Monday, December 17, 2007


A second magnitude star called Polaris (north star) provides a reference for measurement in the northern hemisphere. Polaris has no counterpart in the Southern Hemisphere. Polaris may be located in the northern sky between the constellation Ursa Major (big dipper) and Cassiopeia. The two stars in the bowl of the dipper at the greatest distance from the handle, point toward the north star.

Polaris travels in a diurnal circle of small radius around the celestial north pole. This causes a special circumstance in celestial navigation. If you think of the four arguments in celestial navigation, Ho, Lat, Dec, and LHA. Polaris has some constants that make it a special case in celestial navigation. The declination of Polaris is 90 degree's North, and as long as your are in the northern hemisphere, your LHA will be 0 degree's. Therefore, there is only 2 arguments left - Ho and Latitude. For Polaris, the Nautical Almanac has a special (Polaris) table at the end of the white pages.

You can determine your latitude in the Northern Hemisphere by observing the Hs of Polaris, at a known time. From the time, and the DR or estimated longitude, compute the LHA of Aries. Correct Hs to Ho, and using the LHA Aries, approximate latitude, and date, determine corrections from Polaris tables ao, a1, and a2. Add total correction to Ho, and subtract 1 degree to obtain latitude. An good example is given in the table. The reason Polaris is so important is that you can determine your latitude and check your deviation
or gyro error of the compass.