Principles of Celestial Navigation

The Intercept Method » Observed Altitude (Ho) » Sight Reduction Process

Navy photo of Ensign Jeff Bland using a sextant, GULF OF ADEN (Sept. 21, 2012)

The sight reduction applies each of the corrections to the sextant altitude (Hs) to obtain the observed altitude (Ho). The first correction is the index correction, unique to the sextant being used.

The second correction depends on the observer’s elevation above sea level, and is called the dip correction because the higher we are, the lower—or more dipped—the distant horizon appears. The dip correction accounts for this visual difference. It is in a pre-computed table in the inside front cover of The Nautical Almanac. Lastly, we apply altitude corrections, which are essentially corrections due to Earth's atmosphere bending light rays. These are in pre-computed tables in The Nautical Almanac. For the Moon, Venus, Mars, or if the temperature is extreme, we’ll apply additional corrections.

Watch and listen as Chief Sheedy demonstrates the sight reduction process for an 0800 UT sight of the star Capella.

Chief Sheedy: I’m now going to demonstrate the sight reduction process. In this example we’re going to use the star Capella. The first thing you want to do is have a sextant and a good watch. I like to bring a recorder out with me to record the time and mark my observation.

I will grab my sextant, take my observation, reduce the star down to the horizon, rock it, and then mark the time. In this case, it’s 0800 GMT (Greenwich Mean Time or Universal Time). The observation reads 29 degrees, 46 minutes 0 seconds.

We now have enough information to start our strip form. In this example, the date is 17 July 2018. The body is Capella. Greenwich Mean Time is 0800, 00 seconds. Index correction is 0.4 on. Our dip we get from the Nautical Almanac because our height of eye is 10 feet. Dip for height of eye is 10 feet so we have to interpolate between 9.8 feet and 10.5 feet: -3.1. The sum of the index correction and the dip is -3.5. Our sextant observation was 29 degrees 46 minutes. Our adjusted altitude is 29 degrees 42.5. We go to the Nautical Almanac to Stars and Planet, Apparent Altitude and follow it down. It falls between 28 degrees 54 minutes and 30 degrees 22 minutes. We go to the right to the actual correction, -1.7. Once we apply all the corrections—altitude corrections, dip, and index correction, we now have our Ho, or Height observed: 29 degrees 40 minutes decimal 8.

partially completed strip form for three-star morning fix, showing the sight reduction values for Capella