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.
astronomical unit
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.
asterism
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.
apparent magnitude
the point at which an object is
farthest from the Earth as it orbits
in an ellipse around our planet.
apogee
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.
aphelion
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.
annular
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.
albedo
absolute magnitude
The theoretical 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.
(see distance modulus)
