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Glossary:

Barycentric

Coordinates relative to the centre of mass, the barycenter, of the solar system. The barycenter does not coincide with the centre of the Sun. It is displaced to a point near its surface more or less in the direction of Jupiter. These coordinates can be used to start solving the N-body iteration for the solar system.

Heliocentric

Data is referring to the center of the sun, which is slightly off the center of mass of the solarsystem.

Geocentric

Data is referring to the mass center of the earth. Coordinates do not take into account perspective effects from the parallax.

Topocentric

Data is referring to your selected observing site.

Physical data

Data taking into account the physical dimensions and rotational characteristics of the celestial body.

Radius

Distance of the celestial body from main central body (earth for the moon, the sun otherwise). For the moon the unit is earth radii (ER), otherwise Astronomical Unit (AU), the mean distance between the sun and earth.

Delta

Distance of the celestial body from earth in Astronomical Units (AU). For the moon, Delta is the topocentric distance of the moons mass center from the observer in earth radii (ER). It is also the fourth letter in Greek alphabet

Horizontal Parallax

Difference between the topocentric and geocentric positions of an object, when the object is on the astronomical horizon. Seen from the object, it is the angle of the earth radius.

Light-time

Time it takes for the light to travel from the celestial body to the observer.

Elongation

The elongation is the angular separation of the (ecliptical) longitudes of a celestial body and the central body (Sun, for moons: Jupiter or Saturn), as seen from the earth mass center.

R.A., right ascension, RA

One coordinate used to indicate the position on the sphere. It is the angular distance of the object from the spring equinox measured along the celestial equator, expressed in hours of arc.

Dec., declination, DE

One coordinate used to indicate the position on the sky. It is the angular distance of the object from the celestial equator. North pole, close to Polaris, is 90° north.

Geometric coordinates

Celestial coordinates referring to the center of the Earth without correction of planetary aberration.

Astrometric coordinates

Position of an object corrected in such a way, that it can directly be plotted into a star chart for a given epoch (usually J2000). This is the geometric position, corrected for light-time.

Apparent coordinates

Celestial coordinates which are directly observable. Corrected for various effects: i.e. light-time, light deflection due to effects of relativity, planetary aberration, and corresponding to the true equator and equinox. Topocentric apparent coordinates include effects of refraction (if not assumed airless) and diurnal aberration (perspective displacement from the earth mass center).

Ecliptical coordinates

Position of celestial body referred to the mean plane of Earth's orbit around the sun and the spring equinox, expressed in ecliptical longitude (°) and latitude.

Diameter

Diameter is the geocentric apparent angular diameter of a celestial object (topocentric for artifical satellites). The value is given in seconds of arc for planets and satellites, and in minutes of arc for sun and moon.

Magnitude/Mag

Brightness of an object considered as a point source of light, on a logarithmic scale. Visual limiting magnitude is about 6mag, whereas the brightest star Sirius reaches -1.4mag. The Hubble Space Telescope can image objects as dim as 29mag.

Surface Brightness

The visual magnitude of an average square arc-second area of the illuminated portion of the apparent disk (of a planet).

Phase

Ratio of the illuminated fraction of a planetary disk to its apparent diameter.

Phase angle

Angle between the sun and the observer as seen from the center of the celestial body.

Sub-earth Phi/Lambda

Coordinates (lambda/phi) of the point on the celestial body, which sees the earth directly overhead. The value of Lambda is equal to the central meridian of the body. Planetocentric coordinates refer to a sphere, whereas planetographic coordinates take the flattening of the planet into account (earth in zenith). For Jupiter several rotational periods are known. System I: mean atmospheric equatorial rotation. System II: mean atmospheric rotation north of the south component of the north equatorial belt and south of the north component of the south equatorial belt. System III: rotation of the magnetic field.

Sub-solar Phi/Lambda

Coordinates of the body-graphic point with the sun directly overhead (in local zenith). The latitude phi is taking into account the flattening of the physical body, and hence is not identical to the body-centered latitude.