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Encyclopedia > Orbit of the Moon

The orbit of the Moon around the Earth is completed in approximately 27.3 days. The Earth and Moon orbit about their common center of mass, which lies about 4,700 kilometres from Earth's center (about three quarters of the Earth's radius). On average, the Moon is at a distance of about 385,000 km from the center of the Earth, which corresponds to about 60 Earth radii. With a mean orbital speed of 1.023 km/s, the Moon moves relative to the stars each hour by an amount roughly equal to its angular diameter, or by about 0.5°. The Moon differs from most satellites of other planets in that its orbit is close to the plane of the ecliptic, and not to the Earth's equatorial plane. The lunar orbit plane is inclined to the ecliptic by 5.1°, whereas the Moon's spin axis is inclined by only 1.5°. This article is about Earths moon. ... This article is about Earth as a planet. ... STARS can mean: Shock Trauma Air Rescue Society Special Tactics And Rescue Service, a fictional task force that appears in Capcoms Resident Evil video game franchise. ... The angular diameter of an object as seen from a given position is the diameter measured as an angle. ... For other uses, please see Satellite (disambiguation) A satellite is an object that orbits another object (known as its primary). ... A planet (from the Greek πλανήτης, planetes or wanderers) is a body of considerable mass that orbits a star and that produces very little or no energy through nuclear fusion. ... The plane of the ecliptic is well seen in this picture from the 1994 lunar prospecting Clementine spacecraft. ... In astronomy and navigation, the celestial sphere is an imaginary rotating sphere of gigantic radius, concentric with the Earth. ... A sphere rotating around its axis. ...

Contents

Properties of the lunar orbit

Definition of orbital parameters.
Definition of orbital parameters.
Property Value
Semi-major axis ~384 748 km
Distance at perigee ~364 397 km
Distance at apogee ~406 731 km
Mean eccentricity 0.0549006
(0.044 – 0.067)
Mean inclination of orbit to ecliptic 5.145°
Mean obliquity 6.688°
Mean inclination of lunar equator to ecliptic 1.543°
Period of precession of nodes 18.5996 years
Period of recession of line of apsides 8.8504 years

The distance between the center of masses of the Earth and Moon is, averaged over time, about 385,000 km, corresponding to about 60 Earth radii or 30 Earth diameters. Both bodies orbit about their common center of mass (the barycenter), which is located at about 4,670 kilometres from the center of the Earth, or 1700 km below the Earth's surface. Because of the relatively large size of the Moon and the relatively small Earth/Moon mass ratio of about 81:1 (when compared to other natural satellites in our solar system), some consider the Earth-Moon system to be a double planet. Based on the informal definition that a double planet system must have its barycentre exterior to both bodies, others, however, regard the Earth and Moon as an ordinary planet-moon system. Image File history File links Orbit. ... Image File history File links Orbit. ... The semi-major axis of an ellipse In geometry, the term semi-major axis (also semimajor axis) is used to describe the dimensions of ellipses and hyperbolae. ... In astrodynamics, under standard assumptions any orbit must be of conic section shape. ... For the science fiction novella by William Shunn, see Inclination (novella). ... Axial tilt is an astronomical term regarding the inclination angle of a planets rotational axis in relation to its orbital plane. ... The moons elliptical orbit precesses about once in just under 9 years. ... In physics, the center of mass of a system of particles is a specific point at which, for many purposes, the systems mass behaves as if it were concentrated. ... In physics, the center of mass of a system of particles is a specific point at which, for many purposes, the systems mass behaves as if it was concentrated. ... A natural satellite is an object that orbits a planet or other body larger than itself and which is not man-made. ... Pluto and Charon are sometimes informally considered to be a double (dwarf) planet. ... The barycenter (from the Greek βαρύκεντρον) is the center of mass of two or more bodies which are orbiting each other, and is the point around which both of them orbit. ...


