A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2017 Jun 15, 11:28 -0700
That's right: you need to use your actual longitude converted to time to get the offset of solar Local Mean Time from UT (a.k.a. Greenwich Mean Time). At my current location, the longitude is 71°30' West. Converted to time at the usual rate of 4 minutes per degree, that's a time offset of 4:46:00. So when it's 16:46:00 UT, the local mean time is 12:00:00. Of course, there's one more wrinkle if you want to use this for a compass: the Equation of Time. Today it only amounts to about 37 seconds (June 13 was a "zero day" for EqT this year), but with the Sun's altitude high enough, that can make a small difference in compass direction. And by July 1, the Equation of Time will be as much as four minutes and that will make a noticeable difference even at your latitude. For a sanity check on these things, just double-check the Sun's GHA (for example, using the USNO web app: http://aa.usno.navy.mil/data/docs/celnavtable.php). When the Sun's GHA is exactly equal to your longitude, the Sun's azimuth is necessarily 0.0° or 180.0° exactly. There's no correction for the equation of time required for the GHA since, in effect, it's already in there. That was one of the benefits of the switch to GHA that began in the 1930s. It hid the equation of time from the navigator for nearly all work, burying it in the GHA.