To locate a point on Earth we use latitude and longitude on earth. On Earth the latitude measures the distance between a point and the north and south pole, while the longitude is the distance from east to west, compared to a reference meridian.
The celestial coordinate system works the same way, but the latitude and longitude are called, declination and right ascension. To locate the position of the stars on the celestial sphere, we so use the declination and right ascension. These coordinates are the extension of the latitude and longitude of the earth, projected into the sky, they are used either in equatorial coordinates, or coordinate schedules. Declination is the angle measured on a circle time between a point on the celestial sphere and the celestial equator. As the terrestrial latitude, it is expressed in degrees (°), minutes (') and seconds (") arc, positive north and negative south of the celestial equator. Just as the longitude of a point on Earth measure the angle between the meridian of that place and a reference meridian, the right ascension of a star in the sky, measuring the angle between the hour circle of the star and circle time reference. The zero point on Earth is the intersection of the Greenwich meridian and the equator is the point of origin for terrestrial longitude. The point of origin for right ascension is called the vernal equinox. This is one of two points where the celestial equator and the ecliptic intersect.
The sun moves through these two points of intersection defines the two equinoxes. The vernal equinox is the equinox in March (early spring in the northern hemisphere, autumn in the southern hemisphere).
Right ascension is measured as an angle expressed in hours (H), minutes (M), second. Sidereal time, one hour is 15 degrees (360 ° / 24 = 15 °).
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Image: The vernal equinox is the equinox in March (early spring in the northern hemisphere, autumn in the southern hemisphere).