blown away the delicate pillars.
However, in 2014 the Pillars were imaged a second time by Hubble, in both visible light and infrared light. The new images being 20 years apart provided a new, detailed account of the rate of evaporation occurring within the pillars, it was discovered that there in fact was no supernova explosion within them, it is estimated they will be around for at least 100,000 years longer. List of Messier objects Eagle Nebula in fiction Pillars of Creation The Eagle Nebula on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Sky Map and images The Eagle's EGGs – ESO Photo Release ESO: An Eagle of Cosmic Proportions incl. Photos & Animations ESO: VST Captures Three-In-One incl. Photos & Animations M16 @ Seds.org, SEDS Messier page on M16 Spacetelescope.org, Hubble telescope images on M16 Darkatmospheres.com, Eagle Nebula M16 NASA.gov, APOD February 8, 2009 picture Eagle Nebula Szymanek, Nik. "M16 – Eagle Nebula". Deep Sky Videos. Brady Haran. Eagle Nebula at Constellation Guide
Proxima Centauri, or Alpha Centauri C, is a red dwarf, a small low-mass star, about 4.244 light-years from the Sun in the constellation of Centaurus. It is the nearest-known star to the Sun. With a quiescent apparent magnitude of 11.13, it is too faint to be seen with the naked eye. Proxima Centauri forms a third component of the Alpha Centauri system with a separation of about 12,950 AU and an orbital period of 550,000 years. At present Proxima is 2.18° to the southwest of Alpha Centauri. Because of Proxima Centauri's proximity to Earth, its angular diameter can be measured directly; the star is about one-seventh the diameter of the Sun. It has a mass about an eighth of the Sun's mass, its average density is about 33 times that of the Sun. Although it has a low average luminosity, Proxima is a flare star that undergoes random dramatic increases in brightness because of magnetic activity; the star's magnetic field is created by convection throughout the stellar body, the resulting flare activity generates a total X-ray emission similar to that produced by the Sun.
The mixing of the fuel at Proxima Centauri's core through convection and its low energy-production rate mean that it will be a main-sequence star for another four trillion years, or nearly 300 times the current age of the universe. In 2016, the European Southern Observatory announced the discovery of Proxima Centauri b, a planet orbiting the star at a distance of 0.05 AU with an orbital period of 11.2 Earth days. Its estimated mass is at least 1.3 times that of the Earth. The equilibrium temperature of Proxima b is estimated to be within the range of where water could exist as liquid on its surface, thus placing it within the habitable zone of Proxima Centauri, although because Proxima Centauri is a red dwarf and a flare star, whether it could support life is disputed. Previous searches for orbiting companions had ruled out the presence of brown dwarfs and supermassive planets. In 1915, the Scottish astronomer Robert Innes, Director of the Union Observatory in Johannesburg, South Africa, discovered a star that had the same proper motion as Alpha Centauri.
He suggested. In 1917, at the Royal Observatory at the Cape of Good Hope, the Dutch astronomer Joan Voûte measured the star's trigonometric parallax at 0.755″±0.028″ and determined that Proxima Centauri was the same distance from the Sun as Alpha Centauri. It was found to be the lowest-luminosity star known at the time. An accurate parallax determination of Proxima Centauri was made by American astronomer Harold L. Alden in 1928, who confirmed Innes's view that it is closer, with a parallax of 0.783″±0.005″. In 1951, American astronomer Harlow Shapley announced. Examination of past photographic records showed that the star displayed a measurable increase in magnitude on about 8% of the images, making it the most active flare star known; the proximity of the star allows for detailed observation of its flare activity. In 1980, the Einstein Observatory produced a detailed X-ray energy curve of a stellar flare on Proxima Centauri. Further observations of flare activity were made with the EXOSAT and ROSAT satellites, the X-ray emissions of smaller, solar-like flares were observed by the Japanese ASCA satellite in 1995.
Proxima Centauri has since been the subject of study by most X-ray observatories, including XMM-Newton and Chandra. In 2016, the International Astronomical Union organized a Working Group on Star Names to catalogue and standardize proper names for stars; the WGSN approved the name Proxima Centauri for this star on August 21, 2016 and it is now so included in the List of IAU approved Star Names. Because of Proxima Centauri's southern declination, it can only be viewed south of latitude 27° N. Red dwarfs such as Proxima Centauri are far too faint to be seen with the naked eye. From Alpha Centauri A or B, Proxima would only be seen as a fifth magnitude star, it has an apparent visual magnitude of 11, so a telescope with an aperture of at least 8 cm is needed to observe it under ideal viewing conditions—under clear, dark skies with Proxima Centauri well above the horizon. In 2018, a superflare was observed from Proxima Centauri, the strongest flare seen; the optical brightness increased by a factor of 68 to magnitude 6.8.
