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.
ToolsPolar alignment
Related terms
- 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.
- 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.
- Hour angle
The time since an object crossed the meridian, in hours: hour angle = local sidereal time − right ascension. It is 0h on the meridian and grows towards the west; an object at 2h crossed the meridian two hours ago. Polar scopes and equatorial mounts rely on it.
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