The orbit of the Moon is distinctly elliptical with an average eccentricity of 0.0549. The non-circular form of the lunar orbit causes variations in the Moon's angular speed and apparent size as it moves towards and away from an observer on Earth. The mean angular daily movement relative to an imaginary observer at the barycentre is 13.176358° to the east. The orientation of the orbit is not fixed in space, but precesses over time. One motion is the recession of the line of apsides: the ellipse of the lunar orbit slowly rotates counterclockwise, and completes a full revolution in about 8.850 years (3233 days). The other motion is associated with the (clockwise) precession of the orbital plane itself about an axis perpendicular to the ecliptic. The points where the lunar orbit intersects the ecliptic (the nodes) precess with time, completing one revolution in about 18.6 years (6793 days). (This page refers to eccitricity in astrodynamics. ... The moons elliptical orbit precesses about once in just under 9 years. ... In astrodynamics apse line (or line of apsides) is a common name for an imaginary line defined by orbits eccentricity vector. ... Fig. ... The plane of the ecliptic is well seen in this picture from the 1994 lunar prospecting Clementine spacecraft. ... The lunar nodes are the orbital nodes of the Moon, that is, the points where the orbit of the Moon crosses the ecliptic (which is the apparent path of the Sun across the heavens against the background stars). ...


The mean inclination of the lunar orbit to the ecliptic plane is 5.145°. The rotation axis of the Moon is also not perpendicular to its orbital plane, so the lunar equator is not in the plane of its orbit, but is inclined to it by a constant value of 6.688° (this is the obliquity). One might be tempted to think that as a result of the precession of the Moon's orbit plane, the angle between the lunar equator and the ecliptic would vary between the sum (11.833°) and difference (1.543°) of these two angles. However, as was discovered by Jacques Cassini in 1721, the rotation axis of the Moon precesses with the same rate as its orbit plane, but is 180° out of phase (see Cassini's Laws). Thus, although the rotation axis of the Moon is not fixed with respect to the stars, the angle between the ecliptic and the lunar equator is always 1.543°. Axial tilt is an astronomical term regarding the inclination angle of a planets rotational axis in relation to its orbital plane. ... Jacques Cassini (February 8, 1677 - April 18, 1756) was a French astronomer, son of Giovanni Domenico Cassini Cassini was born at the Paris Observatory. ... Cassinis Laws (1693) are three laws that deal with the motion of the Moon. ...

The Moon orbiting Earth, sizes and distances to scale.

Image File history File links Download high resolution version (14476x480, 396 KB) Summary The below information refers to the original version of this file, which is now half the size it was (i. ... Image File history File links Download high resolution version (14476x480, 396 KB) Summary The below information refers to the original version of this file, which is now half the size it was (i. ...

The lunar month

See also: Month
The Moon's periods
Name Value (d) Definition
sidereal 27.321 661 With respect to the distant stars (13.369 passes per year)
synodic 29.530 589 With respect to the Sun (phases of the Moon, 12.369 cycles per year)
tropical 27.321 582 With respect to the vernal point (precesses in ~26,000 a)
anomalistic 27.554 550 With respect to the perigee (recesses in 3232.6 d = 8.8504 a)
draconic (nodical) 27.212 221 With respect to the ascending node (precesses in 6793.5 d = 18.5996 a)

There are several ways to consider how much time it takes the Moon to complete one orbit. The sidereal month is the time it takes to make one complete orbit with respect to the fixed stars, which is about 27.3 days. In contrast, the synodic month is the time it takes the Moon to reach the same phase, which takes about 29.5 days. The synodic period is longer than the sidereal period because the Earth-Moon system moves a finite distance in its orbit around the Sun during each sidereal month, and a longer time is required to achieve the same relative geometry. Other definitions for the duration of a lunar month include the time it takes to go from perigee to perigee (the anomalistic month), from ascending node to ascending node (the Draconic month), and from two successive passes of the same ecliptic longitude (the Tropical month). As a result of the slow precession of the lunar orbit, these latter three periods are only slightly different than the sidereal month. The average length of a calendric month (1/12 of a year) is about 30.4 days. Look up Month in Wiktionary, the free dictionary. ... A year (from Old English gēr) is the time between two recurrences of an event related to the orbit of the Earth around the Sun. ... The First Point of Aries, also called the vernal equinox point, is one of the two points on the celestial sphere where the celestial equator intersects the ecliptic. ... Look up Month in Wiktionary, the free dictionary. ... Look up Month in Wiktionary, the free dictionary. ... Lunar phase refers to the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. ... Sol redirects here. ... Look up Month in Wiktionary, the free dictionary. ... Look up Month in Wiktionary, the free dictionary. ... Look up Month in Wiktionary, the free dictionary. ...


