Some Terms You Should Know: the Celestial Sphere

These are some terms you will come across in your lab class dealing with the starglobe and locating celestial objects.
Altitude
The height, in degrees, of a star's position with respect to the horizon along a line running from the horizon to the zenith. The coordinates of a star can be given in Azimuth and Altitude. These numbers will change as the star rises and sets.  Go here for more information.
Autumnal Equinoctal (or Fall Equinoctal)
The celestial meridian (an imaginary line) that is overhead at noon on the first day of autumn. Stars on the Autumnal Equinoctal have a Right Ascension of 12 hours.
Azimuth
The angle, in degrees, of a star's position around the horizon clockwise from due north. A star due east has an azimuth of 90 degrees. The coordinates of a star can be given in Azimuth and Altitude. These numbers will change as the star rises and sets.  Go here for more information.
Celestial Sphere
An imaginary, transparent sphere with the Earth at it's center; all of the stars are pictured as being on this sphere. This is what you see on the star globe.
Celestial Pole
The axis around which the celestial sphere rotates. As the Earth rotates, the stars appear to circle the celestial pole.
Celestial Equator
The imaginary line on the celestial sphere directly above the Earth's equator halfway between the north and south clestial poles.
Declination
The angle, in degrees, of a star's position above or below the celestial equator. (Declination is negative for a star below the celestial equator.) A star on the celestial equator has a declination of 0 degrees; a star at the north celestial pole has a declination of 90 degrees. The coordinates of a star can be given in Right Ascension and Declination. These numbers are with respect to the celestial sphere and do not change.  Go here for more information.
Ecliptic
The plane of the Earth's orbit around the sun. On a star map or star globe, the apparent path of the sun across the celestial sphere.
Equator
The imaginary line around the center of the Earth halfway between the north and south poles.
Latitude
The angle, in degrees, of a position on Earth above the equator. (Latitude is negative for a place below the equator.) Points on the Earth can be located by Latitude and Longitude.
Longitude
The angle, in degrees, of a position on Earth around the equator from the Greenwich Meridian. Points on the Earth can be located by Latitude and Longitude.
Meridian
An imaginary line in the celestial sphere that goes from the north celestial pole through the zenith to the south celestial pole. The north-south line directly overhead.
Right Ascension
The angle, in hours, of a star's position around the celestial equator with respect to the Vernal Equinoctal (an imaginary line on the celestial sphere). The coordinates of a star can be given in Right Ascension and Declination. These numbers are with respect to the celestial sphere and do not change.  Go here for more information.
Sidereal Day and Sidereal Time
The day and time measured with respect to the stars.  Go here for more information.
Solar Day and Solar Time
The day and time measured with repect to the sun.  Go here for more information.
Universal Time
The solar time at a longitude of 0 degrees. Often called Greenwich Mean Time, because the British Royal Observatory in Greenwich sits on 0 degrees longitude.
Vernal Equinoctal (or Spring Equinoctal)
The celestial meridian (an imaginary line) that is overhead at noon on the first day of spring. Stars on the Vernal Equinoctal have a Right Ascension of 0 hours.
Zenith
The point in the sky directly above the observer.

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 This page was last updated on 18 January 1998
and is © 1998 by Jim McDonald.