Polar alignment, well...aligns a telescope with the Earth's polar axis. The most common method of achieving polar alignment is to put your telescope on an equatorial mount. Why is polar alignment good? First, it allows you to use celestial coordinates to find objects. Second, an equatorial mount equipped with a clock drive will automatically track objects across the sky. This section deals with the fundamentals of polar aligning an equatorial mount. There are separate instructions for German equatorial and equatorial fork mounts. More experienced observers may want to skip ahead to the declination drift method of polar alignment. Declination drift is the method of choice for many astroimagers who need extremely accurate alignment. First-time telescope owners should stay right here. Let's begin by talking about how we see the sky.
|The Celestial Sphere|
The illustration at left represents how the sky looks to someone standing in an open field. The night sky covers the field like a dome. North, South, East and West are marked to get you oriented. The zenith (Z) is the point directly overhead. The broad arrows represent the apparent east-to-west motion of the stars around the horizon. Polaris, the star at the end of the Little Dipper's handle, is visible to the north.
Let's change the perspective a bit. The illustration at right represents the appearance of the night sky with respect to the whole Earth. Our observer is standing in that same field on a planet surrounded by a celestial sphere of stars. A solid line traces the path Earth's equator would follow if extended against the celestial sphere. The North and South poles are also indicated. Notice that the North pole points right at Polaris. That's how Polaris came to be known as the North star.
Our intrepid observer's local horizon is represented by the dashed line. Notice that the dashed line circle is angled with respect to the solid line. That angle is equal to your local latitude. The line going through the observer's body to a point on the celestial sphere directly overhead marks the local zenith. Notice that the angle between the local horizon and Polaris is the same as in the first figure. That angle also equals your local latitude. This is the celestial sphere.
Layout, design & revisions © W. D. Ferris
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Revised: February 11, 2002 [WDF]