Tidal evolution of the lunar orbit

See also: Tide, Tidal acceleration, and Axial tilt#Long period variations

The gravitational attraction that the Moon exerts on Earth is the cause of tides in the sea. If the Earth possesed a global ocean of uniform depth, the Moon would act to deform both the solid earth (by a small amount) and ocean in the shape of an ellipsoid with high points directly beneath the Moon and on the opposite side of the Earth. However, as a result of the irregular coastline and varying ocean depths, this idealization is only partially realized. While the tidal flow period is generally synchronized to the Moon's orbit around Earth, its phase can vary. In some places on Earth there is only one high tide per day, though this is somewhat rare. This article is about tides in the Earths oceans. ... It has been suggested that Tidal friction be merged into this article or section. ... In astronomy, axial tilt is the inclination angle of a planets rotational axis in relation to a perpendicular to its orbital plane. ... “Gravity” redirects here. ... This article is about tides in the Earths oceans. ... Animated map exhibiting the worlds oceanic waters. ... Typical striping on an old-style suicide lane setup in the United States. ...


The tidal bulges on Earth are carried ahead of the Earth-Moon axis by a small amount as a result of the Earth's rotation. This is a direct consequence of friction and the dissipation of energy as water moves over the ocean bottom and into or out of bays and estuaries. As a result, some of the Earth's rotational momentum is gradually being transferred to the Moon's orbital momentum, and this causes the Moon to slowly recede from Earth at the rate of approximately 38 millimetres per year. Due to conservation of angular momentum, the Earth's rotation is gradually slowing, and the Earth's day thus lengthens by about 17 microseconds every year (this would make each Earth day one second longer every 60,000 years or so, by one minute longer every four million years, and by four hours longer in 1 billion years' time. Looking back, the day was a mere 23h in length when the Dinosaurs roamed the Earth 65 million years ago). See tidal acceleration for a more detailed description and references. For other uses, see Friction (disambiguation). ... An estuary is a semi-enclosed coastal body of water which has a free connection with the open sea and within which sea water mixes with fresh water. ... In physics, angular momentum intuitively measures how much the linear momentum is directed around a certain point called the origin; the moment of momentum. ... Orders & Suborders Saurischia Sauropodomorpha Theropoda Ornithischia Thyreophora Ornithopoda Marginocephalia Dinosaurs were vertebrate animals that dominated the terrestrial ecosystem for over 160 million years, first appearing approximately 230 million years ago. ... It has been suggested that Tidal friction be merged into this article or section. ...


So the Moon is gradually receding from the Earth into a higher orbit, and calculations (by e.g. NASA at the Jet Propulsion Laboratory [1]) suggest that this will continue for about two billion years. By that time, the Earth and Moon will become caught up in what are called a "spin-orbit resonance" in which the Moon will circle the Earth in about 47 days (currently 29 days) and both Moon and Earth will rotate around their axes in the same time, always facing each other with the same side. Beyond this, it is hard to tell what will happen to the Earth-Moon system. This article is about the American space agency. ... For the singer/songwriter, see Jon Peter Lewis. ...


Libration

Animation of the Moon as it cycles through its phases. The apparent wobbling of the Moon is known as libration.

The Moon is in synchronous rotation, meaning that it keeps the same face turned toward the Earth at all times. This synchronous rotation is only true on average because the Moon's orbit has a definite eccentricity. As a result, the angular velocity of the Moon varies as it moves around the Earth, and is hence not always equal to the Moon's rotational velocity. When the Moon is at its perigee, its rotation is slower than its orbital motion, and this allows us to see up to eight degrees of longitude of its Eastern (right) far side. Conversely, when the Moon reaches its apogee, its rotation is faster than its orbital motion and this reveals eight degrees of longitude of its Western (left) far side. This is referred to a longitudinal libration. Image File history File links Lunar_libration_with_phase2. ... Image File history File links Lunar_libration_with_phase2. ... Due to synchronous rotation of their moon, the inhabitants of the central body will never be able to see its green side. ... In astrodynamics, under standard assumptions any orbit must be of conic section shape. ... Perigee is the point at which an object in orbit around the Earth makes its closest approach to the Earth. ... Far side of the Moon. ... This article is about several astronomical terms (apogee & perigee, aphelion & perihelion, generic equivalents based on apsis, and related but rarer terms. ... Not to be confused with Liberation. ...


