The adjustment that points the polar axis of an equatorial mount at the celestial pole. Its angle above the horizon must equal your latitude (51.5° in London), and it must aim at true north, not magnetic north. Good polar alignment keeps stars from drifting, visually and in photos.
ToolsPolar alignment
Related terms
- Equatorial mount
A mount with one axis (the right ascension, or polar, axis) parallel to the Earth’s axis of rotation. Once polar aligned, turning that single axis follows the stars, by hand or with a motor. The “German” type, with counterweights, is the most common.
- Polar scope
A small scope built into the polar axis of an equatorial mount, with a reticle. Since Polaris is about 0.65° from the pole, you place it at a precise spot on the reticle that depends on the time. An app or calculator tells you where.
- Celestial poles
The two points in the sky around which all stars seem to turn: the extension of the Earth’s axis. The north celestial pole lies very close to Polaris; the south pole has no bright star. The pole’s height above the horizon equals your latitude.
- Drift alignment
A very accurate polar alignment method that watches stars drift in declination. You adjust the axis in azimuth with a star near the meridian and the equator, then in altitude with a star low in the east or west. It takes patience but no polar scope.
- Polaris
Alpha Ursae Minoris, a magnitude 2 star about 0.65° from the north celestial pole. It barely seems to move and marks north to within a degree. Find it by extending the front edge of the Plough (Big Dipper) by about five times its length.
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