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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Equinox, 2019 September
From: Paul Hirose
Date: 2019 Sep 26, 20:26 -0700
From: Paul Hirose
Date: 2019 Sep 26, 20:26 -0700
The four different "equinox" times (the third one is the formally correct equinox): 07:49:53 Sun declination = 0 07:50:04 ITRS latitude = 0 07:50:11 ecliptic longitude = 0 07:50:14 right ascension = 12 h Times are UTC on 2019-09-23. computational basis: JPL DE431 ephemeris IAU 2006 precession IAU 2000B nutation 0.2029 0.3185 polar motion x, y (arc sec) If the Sun were exactly on the ecliptic, the first, third, and fourth events would be simultaneous. If not for polar motion, declination and ITRS latitude would be zero at the same time. But Earth's axis of rotation (the celestial pole), which is the basis of declination, is offset a few tenths of an arc second from the geodetic (ITRS / WGS84) north pole. The polar motion parameters I used are from IERS Bulletin A. The numbers constantly change since the celestial pole follows a quasi circular path with respect to the crust. One circle takes about 400 days and the circle is about 20 feet in diameter. It is not centered on the geodetic north pole, which is well outside the circle.
