At night, stars appear as bright points of light on a huge overhead dome, which seems to turn westwards, because of the eastward motion of the Earth. So the stars appear to be mounted on a large sphere with the Earth at its center. This has been called the celestial sphere. At the north pole, Polaris, the North star would be overhead. Stars would appear to move in concentric circles, never dipping below the horizon. However, at a latitude of 40 degrees north Polaris would be 40 degrees above the horizon, and only stars within 40 degrees appear to move in concentric circles. At the equator, Polaris would be on the horizon.
Declination (DEC) is the angular measure in degrees north or south of the celestial equator, of a star with respect to the Sun. Its value ranges from zero at the celestial equator to a maximum of 90 degrees north (+) and 90 degrees south(-) of the equator. Right ascension (RA) is the angular measure in hours, minutes, and seconds. It ranges from 0 hour at the celestial prime meridian and continues eastward a full circle to a maximum of 24 hours which point coincides with the prime meridian. The celestial prime meridian is an imaginary half circle running from the North celestial pole to the South celestial pole and crossing perpendicular to the celestial equator at the point of the vernal equinox.
The distances to the stars are measured in parsecs which is the distance to a star when it exhibits a parallax of one second. One parsec equals 3.26 light years or 2.06 x 105 Astronomical Units. The reciprocal of the parallax angle measured in seconds gives the distance in parsecs. For more details of a parsec, click below:
For more details of the celestial sphere, click below.
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