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Re: When is the Autumnal Equinox?
From: Paul Hirose
Date: 2011 Sep 07, 16:15 -0700
From: Paul Hirose
Date: 2011 Sep 07, 16:15 -0700
Geoffrey Kolbe wrote: >> The equinoxes and solstices are the moments when the geocentric >> apparent Sun reaches 0, 90, 180, and 270 degrees of ecliptical >> longitude. See The Astronomical Almanac glossary: >> http://asa.usno.navy.mil/SecM/Section_M.html > > > If this is the case, how to the equinoxes precess? I'm not sure what Geoffrey is asking. I'm guessing he sees a contradiction between the above definition of "equinox" and the precession of the equinoxes. Actually, the two have almost nothing to do with each other. "Vernal equinox" can mean (1) the time when the Sun passes through a specified coordinate. But it can also mean (2) the point on the celestial sphere where the ecliptic and the celestial equator intersect, and which the Sun is near at the time of the vernal equinox. "Precession of the equinoxes" refers the regular movement of (2). Due mainly to the precession of Earth's rotation axis, the vernal equinox (2) makes an approximate great circle in the sky every 26,000 years. And since this point is the "Greenwich" of the equatorial (right ascension and declination) and ecliptical (longitude and latitude) reference systems, the coordinates of a body with respect to the equinox "of date" change continuously. But this phenomenon occurs regardless of the definition of "equinox" (1). I can understand a reluctance to accept a definition of (1) based on longitude. It's practically unknown. A few years ago I was irked and disbelieving when someone corrected a "precise" equinox time I had computed. But upon consulting the references I most respect, the Almanac and the Explanatory Supplement, I had to admit I'd just learned something new. I'm trying to pass it on. As further evidence I offer the astronomical data sheets from HM Nautical Almanac Office: http://astro.ukho.gov.uk/nao/online/index.html#calendar Here are their autumnal equinox times for the last 10 years. The times are UT. I've added the Sun's geocentric apparent declination and longitude. (Because the longitudes are all very close to 180°, I show only the seconds.) date time dec lon 2002-09-23 04:55 -00.1" 59.1" 2003-09-23 10:47 +00.2" 00.5" 2004-09-22 16:30 -00.7" 00.4" 2005-09-22 22:23 +00.5" 59.6" 2006-09-23 04:03 +00.2" 59.1" 2007-09-23 09:51 +00.2" 59.5" 2008-09-22 15:44 +00.9" 58.8" 2009-09-22 21:19 -00.7" 01.0" 2010-09-23 03:09 +00.9" 59.9" 2011-09-23 09:05 -00.7" 00.9" A little experimentation shows that 5 of those times are in error by a minute, if equinox is defined by declination. But all are correct if equinox is defined by longitude. The thought crossed my mind that perhaps the table was based on the time the Sun crossed the geodetic equator, not the celestial equator. So I recomputed the last 5 years with polar motion (daily values are in IERS Bulletin B). Well, three years got a little better, one stayed the same, and one got a lot worse. So much for that hypothesis. It didn't make much sense, but I had to be sure. --