Polar scope clock

How to use it

1. Level the mount, set the latitude scale roughly, and point the RA axis roughly at the pole.
2. Turn the RA axis until the 12 o'clock mark of the reticle is at the top as you look through the scope.
3. Read the dial reading above. Move the mount with its altitude and azimuth bolts, not the RA or Dec axes, until the star sits on the circle at that clock position.
4. Lock the bolts. The reading moves slowly, so check it again a few minutes later.

New to this? How to read a polar scope reticle.

What the numbers mean

• Dial reading is the star's angle around the pole on a 12 hour clock face, 12 at the top, in the view you picked.
• Hour angle is how long ago the star crossed the meridian above the pole, in hours and minutes of sidereal time.
• Next upper transit is the time until the star is next directly above the pole, which is the 12 o'clock position in the naked eye view.
• Sigma Octantis is used for any latitude below zero, because there is no bright star at the south celestial pole. It is faint, around magnitude 5.4, so you need a dark site.

How accurate is it?

The positions include proper motion, annual aberration, precession and nutation, and the sidereal time comes from your clock. We ran the same code against the astropy library on 260 cases between 2026 and 2035, 130 in each hemisphere. The largest hour angle difference was 0.07 arcminute, under the 0.1 arcminute we aim for. Atmospheric refraction is not applied. Your device clock has to be right: one second of clock error is 0.25 arcminute of hour angle.