# Orbit

**orbitsorbital motionplanetary motionorbitingplanetary orbitrevolutionorbital revolutionrevolveorbit raisingorbital**

In physics, an orbit is the gravitationally curved trajectory of an object, such as the trajectory of a planet around a star or a natural satellite around a planet.wikipedia

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### Planet

**planetsFormer classification of planetsplanemo**

In physics, an orbit is the gravitationally curved trajectory of an object, such as the trajectory of a planet around a star or a natural satellite around a planet. Celestial mechanics treats more broadly the orbital dynamics of systems under the influence of gravity, including spacecraft and natural astronomical bodies such as star systems, planets, moons, and comets.

A planet is an astronomical body orbiting a star or stellar remnant that is 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.

### Natural satellite

**moonmoonssatellite**

In physics, an orbit is the gravitationally curved trajectory of an object, such as the trajectory of a planet around a star or a natural satellite around a planet. Bodies which are gravitationally bound to one of the planets in a planetary system, either natural or artificial satellites, follow orbits about a barycenter near or within that planet.

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 body).

### Geocentric model

**geocentricPtolemaic systemPtolemaic**

Originally geocentric, it was modified by Copernicus to place the Sun at the centre to help simplify the model.

Under the geocentric model, the Sun, Moon, stars, and planets all orbited Earth.

### Trajectory

**trajectoriesflightpathflight path**

In physics, an orbit is the gravitationally curved trajectory of an object, such as the trajectory of a planet around a star or a natural satellite around a planet.

For example, it can be an orbit — the path of a planet, asteroid, or comet as it travels around a central mass.

### Celestial spheres

**celestial sphereplanetary spherescelestial**

Historically, the apparent motions of the planets were described by European and Arabic philosophers using the idea of celestial spheres.

In modern thought, the orbits of the planets are viewed as the paths of those planets through mostly empty space.

### Planetary system

**planetary systemssolar systemssystem**

Within a planetary system, planets, dwarf planets, asteroids and other minor planets, comets, and space debris orbit the system's barycenter in elliptical orbits.

A planetary system is a set of gravitationally bound non-stellar objects in or out of orbit around a star or star system.

### Minor planet

**planetoidminor planetsplanetoids**

Within a planetary system, planets, dwarf planets, asteroids and other minor planets, comets, and space debris orbit the system's barycenter in elliptical orbits.

A minor planet is an astronomical object in direct orbit around the Sun (or more broadly, any star with a planetary system) that is neither a planet nor exclusively classified as a comet.

### Deferent and epicycle

**epicyclesdeferentdeferents and epicycles**

After the planets' motions were more accurately measured, theoretical mechanisms such as deferent and epicycles were added.

The orbits of planets in this system are similar to epitrochoids.

### Urbain Le Verrier

**Le VerrierLeverrierUrbain Jean Joseph Le Verrier**

In a dramatic vindication of classical mechanics, in 1846 Urbain Le Verrier was able to predict the position of Neptune based on unexplained perturbations in the orbit of Uranus.

The calculations were made to explain discrepancies with Uranus's orbit and the laws of Kepler and Newton.

### Satellite

**satellitesartificial satelliteartificial satellites**

Bodies which are gravitationally bound to one of the planets in a planetary system, either natural or artificial satellites, follow orbits about a barycenter near or within that planet.

In the context of spaceflight, a satellite is an object that has been intentionally placed into orbit.

### Barycenter

**barycentrebarycentricbarycentric coordinates**

Within a planetary system, planets, dwarf planets, asteroids and other minor planets, comets, and space debris orbit the system's barycenter in elliptical orbits. To a close approximation, planets and satellites follow elliptic orbits, with the central mass being orbited at a focal point of the ellipse, as described by Kepler's laws of planetary motion.

In astronomy, the barycenter (or barycentre; from the Ancient Greek βαρύς heavy + κέντρον center ) is the center of mass of two or more bodies that orbit one another and is the point about which the bodies orbit.

### Neptune

**NeptunianAtmosphere of NeptuneNeptune-mass**

In a dramatic vindication of classical mechanics, in 1846 Urbain Le Verrier was able to predict the position of Neptune based on unexplained perturbations in the orbit of Uranus. At the present epoch, Mars has the next largest eccentricity while the smallest orbital eccentricities are seen with Venus and Neptune.

