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Orbit Of The Moon
The Moon
Moon
orbits Earth
Earth
in the prograde direction and completes one revolution relative to the stars in about 27.322 days (a sidereal month) and one revolution relative to the Sun
Sun
in about 29.530 days (a synodic month). Earth
Earth
and the Moon
Moon
orbit about their barycentre (common center of mass), which lies about 4,600 km (2,900 mi) from Earth's center (about 3/4 of the radius of Earth). On average, the distance to the Moon
Moon
is about 385,000 km (239,000 mi) from Earth's center, which corresponds to about 60 Earth
Earth
radii
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Declination
In astronomy, declination (abbreviated dec; symbol δ) is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of the celestial equator, along the hour circle passing through the point in question.[1] Right ascension
Right ascension
and declination as seen on the inside of the celestial sphere. The primary direction of the system is the vernal equinox, the ascending node of the ecliptic (red) on the celestial equator (blue). Declination
Declination
is measured northward or southward from the celestial equator, along the hour circle passing through the point in question.The root of the word declination (Latin, declinatio) means "a bending away" or "a bending down"
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Barycenter
The barycenter (or barycentre; from the Ancient Greek βαρύς heavy + κέντρον centre[1]) is the center of mass of two or more bodies that are orbiting each other, which is the point around which they both orbit. It is an important concept in fields such as astronomy and astrophysics. The distance from a body's center of mass to the barycenter can be calculated as a simple two-body problem. In cases where one of the two objects is considerably more massive than the other (and relatively close), the barycenter will typically be located within the more massive object. Rather than appearing to orbit a common center of mass with the smaller body, the larger will simply be seen to wobble slightly. This is the case for the Earth– Moon
Moon
system, where the barycenter is located on average 4,671 km (2,902 mi) from the Earth's center, well within the planet's radius of 6,378 km (3,963 mi)
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Nodal Precession
Nodal precession
Nodal precession
is the precession of the orbital plane of a satellite around the rotation axis of an astronomical body such as Earth. This precession is due to the non-spherical nature of a spinning body, which creates a non-uniform gravitational field. The following discussion relates to low earth orbit of artificial satellites which have no measurable effect on the motion of the Earth. The nodal precession of more massive, natural satellites such as the Moon
Moon
is more complex. Around a spherical body, an orbital plane would remain fixed in space around the central body. However, most bodies rotate, which causes an equatorial bulge
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Apsidal Precession
In celestial mechanics, apsidal precession or orbital precession is the precession (rotation) of the orbit of a celestial body. More precisely, it is the gradual rotation of the line joining the apsides of an orbit, which are the points of closest and farthest approach. Perihelion is the closest point to the Sun
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Quadrature (astronomy)
In spherical astronomy, quadrature is the configuration of a celestial object in which its elongation is perpendicular to the direction of the Sun. It is applied especially to the position of a superior planet or the Moon
Moon
at its first and last quarter phases. As shown in the diagram above, a planet (or other object) can be at the western quadrature (when it is to the west of the Sun
Sun
when viewed from the Earth) or at the eastern quadrature (when it is to the east of the Sun
Sun
when viewed from the Earth)
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Syzygy (astronomy)
In astronomy, a syzygy (/ˈsɪzɪdʒi/; from the Ancient Greek σύζυγος suzugos meaning, "yoked together"[2]) is a (usually) straight-line configuration of three or more celestial bodies in a gravitational system.[3]Mercury transiting the Sun
Sun
as viewed by the Curiosity rover on Mars (June 3, 2014).[4]Contents1 Overview 2 Occultations, transits, and eclipses 3 Einstein ring 4 Tidal variation 5 ReferencesOverview[edit] The word is often used in reference to the Sun, Earth, and either the Moon
Moon
or a planet, where the latter is in conjunction or opposition. Solar and lunar eclipses occur at times of syzygy, as do transits and occultations. The term is often applied when the Sun
Sun
and Moon
Moon
are in conjunction (new moon) or opposition (full moon).[5] The word syzygy is often used to describe interesting configurations of astronomical objects in general
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Opposition (astronomy)
In positional astronomy, two astronomical objects are said to be in opposition when they are on opposite sides of the celestial sphere, as observed from a given body (usually Earth). A planet (or asteroid or comet) is said to be "in opposition" when it is in opposition to the Sun. Because most orbits in the Solar System are nearly coplanar to the ecliptic, this occurs when the Sun, Earth, and the body are configured in an approximately straight line, or syzygy; that is, Earth
Earth
and the body are in the same direction as seen from the Sun
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Conjunction (astronomy And Astrology)
In astronomy, a conjunction occurs when two astronomical objects or spacecraft have either the same right ascension or the same ecliptic longitude, usually as observed from Earth.[1][2] The astronomical symbol for conjunction is ☌ (in Unicode
Unicode
U+260C) and handwritten . The conjunction symbol is not used in modern astronomy. It continues to be used in astrology. When two objects always appear close to the ecliptic—such as two planets, the Moon
Moon
and a planet, or the Sun
Sun
and a planet—this fact implies an apparent close approach between the objects as seen on the sky
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Elongation (astronomy)
