Frequently Used Astronomical Terms
conjunction: a situation in which the Moon or a planet appears to be especially close either to another planet or to a bright star.
constellation: a distinctive pattern of stars used informally to organize a part of the sky. There are 88 official constellations, which technically define sections of the sky rather than collections of specific stars.
culmination: the moment when a celestial object crosses the meridian and is thus as its highest above the horizon.
dark adaptation: the eyes' transition to night vision, in order to see faint objects. Dark adaptation is rapid during the first 5 or 10 minutes after you leave a well-lit room, but full adaptation requires at least a half hour, and can be ruined by a momentary glance at a bright light.
declination: the celestial equivalent of latitude, denoting how far (in degrees) an object in the sky lies north or south of the celestial equator.
Dobsonian: a type of Newtonian reflector made popular by amateur astronomer John Dobson. It uses a simple but highly effective wooden mount. Dobs provide more aperture per dollar than any other telescope design.
double star (binary star): two stars that lie very close to, and often orbit, each other. Line-of-sight doubles are a consequence of perspective and aren't physically related. Most stars, however, are multiples gravitationally bound together. Usually the stars orbit so closely that they appear as a single point of light even when viewed through professional telescopes.
earthshine: sunlight reflected by Earth that makes the otherwise dark part of the Moon glow faintly. It's especially obvious during the Moon's thin crescent phases.
ecliptic: the path among the stars that the Sun traces throughout the year. The Moon and planets never stray far from the ecliptic.
eyepiece: the part of a telescope that you look into.
field of view: what you see when you loook through a telescope or binoculars. Generally, the lower the magnification, the wider the field of view.