Astronomical Definitions


Altitude  For any object in the sky, the angular measure of its position above the horizon 
Astronomical Unit  Mean distance from the Earth to the Sun. Numerically equal to 93 million miles, or about 1.5 x 1011
Azimuth  For any object in the sky, the angular measure of its position east of the northern point on the horizon 
Celestial Coordinates  Declination and Right Ascension 
Celestial Equator  The circle around the Celestial Sphere midway between the two poles. Lies directly above the Earth's equator 
Celestial Pole  One of two points on the Celestial Sphere which do not appear to move during the course of one day. Lies directly above the Earth's pole 
Circumpolar  Any star which completes a full circle around the Celestial Pole during the course of a day, without setting 
Cluster  A small (commonly a few thousand) cluster of stars which orbit around a galaxy 
Crescent Moon  The Moon when viewed such that we see less than half of the illuminated potion. It corresponds to an elongation between 0 and 90o
Declination  For a point on the Celestial Sphere, the angular measure of the position north or south of the Celestial Equator 
Density The ratio mass/volume
Ecliptic Either - the plane of the Earth's orbit around the Sun
Or - the path of the Sun across the sky
Elongation  For any object in the sky, its angle from the Sun 
Equinox  Equal night. Occurring once in spring and once in fall the equinox days have 12 hours daylight and 12 hours night 
Full Moon  The Moon when viewed such that we see the illuminated half. It corresponds to an elongation of 180o
Galaxy  A large (commonly of the order of 100 billion) collection of stars 
Gibbous Moon  The Moon when viewed such that we see more than half of the illuminated potion. It corresponds to an elongation between 90o and 180o
Latitude  For a point on the Earth's surface, the angular measure of the position north or south of the equator 
Light year  Distance that light travels in one year. Numerically equal to 5.88 x 1012 miles, or about 9.46 x 1015
Local Coordinates  Altitude and Azimuth 
Longitude  For a point on the Earth's surface, the angular measure of the position east or west of the Prime Meridian 
Mass The amount of material making up an object
Meridian  A line drawn from the North Pole to the South Pole 
Nadir  The point on the Celestial Sphere which is directly below you. It declination is equal to -(your latitude) 
Nebula  A large interstellar cloud made of gas or dust 
New Moon  The Moon when viewed such that we see the unilluminated half. It corresponds to an elongation of 0o
Prime Meridian  The Meridian which corresponds to 0o longitude. 
Quarter Moon  The Moon when viewed such that we see half of the illuminated potion. It corresponds to an elongation of 90o E (first quarter) or 90o W (third quarter) 
Reflecting telescope A telescope which uses a mirror for focussing light
Refracting telescope A telescope which uses lenses for focussing light
Right Ascension  For a point on the Celestial Sphere, the angular measure of the position east of the vernal equinox 
Sidereal Day  The time for the Earth to rotate on its axis exactly once, relative to the stars. Numerically equal to 23 h 56 m 
Sidereal Month  The time for the Moon to orbit the Earth once, as measured relative to the stars. Numerically equal to 27.3 days 
Solar Day  The time form noon one day to noon the next day. Defines the 24 hour period 
Solstice  Sun stopped. Occurs once in summer when the noontime sun is at its highest position in the sky, and in winter when the noontime Sun is at its lowest position in the sky. 
Synodic Month  The time one phase of the Moon (e.g. a full Moon) to the next time the same phase occurs. Numerically equal to 29.6 days 
Volume The amount of space an object occupies
Waning  Those phases of the Moon when the portion that is illuminated decreases night by night. All waning phases have an elongation to the west 
Waxing  Those phases of the Moon when the portion that is illuminated increases night by night. All waning phases have an elongation to the east 
Weight The force of gravity
Zenith  The point on the Celestial Sphere which is directly above you. It declination is equal to your latitude 


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