It is estimated that similar flares occur around five times every year but are of such short duration, just a few minutes, that they have never been observed before. Proxima Centauri is a red dwarf, because it belongs to the main sequence on the Hertzsprung–Russell diagram and is of spectral class M5.5. M5.5 means. Its absolute visual magnitude, or its visual magnitude as viewed from a distance of 10 parsecs, is 15.5. Its total luminosity over all wavelengths is 0.17% that of the Sun, although when observed in the wavelengths of visible light the eye is most sensitive to, it is only 0.0056% as luminous as the Sun. More than 85% of its radiated power is at infrared wavelengths, it has a regular activity cycle of starspots. In 2002, optical interferometry with the Very Large Telescope found that the angular diameter of Proxima Centauri was 1.02±0.08 mas. Because its distance is known, the actual diameter of Proxima Centauri can be calculated to be about 1/7 that of the Sun, or 1.5 times that of Jupiter.
The star's mass, estimated from stellar theory, is 129 Jupiter masses. The mass has been calculated directly, although with less precision, from observations of microlensing events to be 0.150+0.062−
Mercury is the smallest and innermost planet in the Solar System. Its orbital period around the Sun of 87.97 days is the shortest of all the planets in the Solar System. It is named after the messenger of the gods. Like Venus, Mercury orbits the Sun within Earth's orbit as an inferior planet, never exceeds 28° away from the Sun when viewed from Earth; this proximity to the Sun means the planet can only be seen near the western or eastern horizon during the early evening or early morning. At this time it may appear as a bright star-like object, but is far more difficult to observe than Venus; the planet telescopically displays the complete range of phases, similar to Venus and the Moon, as it moves in its inner orbit relative to Earth, which reoccurs over the so-called synodic period every 116 days. Mercury is tidally locked with the Sun in a 3:2 spin-orbit resonance, rotates in a way, unique in the Solar System; as seen relative to the fixed stars, it rotates on its axis 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, it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see only one day every two Mercurian years. Mercury's axis has the smallest tilt of any of the Solar System's planets, its orbital eccentricity is the largest of all known planets in the Solar System. Mercury's surface appears cratered and is similar in appearance to the Moon's, indicating that it has been geologically inactive for billions of years. Having no atmosphere to retain heat, it has surface temperatures that vary diurnally more than on any other planet in the Solar System, ranging from 100 K at night to 700 K during the day across the equatorial regions; the polar regions are below 180 K. The planet has no known natural satellites. Two spacecraft have visited Mercury: Mariner 10 flew by in 1974 and 1975; the BepiColombo spacecraft is planned to arrive at Mercury in 2025. Mercury appears to have a solid silicate crust and mantle overlying a solid, iron sulfide outer core layer, a deeper liquid core layer, a solid inner core.
Mercury is one of four terrestrial planets in the Solar System, is a rocky body like Earth. It is the smallest planet in the Solar System, with an equatorial radius of 2,439.7 kilometres. Mercury is smaller—albeit more massive—than the largest natural satellites in the Solar System and Titan. Mercury consists of 70% metallic and 30% silicate material. Mercury's density is the second highest in the Solar System at 5.427 g/cm3, only less than Earth's density of 5.515 g/cm3. If the effect of gravitational compression were to be factored out from both planets, the materials of which Mercury is made would be denser than those of Earth, with an uncompressed density of 5.3 g/cm3 versus Earth's 4.4 g/cm3. Mercury's density can be used to infer details of its inner structure. Although Earth's high density results appreciably from gravitational compression at the core, Mercury is much smaller and its inner regions are not as compressed. Therefore, for it to have such a high density, its core must be rich in iron.
Geologists estimate. Research published in 2007 suggests. Surrounding the core is a 500–700 km mantle consisting of silicates. Based on data from the Mariner 10 mission and Earth-based observation, Mercury's crust is estimated to be 35 km thick.. One distinctive feature of Mercury's surface is the presence of numerous narrow ridges, extending up to several hundred kilometers in length, it is thought that these were formed as Mercury's core and mantle cooled and contracted at a time when the crust had solidified. Mercury's core has a higher iron content than that of any other major planet in the Solar System, several theories have been proposed to explain this; the most accepted theory is that Mercury had a metal–silicate ratio similar to common chondrite meteorites, thought to be typical of the Solar System's rocky matter, a mass 2.25 times its current mass. Early in the Solar System's history, Mercury may have been struck by a planetesimal of 1/6 that mass and several thousand kilometers across.
