NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2022 Oct 22, 18:45 -0700
Ed Popko, you wrote:
"I'm looking for a reliable source of the daily value of the Equation of Time (or twice daily, better yet)."
You attended my "Celestial Navigation in the Age of Sail" workshop at some point, didn't you? The daily almanac data was Sun's Equation of Time and Declination. The tables repeat nearly enough every four years. Note that it's a nap to calculate the Equation of Time from standard GHA data. Let's take the time right now. By the time this message distributes it should be 10:00pm Eastern US time on 22 October which is 0200 GMT/UT on 23 October. Visiting my web app for Nautical Almanac data, I find that the Sun's GHA at 0200 UT is 213.9041°. Of course the Mean Sun that we're comparing against starts at 180° longitude at midnight and advances at exactly 15° per hour. So if the Sun kept good time, its longitude would be (that is, the GHA of the Mean Sun would be) 210°. The actual GHA of the Sun is ahead of the Mean Sun by 3.9041°. We convert that to time at the usual (solar) rate of 15° per hour or 240 seconds per degree. That gives 937 seconds or 15m37s. The Sun is fast right now by that much.
Frank Reed
