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Re: Today’s the Spring Equinox
From: Paul Hirose
Date: 2019 Mar 22, 22:52 -0700
From: Paul Hirose
Date: 2019 Mar 22, 22:52 -0700
The IMCCE says the equinox occured at March 20 21:58:27.05 UTC: "La date de l'équinoxe de printemps est, en 2019, le mercredi 20 mars à 21h 58m 27,05s soit le mercredi 20 mars à 22h 58m 27,05s heure légale française (UTC + 1h)." They explain that the equinox (zero ecliptical longitude), zero declination, and zero right ascension occur at three different times: "À cet instant, la latitude apparente géocentrique du centre du Soleil est de 0,72", son ascension droite est de 23h 59m 59,981s et sa déclinaison est de 0,66". Comme on le constate, ces valeurs sont toutes très proches de zéro. C'est pourquoi on dit souvent que le centre du Soleil est dans la direction du point gamma. Néanmoins pour un calcul à la seconde de temps près, le choix de la définition est important, en effet la déclinaison du centre du Soleil est nulle à 21h 57m 46,69s UTC et l'ascension droite du centre du Soleil est nulle à 21h 58m 34,64s UTC." https://www.imcce.fr/lettre-information/archives/154 I have never before seen an equinox computed to .01 second of time. Unfortunately, there is some disagreement on the geocentric apparent ecliptical longitude and latitude of the Sun at 21:58:27.05 UTC: .00″ .72″ IMCCE article .00″ .70″ JPL Horizons .11″ .70″ Miriade (IMPOP ephemeris) .05″ .68″ me JPL Horizons uses the DE431 ephemeris, IAU 1976/80 precession / nutation, plus nutation corrections published by the IERS. I use the same ephemeris and IAU 2006/00B precession / nutation, which yield a pole within 1 mas of the true pole and therefore virtually identical to the Horizons pole. Nevertheless, the coordinates don't coincide because the different precession / nutation models yield different equinoxes. (Here I use "equinox" as the point on the celestial sphere where right ascension, declination, ecliptical longitude, and latitude are all zero.) Miriade is the IMCCE equivalent of JPL Horizons. The table above shows it doesn't agree with the article on the equinox. Since the Sun moves about .04″ of longitude per second of time, the equinox time discrepancy between the two different IMCCE sources is about 3 seconds! http://vo.imcce.fr/webservices/miriade/?forms If anyone can replicate the top line in the table, I'd like to know how you do it.
