Technical data

Glossary 51
Glossary
absolute magnitude - the apparent magnitude an object would have if it were moved to a distance of 10 parsecs.
altitude - angular distance of an object above or below the horizon expressed in deg, min, sec. Positive values are
above the horizon, negative values are below.
angular diameter - the apparent diameter of an object expressed in angular measure i.e., deg, min, sec.
aperture - the diameter of the objective lens or primary mirror of a telescope.
aphelion - the point on an object's orbit that is furthest from the sun.
apparent time - time by the real Sun, basically sundial time. Because the Earth's motion is not uniform, the
accumulated error can be as much as 16 min.
apparent magnitude - the observed brightness of an object without any correction for distance.
asterism - a grouping of stars that have no association with each other, except that they appear to be associated.
asteroid - one of thousands of small bodies orbiting the Sun between the orbits of Mars and Jupiter, synonymous with
Minor Planet.
astrometry - the branch of astronomy concerned with the precise measurement of positions and motions of celestial
objects.
astronomical unit (AU) - 1.46x10
11
meters, approximately the average Earth-Sun distance.
autumnal equinox - the point where the Sun crosses the celestial equator going from North to South. This occurs on
approximately September 23.
azimuth - the angle along the celestial horizon intersecting the vertical circle (meridian) containing the object, it's
measured from the South with positive to the West. This is contrary to the Geographical orientation which starts at
North and increases to the East.
binary stars - two stars gravitationally bound about a common center of mass (barycentre).
blackbody - theoretically a body which absorbs all radiation that falls upon it and then re-emits it in its entirety. The
intensity of the emitted radiation as a function of wavelength is solely dependent on it's temperature and is described by
Plank's Law.
black hole - a theoretical body in which the escape velocity is equal to or greater than the speed of light.
B-V - see color index.
celestial equator - the projection of the Earth's equator onto the celestial sphere.
celestial poles - the two points about which the celestial sphere appears to rotate.
celestial sphere - an imaginary sphere with the Earth at it's center onto which all celestial objects are projected.
cepheid variables - stars having a long periodic variation in brightness.
chromatic aberration - an optical defect in which the colors are not all brought to the same focus resulting in
unfocused color halos around the brighter objects.