In 1821, Alexis Bouvard published astronomical tables of the orbit of Neptune's neighbour Uranus.

### Geodesic

**geodesicsgeodesic flowgeodesic equation**

However, Albert Einstein's general theory of relativity, which accounts for gravity as due to curvature of spacetime, with orbits following geodesics, provides a more accurate calculation and understanding of the exact mechanics of orbital motion.

In particular, the path taken by a falling rock, an orbiting satellite, or the shape of a planetary orbit are all geodesics in curved spacetime.

### Circular orbit

**circularcircular orbitscircular Keplerian orbits**

For any specific combination of height above the center of gravity and mass of the planet, there is one specific firing speed (unaffected by the mass of the ball, which is assumed to be very small relative to the Earth's mass) that produces a circular orbit, as shown in (C).

A circular orbit is the orbit with a fixed distance around the barycenter, that is, in the shape of a circle.

### Sub-orbital spaceflight

**suborbitalsub-orbitalsuborbital flight**

A sub-orbital spaceflight is a spaceflight in which the spacecraft reaches outer space, but its trajectory intersects the atmosphere or surface of the gravitating body from which it was launched, so that it will not complete one orbital revolution.

### Moon

**lunarthe MoonLuna**

Celestial mechanics treats more broadly the orbital dynamics of systems under the influence of gravity, including spacecraft and natural astronomical bodies such as star systems, planets, moons, and comets.

The Moon is an astronomical body that orbits the Earth as its only permanent natural satellite.

### Conic section

**conicconic sectionsconics**

Isaac Newton demonstrated that Kepler's laws were derivable from his theory of gravitation and that, in general, the orbits of bodies subject to gravity were conic sections (this assumes that the force of gravity propagates instantaneously).

Conic sections are important in astronomy: the orbits of two massive objects that interact according to Newton's law of universal gravitation are conic sections if their common center of mass is considered to be at rest.

### Apse line

**line of apsesline of apsidesline-of-apsides**

A line drawn from periapsis to apoapsis is the line-of-apsides.

An apse line, or line of apsides, is an imaginary line defined by an orbit's eccentricity vector.

### Orbital plane (astronomy)

**orbital planeorbital planesplane of its orbit**

A transverse impulse (out of the orbital plane) causes rotation of the orbital plane without changing the period or eccentricity.

The orbital plane of a revolving body is the geometric plane in which its orbit lies.

### Apsis

**perigeeperihelionapogee**

As two objects orbit each other, the periapsis is that point at which the two objects are closest to each other and the apoapsis is that point at which they are the farthest.

Apsis (ἁψίς; plural apsides, Greek: ἁψῖδες; "orbit") denotes either of the two extreme points (i.e., the farthest or nearest point) in the orbit of a planetary body about its primary body (or simply, "the primary").

### Lagrangian point

**Lagrange pointLagrange pointsLagrangian points**

Lagrange (1736–1813) developed a new approach to Newtonian mechanics emphasizing energy more than force, and made progress on the three body problem, discovering the Lagrangian points.

In celestial mechanics, the Lagrangian points ( also Lagrange points, L-points, or libration points) are the points near two large bodies in orbit where a smaller object will maintain its position relative to the large orbiting bodies.

### Star

**starsstellarmassive star**

Binary and multi-star systems consist of two or more stars that are gravitationally bound and generally move around each other in stable orbits.

### Newton's cannonball

As an illustration of an orbit around a planet, the Newton's cannonball model may prove useful (see image below).

Newton's cannonball was a thought experiment Isaac Newton used to hypothesize that the force of gravity was universal, and it was the key force for planetary motion.

### Hyperbola

**hyperbolicrectangular hyperbolahyperbolas**

An open orbit will have a parabolic shape if it has velocity of exactly the escape velocity at that point in its trajectory, and it will have the shape of a hyperbola when its velocity is greater than the escape velocity.

### Venus

**Morning Starevening starplanet Venus**

At the present epoch, Mars has the next largest eccentricity while the smallest orbital eccentricities are seen with Venus and Neptune.

Although all planetary orbits are elliptical, Venus's orbit is the closest to circular, with an eccentricity of less than 0.01.