Albedo: The amount of light reflected by a body, such as a planet or part of a planet’s surface.
Aperture: The clear diameter of the objective lens or primary mirror of a telescope or other optical instrument.
Aphelion: The point on its elliptical orbit around the Earth at which a body such as a planet, asteroid or comet is at its greatest distance from the Sun.
Apogee: The point on its elliptical orbit around the Earth at which a body such as the Moon or spacecraft is at its greatest distance from the Earth.
Asterism: A pattern of stars that itself is not a constellation. An example of which is the Plough or Big Dipper, which forms part of the constellation ursa major (the Great Bear).
Asteroid: One of a vast number of small bodies that revolve around the Sun. The main concentration of these lie between the orbits of Mars and Jupiter in what is called the Main Belt.
Astronomical Unit (AU): A unit of distance measurement equal to the average distance between the Earth and the Sun (92,956,000 miles).