Understanding the Celestial Coordinate System

The illustration at left diagrams Earth's coordinate system of latitude and longitude. The equator is marked by a great circle around the planet's waist. This is also our reference point for latitude measurements. Latitude measures distance from the equator to each pole. The equator is at 0 degrees and each pole is at 90 degrees latitude. The lines extending from pole-to-pole indicate longitude. Longitude measures distance around the equator. There are 360 degrees of longitude. This grid work system of lines is fixed against the planet. Despite the fact that Earth moves, your hometown has the same latitude/longitude coordinates today as the day you were born.

Astronomers borrowed from the Earth system to build a framework for the coordinate system used to locate stars and planets in the night sky. This astronomical framework is called the celestial coordinate system. It is illustrated at right. The lines of latitude and longitude have been extended outward against the celestial sphere. The Earth's equator becomes the celestial equator. Latitude is now called declination. There are 90 degrees of declination from the celestial equator to each celestial pole. And guess what, Polaris sits smack dab at the location of the celestial north pole. Longitude is now called right ascension. Astronomers don't measure right ascension in degrees but in hours. There are 24 hours of right ascension.

The celestial coordinate system is fixed against the celestial sphere. Just like your hometown, Polaris and the other bright stars have essentially the same coordinate locations today as they did the day you were born. But as Galileo figured out, the Earth moves. As a result, your earthbound telescope is constantly moving with respect to the night sky. The stars star at the same coordinates throughout the night but we're constantly moving.

Polar alignment allows your telescope to track celestial objects in their apparent east-to-west motion across the sky. How? Consider why the stars appear to move. They follow that east-west path because Earth rotates about an axis. If you could align your telescope with that axis and move the telescope in the opposite direction at the exact same rate as Earth's rotation, then your telescope would stay fixed on whichever star or planet you want to observe. Polar alignment takes care of the first part of that challenge. And it's not that tough to do.

Whether you own a telescope with German equatorial mount (GEM) or an equatorial fork mount, you can spend as little as five minutes on polar alignment and achieve good tracking at high magnification. Click on one of the buttons above for instructions on how to polar align your telescope. Or, if you need extremely accurate alignment, click on the right arrow below to get to my instructions for the declination drift method.