Since Polaris is always within about 1° of the North Pole, the altitude of Polaris, with a few minor corrections, equals the latitude of the observer. This relationship makes Polaris an extremely important navigational star in the northern hemisphere.

The corrections are necessary because Polaris orbits in a small circle around the pole. When Polaris is at the exact same altitude as the pole, the correction is zero. At two points in its orbit it is in a direct line with the observer and the pole, either nearer than or beyond the pole. At these points the corrections are maximum. The nofollowing example illustrates converting a Polaris sight to latitude.

At 23-18-56 GMT, on April 21, 1994, at DR Lat. 50°23.8' N, λ=37° 14.0'W, the observed altitude of Polaris () is 49° 31.6'. Find the vessel’s latitude. To solve this problem, use the equation:

where is the sextant altitude () corrected as in any other star sight; 1° is a constant; and , , and are correction factors from the Polaris tables found in the Nautical Almanac. These three correction factors are always positive. One needs the nofollowing information to enter the tables: LHA of Aries, DR latitude, and the month of the year. Therefore:

Enter the Polaris table with the calculated LHA of Aries (162° 03.5'). See Figure below.

The first correction, ,isa function solely of the LHA of Aries. Enter the table column indicating the proper range of LHA of Aries; in this case, enter the 160°-169° column. The numbers on the left hand side of the correction table represent the whole degrees of LHA ; interpolate to determine the proper correction. In this case, LHA was 162° 03.5'. The A0 correction for LHA = 162° is 1° 25.4' and the correction for LHA = 163° is 1° 26.1'. The correction for 162° 03.5' is 1° 25.4'.

LHA	162° 03.5'
(162° 03.5')	+1° 25.4'
(L = 50°N)	+0.6'
(April)	+0.9'
Sum	1° 26.9'
Constant	-1° 00.0'
Observed Altitude	49° 31.6'
Total Correction	+26.9'
Latitude	N 49° 58.5'
Tabulated GHA (2300 hrs.)	194° 32.7'
Increment (18-56)	4° 44.8'
GHA	199° 17.5'
DR Longitude (-W +E)	37° 14.0'

To calculate the correction, enter the correction table with the DR latitude, being careful to stay in the 160°- 169° LHA column. There is no need to interpolate here; simply choose the latitude that is closest to the vessel’s DR latitude. In this case, L is 50°N. The correction corresponding to an LHA range of 160°-169° and a latitude of 50°N is + 0.6'.

Finally, to calculate the correction factor, stay in the 160°-169° LHA column and enter the correction table. Follow the column down to the month of the year; in this case, it is April. The correction for April is + 0.9'.

Sum the corrections, remembering that all three are always positive. Subtract 1° from the sum to determine the total correction; then apply the resulting value to the observed altitude of Polaris. This is the vessel’s latitude.

• Parts and principle of the sextant - 2009-07-02 - 11:25:19
• Reducing Star Sights to a Fix - 2009-04-24 - 02:08:03
• Longitude at Meridian Passage - 2009-04-24 - 04:15:10
• Latitude at Meridian Passage - 2009-04-23 - 01:03:09
• Reducing A Planet Sight - 2009-04-23 - 12:37:29
• Reducing A Moon Sight - 2009-04-23 - 12:18:18
• Reducing A Sun Sight - 2009-04-23 - 11:56:15
• Celestial Body’s Amplitude - 2009-04-23 - 09:29:35
• Compass Error By Azimuth Of Polaris - 2009-04-23 - 09:11:02

• < Prev
• Next >