absolute magnitude - The actual brightness of an object if it were at a standard ten parsec distance.  We can know the absolute magnitude of a star if its physical characteristics are known.  The difference between absolute and apparent magnitude can be used to calculate the distance to an object. albedo - The reflectivity of a solid object, on a scale of 0 to 1.  Snow has an albedo of essentially 1.0 whereas coal would have an albedo near 0. annular - referring to a type of solar eclipse.  When the Moon is too far away in its elliptical orbit to completely cover the Sun, we see an annular eclipse where a ring of light from the photosphere remains.  The sky does not darken dramatically and all of the other fun things about a total solar eclipse do not occur. aphelion - For a body orbiting the Sun, aphelion is that point where it is farthest from the Sun.  All objects orbit in paths that are ellipses, not circles. apogee - the point at which an object is farthest from the Earth as it orbits in an ellipse around our planet. apparent magnitude - The brightness of a celestial body as seen by an observer on Earth.  The magnitude system used by astronomers is a ranking system, so smaller numbers mean brighter objects. asterism - A small group of stars that forms a simple geometrical pattern.  The Big Dipper is a well-known asterism that is formed by the seven brightest stars of the constellation of Ursa Major. astronomical unit - The average distance between the Sun and the Earth.  It is our unit of distance for orbits in our solar system.  One A.U. is about 147 million kilometers.
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Carpe Caelum