Click on a term, or a section heading below to see it's meaning, or read down through the terms in alphabetical order.
AU, Absolute Magnitude (H), Albedo, Argument of Perihelion, Asteroid, Astrometry, Comet, Declination, Eccentricity, Ephemeris, Geocentric, Geodetic, Heliocentric, Inclination, Laplace Plane, Longitude of Perihelion, Longitude of the Ascending Node, Mean Anomaly, Mean Longitude, Mean Motion, Meridian, Nutation, Obliquity, Orbit, Orbital Elements, Orbital Period, Perihelion Distance, Right Ascension, Semi-major Axis, Time of Perihelion Passage, Topocentric, Vernal Equinox.
General Terms, Orbital Terms, Coordinate and Reference System Terms, Common Units, Miscellaneous Terms.
Absolute Magnitude (H)
The magnitude of an asteroid at zero phase angle and at unit heliocentric and geocentric distances.
Albedo
Geometric albedo is the ratio of a body's brightness at zero phase angle to the brightness of a perfectly diffusing disk with the same position and apparent size as the body.
Asteroid
An asteroid is a planetesimal in orbit around the sun. Generally, asteroids are irregularly shaped, rocky bodies concentrated in orbits between Mars and Jupiter. The largest known asteroid is 1 Ceres which is about 933 km in diameter.
Astrometry
Astrometry pertains to the measurement of the position in the sky of solar-system bodies. Position measurements are typically made in right ascension and declination.
Comet
A comet is a planetesimal in orbit around the sun. Comets are believed to be composed of dust and volatile ices. When close to the sun, comets become heated enough to produce a coma of gas and dust. As this gas and dust moves outward from the comet, it is "blown" away by the solar wind and forms the comet's tail.
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Orbit
The path followed by a celestial body.
Orbital Elements
The parameters that specify the position and motion of a body in orbit are called the orbital elements. Six parameters are required to uniquely define a body's orbit. All terms are defined in the context of bodies in orbit around the sun.
Argument of Perihelion
The angle (in the body's orbit plane) between the ascending node line and perihelion measured in the direction of the body's orbit.
Eccentricity
An orbital parameter describing the eccentricity of the orbit ellipse. Eccentricity e is the ratio of half the distance between the foci c to the semi-major axis a: e=c/a. For example, an orbit with e=0 is circular, e=1 is parabolic, and e between 0 and 1 is elliptic.
Inclination
The angle between the vectors normal to the body's orbit plane and the specified reference plane. Typical reference planes are the ecliptic plane and the equatorial plane (referred to a specific epoch).
Longitude of Perihelion
The sum of the longitude of the ascending node and the argument of perihelion.
Longitude of the Ascending Node
The angle between the reference X-direction (typically the vernal equinox) and the point at which the body passes up (north) through the reference plane. This angle is often referred to as capital omega.
Mean Anomaly
The product of an orbiting body's mean motion and time past perihelion passage.
Mean Longitude
The sum of the mean anomaly and the longitude of perihelion.
Mean Motion
The angular speed required for a body to make one orbit around an ideal ellipse with a specific semi-major axis.
Orbital Period
The time required for an object to make a complete revolution along its orbit. For example, the orbital period for a typical main-belt asteroid is about 4 years.
Perihelion Distance
The distance between the orbiting body and the sun at it's closest approach.
Semi-major Axis
The length of the semi-major axis of an orbit ellipse. For solar system bodies, the value of semi-major axis is typically denoted by a and expressed in units of AU.
Time of Perihelion Passage
The time at which an object is at perihelion (it's closest distance to the sun).
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Coordinate and Reference System Terms
Declination
Declination is the angular distance on the celestial sphere north or south of the celestial equator. It is measured along the hour circle passing through the celestial object.
Laplace Plane
The Laplace plane (as used in our satellite orbital elements tables) is defined as the plane in which the satellite's nodal precession is contained (on average). An equivalent definition is the plane normal to the satellite's orbital precession pole.
The typical application for the Laplace plane is in describing the orbits of giant-planet satellites which are close enough to the planet to be perturbed primarily by the sun, planet, and the planet's gravitational harmonics. Is such cases, the Laplace plane is constrained to be between the planet's orbital plane and it's equatorial plane. However, in the case of the "outer" Jovian satellites (such as Pasiphae), the perturbations by Saturn and even the Galilean satellites dominate those due to Jupiter's gravitational harmonics. The result of those "external" perturbations (which are not normally considered in typical descriptions of the Laplace plane) is a Laplace plane which is beyond the confines of Jupiter's equatorial and orbital planes.
Right Ascension
Right ascension (or R.A.) is the angular distance on the celestial sphere measured eastward along the celestial equator from the equinox to the hour circle passing through the celestial object.
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AU
AU is short for Astronomical Unit and defined as: the radius of an unperturbed circular orbit around the Sun of a massless body having an orbital period of 2*(pi)/k days (k is the Gaussian gravitational constant). One AU is slightly less than the average distance between the Earth and the Sun (approximately 1.5x10^11 m). See the table of astrodynamic constants for precise values.
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Ephemeris
An ephemeris is a tabulation of computed positions and velocities (and/or various derived quantities such as right ascension and declination) of an orbiting body at specific times. The plural form of ephemeris is ephemerides.
Geocentric
Refers to a reference system centered at the Earth's center. A major difference between geocentric and topocentric ephemerides is that no local horizon exists (and therefore derived quantities such as azimuth and elevation are not defined) for a geocentric ephemeris.
Geodetic
Geodetic coordinates, latitude and longitude, specify a location on the Earth's oblate (non-spherical) surface. Latitude, unless otherwise specified, is generally the geodetic latitude. Geodetic latitude is defined as the angle between the equatorial plane and a line normal to the surface at that location. Geodetic longitude is the angular distance between the location's meridian and the Greenwich meridian.
Heliocentric
Refers to a reference system centered at the sun. For example, the heliocentric distance to an object is the distance between the object and the sun's center.
Meridian
A great circle passing through the Earth's rotational poles and through the zenith (the point directly overhead) of a location on the Earth.
Nutation
the short-period oscillations in the motion of the pole of rotation of a freely rotating body that is undergoing torque from external gravitational forces. Nutation of the Earth's pole is discussed in terms of components in obliquity and longitude.
Obliquity
The angle between the equatorial and orbital planes (or the rotational and orbital poles) of a body. The obliquity of the ecliptic for the Earth is the angle between the equatorial and ecliptic planes.
Topocentric
Refers to a reference system centered at some location on the Earth (for example, the geographic location of an observatory). The topocentric distance to an object is the distance between the object and the topocenter (for example, the observer).
Vernal Equinox
The point on the celestial sphere where the Sun crosses the Earth's equatorial plane from south to north. Also called the first point of Aries.
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