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Re: Equinoxes: when Lon=0 (or 180) but not when Dec=0?
From: Paul Hirose
Date: 2017 Aug 3, 12:45 -0700
From: Paul Hirose
Date: 2017 Aug 3, 12:45 -0700
On 2017-08-01 19:18, Lu Abel wrote: > The Wikipedia article on the annalemma clearly labels the equatorial crossings as defining the equinoxes, not the meridian crossings. At the scale of the illustration, the Sun's declination is indeed zero at the equinoxes. But according to the formal definition, the equinoxes occur when the Sun's ecliptic longitude is 0 or 180°. E.g., see the Astronomical Almanac glossary: http://asa.usno.navy.mil/SecM/Glossary.html#equinox To be on the celestial equator at the time of an equinox, the Sun must be exactly on the ecliptic. That's virtually impossible since the Sun has a monthly oscillation of about one second of arc either side of the ecliptic.
