A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2019 Sep 25, 21:23 -0700
Peter Hakel, you wrote:
"Bracketing the moment when Sun's declination was changing from North to South in 2019"
[7:49:58 and 7:49:59 UT]
This is interesting to know, of course, but it's not quite the formal astronomical definition of the instant of the equinox. We can a bit closer to the definition by looking at SHA. Can you find that second of time when the Sun's SHA was 180°? How much does it differ this year from the instant when the Sun's center was directly above the equator (Dec=0)? How much next year?
For navigators, or indeed for anyone with any practical concerns, these numbers are, of course, mere trivia. Around the equinoxes the Sun is moving North/South at a rate of just about 1 knot. This means that the Sun's declination is changing by 1 minute of arc per hour. If we divide by sixty, that's one second of arc change in Dec per minute of time. And dividing again by sixty, that's one "third" of arc (about 17 milli-arcseconds) in one second of time. Since navigators never worry about anything smaller than a tenth of a minute of arc (and even that's usually over-kill), the timing of the equinox needs to be known only within a few minutes at best for celestial navigation concerns.