Because the lunar orbit is also inclined to the Earth's ecliptic plane by 5.1° , the rotation axis of the Moon seems to rotate towards and away from us during one complete orbit. This is referred to as a latitudinal libration, which allows one to see almost 7° of latitude beyond the pole on the far side. Finally, because the Moon is only about 60 Earth radii away from the Earth's center of mass, an observer at the equator who observes the Moon throughout the night moves laterally by one Earth diameter. This gives rise to a diurnal libration, which allows one to view an additional one degree's worth of lunar longitude. For the same reason, observers at both geographical poles of the Earth would be able to see one additional degree's worth of libration in latitude. A geographical pole is either of two fixed points on the surface of a spinning body or planet, at 90 degrees from the equator, based on the axis around which a body spins. ...


The Earth & Moon's path around the Sun

The Earth & Moon's path around the Sun is always concave to the Sun (far left down)
The Earth & Moon's path around the Sun is always concave to the Sun (far left down)

In representations of the Solar system, it is common to draw the trajectory of the Earth from the point of view of the Sun, and the trajectory of the Moon from the point of view of the Earth. This could give the impression that the trajectory of the Moon circles around the Earth in such a way that sometimes it goes backwards when viewed from the Sun's perspective. Since the orbital velocity of the Moon about the Earth (1 km/s) is small compared to the orbital velocity of the Earth about the Sun (30 km/s), this never occurs. Image File history File links This is a lossless scalable vector image. ... Image File history File links Moon_trajectory1. ... Image File history File links Moon_trajectory1. ...


Unlike most other moons in the Solar System, the annual trajectory of the Moon is very similar to the one of the Earth. It is always concave towards the Sun, and is nowhere convex or looped.[2][3] Nevertheless, this behaviour should not be taken to mean that the Moon orbits the Sun, and not the Earth. If the gravitational attraction of our Sun could be "turned off," the Moon would continue to make one orbit about the Earth with its current sidereal period. If the Earth-Moon system could be transported to the far reaches of our solar system where the orbital velocity was less than 1 km/s, then the Moon would indeed sometimes move backwards from the vantage point of non-rotating coordinate system centered at the Sun. The orbital period is the time it takes a planet (or another object) to make one full orbit. ... Direct motion is the motion of a planetary body in a direction similar to that of other bodies within its system, and is sometimes called prograde motion. ...


See also

Ernest William Brown (November 29, 1866 – July 22, 1938) was a British astronomer. ... Lunar phase refers to the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. ... The elements of an orbit are the parameters needed to specify that orbit uniquely, given a model of two ideal masses obeying the Newtonian laws of motion and the inverse-square law of gravitational attraction. ... Precession of a gyroscope Precession refers to a change in the direction of the axis of a rotating object. ... The following is a list of types of orbits: // Box orbit Circular orbit Ecliptic orbit Elliptic orbit Highly Elliptical Orbit Graveyard orbit Hohmann transfer orbit Hyperbolic trajectory Inclined orbit Osculating orbit Parabolic trajectory Capture orbit Escape orbit Semi-synchronous orbit Subsynchronous orbit Synchronous orbit Geocentric orbit Geosynchronous orbit Geostationary orbit... In astronomy, axial tilt is the inclination angle of a planets rotational axis in relation to a perpendicular to its orbital plane. ... Milankovitch cycles are the collective effect of changes in the Earths movements upon its climate, named after Serbian civil engineer and mathematician Milutin Milanković. The eccentricity, axial tilt, and precession of the Earths orbit vary in several patterns, resulting in 100,000 year ice age cycles of the... Position of vernal equinox occurring in Pisces after leaving Aries constellation (through the precession of the equinoxes backward motion). ... A Great year (also known as a Platonic year or Equinoctial cycle) is the time required for one complete cycle of the precession of the equinoxes: about 25700 years. ... (This page refers to eccitricity in astrodynamics. ... Illumination of the Earth by the Sun on the day of equinox, (ignoring twilight). ...

References

  1. ^ Dickinson, Terence (1993). From the Big Bang to Planet X. Camden East, Ontario: Camden House, 79-81. ISBN 0-921820-71-2. 
  2. ^ The Orbit of the Moon around the Sun is Convex!. Retrieved on 2006-04-21.
  3. ^ Vacher, H.L. (Nov 2001). "Computational Geology 18 - Definition and the Concept of Set" (PDF). Journal of Geoscience Education (5): 470-479. Retrieved on 2006-04-21. 

 
 

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