Sky Coordinates

From "Words Ya Gotta Know"

By Alan M. MacRobert

Seen from Earth, the night sky looks like a huge dome with stars stuck on its inside surface. If the Earth beneath us vanished, we'd see stars all around us — and we'd have the breathtaking sensation of hanging at the center of an immense, star-speckled sphere.

Astronomers designate the positions of stars by where they are on this celestial sphere.

Picture the Earth hanging at the center of the celestial sphere. Imagine the Earth's latitude and longitude lines expanding outward and printing themselves on the celestial sphere's inside. They now provide a coordinate grid on the sky that tells the position of any star, just as latitude and longitude tell the position of any point on Earth. In the sky, "latitude" is called declination and "longitude" is called right ascension.

Declination is expressed in degrees, arcminutes, and arcseconds north (+) or south (–) of the celestial equator.

Right ascension is expressed not in degrees but in hours (h), minutes (m), and seconds (s) of time, from 0 to 24 hours. Astronomers set up this arrangement long ago because the Earth completes one turn in about 24 hours, so the celestial sphere appears to take about 24 hours to complete one turn around Earth.

There's a slight complication. A star's celestial coordinates gradually change over the years, due to a slow shift of the Earth’s orientation in space called precession. When right ascension and declination are given in books and atlases, you might see them accompanied with a year date such as 2000.0. (The ".0" means the beginning of the year.) This is the moment for which the coordinates are strictly correct. For most amateur purposes this refinement is too small to matter.

Alan MacRobert is a senior editor of Sky & Telescope magazine and an avid backyard astronomer.

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