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Celestial Coordinates

Geographic coordinates - latitude φ and longitude λ - are integral for determining terrestrial locations. Equatorial coordinates - declination δ and right ascension α - accomplish the same purpose for positioning of the stars on a celestial sphere. Celestial equator plane serves as a base plane for the equatorial coordinates.

Right ascension is counted from the spring equinox in the direction that is opposite to the daily rotation of the celestial sphere. Right ascension is usually measured in angular change per hour, although it is also sometimes measured in degrees.

Declination is measured in degrees, minutes and seconds. Celestial equator divides the celestial sphere into northern and southern hemispheres. Declinations of stars of the northern and southern hemisphere may range from zero to 90? and from 0 to -90?, respectively.

Geographical latitude of the observation point determines the coordinate of the celestial pole above the horizon:

hP = φ.

This model is an illustration of a celestial sphere. Observe the polar axis P1P2, zenith and nadir points Z and Z', the horizon, celestial equator (circumference inclined to the horizon) and the celestial meridian (perpendicular to the equator). Color arrows display the equatorial coordinates of point A. Right ascension α is counted from the spring equinox point γ. Declination δ is counted from the celestial equator.

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