In astronomy, a planet's elongation is the angular separation between the Sun
Sun
and the planet, with Earth
Earth
as the reference point. The greatest elongation of a given inferior planet occurs when this planet’s position, in its orbital path around the Sun, is at tangent to the observer on Earth. Since an inferior planet is well within the area of Earth's orbit
Earth's orbit
around the Sun, observation of its elongation should not pose that much a challenge (compared to deep-sky objects, for example). When a planet is at its greatest elongation, it appears farthest from the Sun
Sun
as viewed from Earth, so its apparition is also best at that point. When an inferior planet is visible after sunset, it is near its greatest eastern elongation. When an inferior planet is visible before sunrise, it is near its greatest western elongation
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Julian Day
Julian day is the continuous count of days since the beginning of the Julian Period and is used primarily by astronomers. The Julian Day
Day
Number (JDN) is the integer assigned to a whole solar day in the Julian day count starting from noon Universal time, with Julian day number 0 assigned to the day starting at noon on Monday, January 1, 4713 BC, proleptic Julian calendar
Julian calendar
(November 24, 4714 BC, in the proleptic Gregorian calendar),[1][2][3] a date at which three multi-year cycles started (which are: Indiction, Solar, and Lunar cycles) and which preceded any dates in recorded history.[4] For example, the Julian day number for the day starting at 12:00 UT on January 1, 2000, was 2 451 545.[5] The Julian date (JD) of any instant is the Julian day number plus the fraction of a day since the preceding noon in Universal Time
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Planetesimal
Planetesimals /plænɪˈtɛsɪməlz/ are solid objects thought to exist in protoplanetary disks and in debris disks. Debris disks
Debris disks
detected in HST archival images of young stars, HD 141943 and HD 191089, using improved imaging processes (24 April 2014).[1]A widely accepted theory of planet formation, the so-called planetesimal hypotheses, the Chamberlin–Moulton planetesimal hypothesis and that of Viktor Safronov, states that planets form out of cosmic dust grains that collide and stick to form larger and larger bodies. When the bodies reach sizes of approximately one kilometer, then they can attract each other directly through their mutual gravity, enormously aiding further growth into moon-sized protoplanets. This is how planetesimals are often defined. Bodies that are smaller than planetesimals must rely on Brownian motion
Brownian motion
or turbulent motions in the gas to cause the collisions that can lead to sticking
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Equatorial Plane
The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth. This plane of reference bases the equatorial coordinate system. In other words, the celestial equator is an abstract projection of the terrestrial equator into outer space.[1] As a result of the planet's axial tilt, the celestial equator is currently inclined by about 23.44° with respect to the ecliptic plane. An observer standing on Earth's equator visualizes the celestial equator as a semicircle passing through the zenith, the point directly overhead. As the observer moves north (or south), the celestial equator tilts towards the opposite horizon. The celestial equator is defined to be infinitely distant (since it is on the celestial sphere); thus, the observer always sees the ends of the semicircle disappear over the horizon exactly due east and due west, regardless of the observer's position on Earth
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Planet
Shown in order from the Sun
Sun
and in true color. Sizes are not to scale.A planet is an astronomical body orbiting a star or stellar remnant thatis massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.[a][1][2]The term planet is ancient, with ties to history, astrology, science, mythology, and religion. Several planets in the Solar System
Solar System
can be seen with the naked eye. These were regarded by many early cultures as divine, or as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union
International Astronomical Union
(IAU) officially adopted a resolution defining planets within the Solar System
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Natural Satellite
A natural satellite or moon is, in the most common usage, an astronomical body that orbits a planet or minor planet (or sometimes another small Solar System
Solar System
body). In the Solar System
Solar System
there are six planetary satellite systems containing 175 known natural satellites.[1][2] Four IAU-listed dwarf planets are also known to have natural satellites: Pluto, Haumea, Makemake, and Eris.[3] As of October 2016[update], there are over 300 minor planets known to have moons.[4] The Earth– Moon
Moon
system is unique in that the ratio of the mass of the Moon
Moon
to the mass of Earth
Earth
is much greater than that of any other natural-satellite–planet ratio in the Solar System
Solar System
(although there are minor-planet systems with even greater ratios, notably the Pluto–Charon system)
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Celestial Sphere
In astronomy and navigation, the celestial sphere is an abstract sphere, with an arbitrarily large radius, that is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, which may be centered on Earth or the observer. If centered on the observer, half of the sphere would resemble a hemispherical screen over the observing location. The celestial sphere is a practical tool for spherical astronomy, allowing astronomers to specify the apparent positions of objects in the sky if their distances are unknown or irrelevant.Contents1 Introduction 2 Celestial coordinate systems 3 History 4 Star globe 5 Bodies other than Earth 6 See also 7 Notes 8 References 9 External linksIntroduction[edit]Celestial Sphere, 18th century. Brooklyn Museum.Because astronomical objects are at such remote distances, casual observation of the sky offers no information on their actual distances
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