Blue Planet Project

Coordinates in the Sky

With this article, I hope you will learn how astronomers measure positions of stars in the sky. It's a lot like measuring longitude and latitude on Earth, but of course there's a bit of complication in that the sky turns overhead. Knowing where the imaginary lines are can add interest to your observing; you'll be able to look up and "see" these imaginary lines among the stars.

Positions in the sky are given on a grid of lines of Declination and lines of Right Ascension. These correspond to the Earthly measures of latitude and longitude, respectively. Lines of latitude on Earth, measuring degrees North and South from the Equator, are fairly easy to establish. All one need do is measure the angle to the North Celestial Pole, near Polaris. Lines of Declination in the sky correspond to the latitude lines.

Lines of longitude, which connect the north and south poles, mark the earthly East-West measure in degrees, but really measure time. This is much is more difficult to gauge than is latitude - there are no east and west poles. Longitude has to start from an arbitrary North-South "zero-hour" line upon which nations must agree. Once this "prime meridian" is established, the trouble really begins. The trick is to start at the prime meridian with a really accurate watch and a refined knowledge of the stars, then head out. Observe stars carefully as you travel, using the watch to time when they reach their highest point. These times will be early or late relative to their times at the prime meridian, and the time differences will tell you your longitude. Since the Earth turns a full 360-degrees circle in a 24 hour day, the sky must turn through 15 degrees in one hour. If the stars are passing overhead an hour late, you are 15 degrees west of the prime meridian. In the sky, separation from the zero meridian is measured in hours of Right Ascension, not in degrees. Since the sky rotates overhead, the lines of Right Ascension do not stay put over any corresponding line of Longitude. Imagine a star passing its highest point right now. In an hour, it will have moved to the West, and some new stars will be passing their highest points; these stars are said to be one hour of Right Ascension later than the original star.

Here’s a guide to observing Right Ascension (RA). All lines of RA meet at Polaris (nearly). Zero hours runs through the star in Andromeda’s head and lies along that edge of the Great Square; 6 hours is at Orion’s easterly shoulder; 12 hours is near the tip of the Lion’s tail and the top of Virgo’s head; 18 hours touches the tip of the Sagittarius teapot, and passes very near to Vega. These constellations are shown on the map.


To learn more about the science, the scientists, and the politicians behind measuring Earthly longitude and latitude, read Longitude, by Dava Sobel, and The Measure of All Things, by Ken Alder.