Frequently Used Astronomical Terms
phase: the fraction of the Moon or other body that we see illuminated by sunlight.
reflector: a telescope that gathers light with a mirror.
refractor: a telescope that gathers light with a lens.
retrograde: when an object moves in the reverse sense of "normal" motion. For example, most bodies in the solar system revolve around the Sun and rotate counterclockwise as seen from above (north of) Earth's orbit; those that orbit or spin clockwise have retrograde motion. This term also describes the period when a planet or asteroid appears to backtrack against background stars because of the changing viewing perspective caused by Earth's orbital motion.
right ascension: the celestial equivalent of longitude, denoting how far (in 15 degree wide hours) an object lies east of the Sun's location during the March equinox.
seeing: a measure of the atmosphere's stability. Poor seeing makes objects waver or blur when viewed in a telescope at high magnification. The best seeing often occurs on hazy nights, when the sky's transparency is poor.
star: a massive ball of gas that generates prodigious amounts of energy (including light) from nuclear fusion in its hot, dense core. The Sun is a star.