The impact would have stripped away much of the original crust and mantle, leaving the core behind as a major component. A similar process, known as the giant impact hypothesis, has been proposed to explain the formation of the Moon. Alternatively, Mercury may have formed from the solar nebula before the Sun's energy output had stabilized, it would have had twice its present mass, but as the protosun contracted, temperatures near Mercury could have been between 2,500 and 3,500 K and even as high as 10,000 K. Much of Mercury's surface rock could have been vaporized at such temperatures, forming an atmosphere of "rock vapor" that could have been carried away by the solar wind. A third hypothesis proposes that the solar nebula caused drag on the particles from which Mercury was accreting, which meant that lighter particles were lost from the accreting material and not gathered by Mercury; each hypothesis predicts a different surface composition, there are two space missions set to make observations.
Earth orbits the Sun at an average distance of 149.60 million km, one complete orbit takes 365.256 days, during which time Earth has traveled 940 million km. Earth's orbit has an eccentricity of 0.0167. Since the Sun constitutes 99.76% of the mass of the Sun–Earth system, the center of the orbit is close to the center of the Sun. As seen from Earth, the planet's orbital prograde motion makes the Sun appear to move with respect to other stars at a rate of about 1° eastward per solar day. Earth's orbital speed averages about 30 km/s, fast enough to cover the planet's diameter in 7 minutes and the distance to the Moon in 4 hours. From a vantage point above the north pole of either the Sun or Earth, Earth would appear to revolve in a counterclockwise direction around the Sun. From the same vantage point, both the Earth and the Sun would appear to rotate in a counterclockwise direction about their respective axes. Heliocentrism is the scientific model that first placed the Sun at the center of the Solar System and put the planets, including Earth, in its orbit.
Heliocentrism is opposed to geocentrism, which placed the Earth at the center. Aristarchus of Samos proposed a heliocentric model in the 3rd century BC. In the 16th century, Nicolaus Copernicus' De revolutionibus presented a full discussion of a heliocentric model of the universe in much the same way as Ptolemy had presented his geocentric model in the 2nd century; this "Copernican revolution" resolved the issue of planetary retrograde motion by arguing that such motion was only perceived and apparent. "Although Copernicus's groundbreaking book...had been over a century earlier, Joan Blaeu was the first mapmaker to incorporate his revolutionary heliocentric theory into a map of the world." Because of Earth's axial tilt, the inclination of the Sun's trajectory in the sky varies over the course of the year. For an observer at a northern latitude, when the north pole is tilted toward the Sun the day lasts longer and the Sun appears higher in the sky; this results in warmer average temperatures. When the north pole is tilted away from the Sun, the reverse is true and the weather is cooler.
North of the Arctic Circle and south of the Antarctic Circle, an extreme case is reached in which there is no daylight at all for part of the year, continuous daylight during the opposite time of year. This is called midnight sun; this variation in the weather results in the seasons. By astronomical convention, the four seasons are determined by the equinoxes; the solstices and equinoxes divide the year up into four equal parts. In the northern hemisphere winter solstice occurs on or about December 21; the effect of the Earth's axial tilt in the southern hemisphere is the opposite of that in the northern hemisphere, thus the seasons of the solstices and equinoxes in the southern hemisphere are the reverse of those in the northern hemisphere. In modern times, Earth's perihelion occurs around January 3, the aphelion around July 4; the changing Earth–Sun distance results in an increase of about 6.9% in total solar energy reaching the Earth at perihelion relative to aphelion. Since the southern hemisphere is tilted toward the Sun at about the same time that the Earth reaches the closest approach to the Sun, the southern hemisphere receives more energy from the Sun than does the northern over the course of a year.
However, this effect is much less significant than the total energy change due to the axial tilt, most of the excess energy is absorbed by the higher proportion of water in the southern hemisphere. The Hill sphere of the Earth is about 1,500,000 kilometers in radius, or four times the average distance to the Moon; this is the maximal distance at which the Earth's gravitational influence is stronger than the more distant Sun and planets. Objects orbiting the Earth must be within this radius, otherwise they can become unbound by the gravitational perturbation of the Sun; the following diagram shows the relation between the line of solstice and the line of apsides of Earth's elliptical orbit. The orbital ellipse goes through each of the six Earth images, which are sequentially the perihelion on anywhere from January 2 to January 5, the point of March equinox on March 19, 20, or 21, the point of June solstice on June 20, 21, or 22, the aphelion on anywhere from July 3 to July 5, the September equinox on September 22, 23, or 24, the December solstice on December 21, 22, or 23.
The diagram shows an exaggerated shape of Earth's orbit. Because of the axial tilt of the Earth in its orbit, the maximal intensity of Sun rays hits the Earth 23.4 degrees north of equator at the June Solstice, 23.4 degrees south of equator at the December Solstice (at t