# Glossary: Astronomical

a.u.
An a.u. is an Astronomical Unit. An Astronomical Unit is 149 597 870 691 ± 30 meters (about 150 million kilometers or 93 million miles).

absolute magnitude
The absolute magnitude of an asteroid is how bright an asteroid would be if it were 1 .a.u. away from the Sun, 1 a.u. away from the observer, and the asteroid in question is fully illuminated.

Note that this absolute magnitude definition differs from the absolute magnitude used in stars.

Asteroid absolute magnitude calculated for a theoretical position of an asteroid. One may estimated current brightness of an asteroid by adjusting the absolute magnitude for factors such as the actual distance from the observer, the asteroid's distance from the Sun, and how much of the observable surface is illuminated by the Sun.

See NEO absolute magnitude glossary and absolute magnitude for planets for more information.

aphelion
Aphelion is the point of farthest approach of a body that is orbiting the Sun.

arcminute
An arcminute is 1/60-th of a degree. Humans with normal visual acuity can just barely distingush two objects that are separated by one arcminute. A line that is 2.91 cm long (about 1.15 inches) will span about one arcminute at a distance of 100m (about 328 feet).
arcsecond
An arcsecond is 1/3600-th or a degree. An arcminute is 1/60-th of an arcminute. A line that is 4.85 cm long (about 1.91 inches) will span about one arcsecond at a distance of 10km (about 6.21 miles).
ecliptic
The ecliptic is the plane in with the Earth orbits around the Sun.

greatest elongation

When an object that is closer to the Sun appears to be at a maximum separation angle from the Sun, then the object is said to be at greatest elongation. The orbit of such interior objects will have two greatest elongation points. One greatest elongation point will correspond to the point where its rise will be the earliest relative to sunrise. The other greatest elongation point will correspond to the point where its set will be the latest relative to sunset.

Gyr
1 Gyr = 1 billion (109 = 1 000 000 000) years.
ideal horizon
The ideal horizon is the apparent junction of the sky and the sea level surface of the Earth. The ideal horizon ignores any obstructions such as hills, buildings, trees, etc. The ideal horizon takes into account atmospheric refraction.

The angle in the sky between the zenith and the ideal horizon will be more than 90° because of atmospheric refraction. For example, a satellite that appears at zenith will need to move down the celestial sphere by more than 90° before it disappears below the ideal horizon. This is because air acts as a lens and refracts light passing through it. This refractive bending of the light is most noticeable near the horizon where light may be net by as much as 0.5° or so.

The ideal horizon is sometimes called the sea level horizon. It is also sometimes called the ideal horizon, the sensible horizon, or the apparent horizon. However, those last 3 terms should be avoided because in some circles those 3 terms mean a somewhat different type of horizon.

inferior conjunction
When object (planet, asteroid, comet, etc.) that orbits closer to the Sun than the observer (Earth) is in front of the Sun, it is said to be in inferior conjunction. Because objects orbit in different planes, objects in inferior conjunction are frequently do not cross in front of the solar disk (as view by the observer) but instead appear to pass above or below the Sun.

mean distance
The mean distance arithmetic is the mean of the maximum and minimum distance between two bodies. For a body orbiting the sun, its mean distance is half way between the maximum distance to the Sun (aphelion) and the minimum distance to the Sun (perihelion). See also: semi-major axis
Mercury
Mercury is the closest planet to the Sun. As of 15 October 2005, it is the object that has the closest known mean distance to the Sun.

Myr
1 Myr = 1 million (106 = 1 000 000) years.
perihelion
Perihelion is the point of closest approach of a body that is orbiting the Sun.

power-law
A power-law describes an exponential relationship between x and y such: y = axk where "a" is the constant of proportionality and "k" is the exponent constant. In 1969, J. W. Dohnanyi suggested that a collision-based system such as the asteroid belt would have a "k" very close to -11/6. Others have suggested somewhat different values of "k" for large and small asteroids.

semi-major axis
Based on Kepler's third law, the semi-major axis of a planet is equal to the mean distance of the planet.

stable Vulcanoid zone
If an object has a mean distance of ≥ 0.08 a.u and ≤ 0.18 a.u. then it resides inside the stable Vulcanoid zone.

See FAQ 1.3: What is the stable Vulcanoid zone? for more information.

superior conjunction
When object (planet, asteroid, comet, etc.) that orbits closer to the Sun than the observer (Earth) is behind the Sun, it is said to be in superior conjunction. Because objects orbit in different planes, objects in superior conjunction are frequently not covered by the solar disk (as view by the observer) but instead appear to pass above or below the Sun.