Astronomical Dictionary on Letter #A

Absolute Magnitude

The apparent magnitude that a star would possess it if were placed at a distance of 10 parsecs from ...

Accretion

An accumulation of dust and gas into larger bodies such as stars, planets and moons, or as discs aro...

Albedo

The total ratio of sunlight reflected by a planet or body compared to what it receives from the S...

Aphelion

The point in the orbit of a planet or comet when it is furthest from the center of the Sun. (Opposi...

Apogee

Similar to aphelion. The point in the orbit of the moon furthest from the center of the earth. (oppo...

Apoapsis

The point in an orbit when a planet is farthest from any body other than the Sun or the Earth. ...

Arc Minute

A measure of angular separation. Arc is measured in a number of different units designed for various...

Asteroid

(Also "planetoid") These are rocky bodies, the vast majority of which orbit the Sun between Mars and...

Ascending Node

The point in the orbit of an object, when it crosses the ecliptic, (or celestial equator) whilst mov...

Astronomical Unit

A system of measurement used within our solar system based on the mean distance between the Earth an...

Aurora

The Northern Lights. The brilliant displays of colored light in the sky at extreme northern and sout...

Atlas

In Greek mythology, brother of Prometheus and grandfather of Hermes (Mercury). Condemned to stand fo...

Atla

In Norse mythology, a giantess, mother of Heimdall....

Arachnoid

Spider or cobweblike feature on the surface of Venus, typically having a diameter of about 100-km an...

Apollo

In Greek mythology, one of the twelve Olympian gods. God of prophecy, healing, archery, music, youth...

Aphrodite

One of the twelve Greek Olympian gods. Goddess of Love (Roman name, Venus), daughter of Zeus and Dio...

Antipodal point

The opposite point with respect to any given point....

Anorthosite

A type of igneous rock composed almost entirely of feldspar, a group of minerals that make up about...

Altimetry

The measurement of elevation or altitude....

Albedo

The ratio of the amount of solar radiation reflected from an object to the total amount incident upo...

Akna

In Native American traditions from Mexico and from the Arctic, "Moon" (Wife of the Sun) and "The Mot...

Adams

John Couch Adams (1819-1892) English astronomer. One of the discoverers of the planet Neptune....

Achondrite

A stony meteorite, coarsely crystallized, with sizable fragments of various minerals visible to the ...

Aa

A basaltic lava with a rough, jagged surface....