A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
From: Paul Hirose
Date: 2018 Jun 23, 13:24 -0700
On 2018-06-21 14:15, Bill B wrote: > Recalling Herbert Prinz's March 2006 explanation of how the equinox time > to the second is determined relative to the two fundamental references, > how is determining the exact time of solstice rigorously defined? According to the formal definition, a solstice occurs when the geocentric apparent ecliptic longitude of the Sun is 90° or 270°. (That's identical to 6h or 18h right ascension.) See the Astronomical Almanac glossary: http://asa.usno.navy.mil/SecM/Glossary.html#solstice On 2018-06-21 23:45, Geoffrey Kolbe wrote: > So, the first point of Aries (longitude zero) is defined as the longitude of the sun as it crosses the equator at the March equinox. Actually, the zero point on the ecliptic is the intersection with the celestial equator *near* where the Sun crosses the equator in March. Since the Sun oscillates several tenths of a second north and south of the ecliptic on a monthly cycle, it doesn't pass exactly through that point. Therefore, it's at zero longitude, zero declination, and zero right ascension at three different times.