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May 9, 2012

Celestial Navigation Primer

Filed under: Navigating — Richard Jordan @ 12:00 am

My Sextant

Getting your latitude by celestial navigation can be actually relatively easy. It also can be totally overwhelming if you read the incredibly obtuse chapter 8 of Bowditch. My advise when learning is to stick to a few basics as outlined below.

Equipment

  • Sextant – Any sextant will do including a plastic one from Davis instruments.
  • Current Nautical Almanac – Every year a new version is published with the pertinent numbers for making simple and complex corrections to your sights. You can download in PDF form for free online.
  • Artificial Horizon – While unnecessary for daylight sighting, an artificial horizon is critical for accurate Polaris or Moon sights.

0. Preparation Calculations
All sights are subject to corrections for index error and height of above water (dip). Make sure to line up and note the index error on your sextent prior to use. Consult online for a table for dip correction values. These numbers are unlikely to change.

1. Noon Sights
Taking a noon sight is the simplest and most rewarding way to get a lock on your position. To complete a noon sight, I start at 11am and sporadically shoot the altitude of the sun with my sextant. I usually get a generally idea of when local noon is on my first day at sea by noting when the altitude starts decreasing. On the second day, I am more methodical. I start a half hour before the expected local noon, take shots every 15 minutes, and jot down the altitude at each reading. Once I have all my data, I look up in the nautical almanac the declination of the sun at noon on the current day. Finally I subtract my reading at noon from 90 degrees, then add or subtract the declination correction to get my latitude. I have not memorized the add / subtract rule, but it is easy to guess using common sense whether, if the altitude = 70 and declination = 10, the latitude should be 90 – 70 – 10 = 10 or 90 – 70 + 10 = 30. Adjust for index and dip corrections.

2. Polaris Sights
Finding Polaris may be the most difficult part of this reading because your altitude measurement of Polaris is your latitude! Look for the big dipper and check a map of the stars. Polaris is nearly exactly above the north pole, so there is no need for correction for our purposes. You will however need your artificial horizon as it is near impossible to gauge the horizon at night offshore even with a full moon to light up the ocean. Adjust for index and dip corrections.

Conclusion
After you get these two methods down for latitude, you can move on to moon sights and figuring your longitude from the noon sight readings. Celestial navigation is pretty pointless with GPS, but if you are mathematically inclined, it is an amazing opportunity to see the practical use of mankind’s genius.

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