Enter the time and sextant altitude of a Polaris sight and your DR longitude. You get your latitude, with the a0, a1 and a2 corrections shown.

Latitude from the Pole Star

Polaris sits less than a degree from the celestial pole, so its altitude is close to your latitude. Close, but not equal: it circles the pole once a day, so its altitude can be up to about 40′ more or less than your latitude, depending on where it is on that circle.

The Nautical Almanac's Polaris tables turn the altitude into a latitude with three small corrections: a0, the main one, from the LHA of Aries (which says where Polaris is on its circle); a1, from your latitude; and a2, from the month. Each has a constant added so it is never negative, and the three constants add up to exactly 1°, which is why the formula subtracts 1°: Latitude = Ho − 1° + a0 + a1 + a2.

Polaris is only visible from the Northern Hemisphere, and it is not bright, so shoot it at twilight when the horizon is still sharp. Set the sextant to your DR latitude and it will be near the horizon mirror when you look north.

It is a learning aid, not a navigation tool: at sea, use the official Nautical Almanac and tables, and check every position by other means.

Learn to do it yourself

Latitude from Polaris: Aries, the Polaris tables and practice.
First point of Aries and SHA: how the almanac places the stars.
Star finder: where Polaris and the other stars will be at twilight.