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.
ToolsPolar alignment, Interactive atlas, Celestial fix
Related terms
- 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.
- Polar alignment
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.
- 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.
- Precession
The Earth’s axis slowly sweeps out a cone, like a spinning top, in about 26,000 years. The celestial poles therefore wander among the stars: in about 12,000 years Vega will be near the north pole. Star coordinates slowly change, which is why an epoch such as J2000 is given.
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