Polaris Position Calculator: Polar Scope Clock, UK

Polaris is not at the north celestial pole. It sits about two-thirds of a degree away and circles the true pole once every sidereal day, which is why a polar scope reticle has a ring with a clock face on it rather than a simple crosshair. Pick your town, the date and time, and this works out where Polaris should sit on that ring. The position moves a full twelve "hours" of the reticle clock in under a day, so a reading taken an hour before you set up is already half a reticle hour out.

Why Polaris is not on the crosshair

The north celestial pole is the point the Earth's axis aims at, and everything in the northern sky turns around it. Polaris happens to be the bright star closest to it, but it is not on it: precession of the Earth's axis is still carrying the pole towards Polaris, and the gap this year works out at roughly 37 to 38 arcminutes, a little over the width of the full Moon. For visual observing with a manual mount, pointing the axis at Polaris is close enough. For a motor drive at high power or long-exposure photography, that gap shows up as drift, which is why the polar scope reticle puts a ring at that distance and asks you to place Polaris at the right point on it.

That point depends on the star's hour angle: how far it has turned since it last crossed the meridian above the pole. Hour angle depends on local sidereal time, which runs about four minutes a day faster than clock time and also shifts with your longitude. Between Norwich and Belfast the same clock time gives an hour angle about half an hour different, which moves Polaris a quarter of an hour on the reticle clock.

Reading the result

The reticle clock is not a time of day. It is simply a way of naming a direction round the ring: 12 at the top, 3 to the right, 6 at the bottom, 9 to the left. A full turn of the ring is one sidereal day, so the clock position goes backwards (anticlockwise) by one reticle "hour" roughly every two hours of real time.

• Through a polar scope: most polar scopes on Sky-Watcher and similar EQ mounts and star trackers are small straight-through refractors, which show an image turned through 180°. This is the default view in the calculator and the one to match against your reticle.
• With the naked eye: useful if your app or handset shows the sky the right way round, or if you want to picture where Polaris sits on its little circle tonight.
• Upper and lower transit: at upper transit Polaris is directly above the pole in the sky; at lower transit it is directly below. They make handy reference points: around those times the offset is almost all up or down, so a rough by-eye alignment only needs the altitude adjusted for it.

Mounts and apps do not all draw the reticle the same way up, and some electronic polar scopes or camera-based aligners show a non-inverted view. If your handset or app gives a different answer by exactly six clock hours, it is using the other view; check which once in daylight or against your mount's manual and stick with it.

How accurate is this?

The calculator precesses Polaris's catalogue position to the date you enter and uses the standard formula for Greenwich sidereal time. It leaves out nutation and aberration, which shift the star by a few tens of arcseconds. At Polaris's distance from the pole, that is worth a fraction of a degree round the ring, or a minute or two on the reticle clock, which is finer than you can place a star by eye on an etched circle. Your watch being five minutes out makes a similar difference. For the full step-by-step routine, including the rough alignment you do before using the polar scope, see our guide to polar aligning a telescope.