The celestial sphere

Although the ability to measure distances to celestial objects is a relatively recent development in astronomy, the actual positions of the stars in the night sky have been defined and recorded on maps for thousands of years. Irrespective of whether a star is 1 or 1,000 parsecs from earth, its position can be pinpointed on an imaginary sphere surrounding the earth— the celestial sphere.

The concept of a star-studded celestial sphere was designed to enable the positions of the stars to be located in the same way that positions on earth are defined, in terms of latitude and longitude. The imaginary celestial sphere is divided into a grid system that corresponds directly to the terrestrial grid of lines of latitude and longitude. In the astronomical system, however, the two coordinates are right ascension (abbreviated to RA), equivalent to celestial longitude, and declination, equivalent to celestial latitude. These coordinates are measured with respect to an imaginary celestial equator, which is the extension into space of the earth’s equator. Similarly, the celestial poles are extensions of the earth’s axis and lie, therefore, directly above the earth’s true North and South poles. Thus a star on the celestial equator has a declination of 0°. Just as the earth’s North Pole has a latitude of 90° N, Polaris (the polestar), located almost exactly at the north celestial pole, has a declination of about 90° N.

The ecliptic (an imaginary circle on the celestial sphere) represents the plane of the earth’s orbit around the sun. Because the earth’s axis is inclined at an angle of 23.44° to the plane of the ecliptic, the celestial equator is also inclined by 23.44° to the ecliptic. The celestial equator and the ecliptic meet at two opposite points, which mark the equinoxes. The sun rises at one of these points (the vernal, or spring, equinox, also called the First Point of Aries), on March 20 or 21, and at the other (the autumnal equinox, or the First Point of Libra) on September 22 or 23.

Just as the terrestrial longitude system needs a zero point, so does the celestial sphere, from which all measurements of right ascension can be made. The earth’s was arbitrarily chosen, and internationally agreed upon in 1884, to be the Greenwich meridian. The celestial sphere’s point was defined—in a similarly arbitrary way—by the ancient Greeks, who chose the vernal equinox as the zero point of right ascension, a system that has remained unchanged to this day.

Instead of measuring right ascension in degrees, it is usually expressed in terms of hours, minutes, and seconds, so that the position of a star is related to its apparent motion across the sky. Thus, the imaginary lines of right ascension on the celestial sphere are spaced at intervals of one hour.
The declination of a heavenly body is measured in degrees north (designated by a plus sign) or south (designated by a minus sign) of the celestial equator—in the same way that latitude on earth is measured relative to the terrestrial equator.

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