# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Winter solstice this weekend**

**From:**Paul Hirose

**Date:**2014 Dec 19, 23:56 -0800

According to the formal definition, solstice occurs when the Sun's apparent ecliptic longitude is 90° or 270°. http://asa.usno.navy.mil/SecM/Glossary.html#_S Longitude increases at a nearly constant rate of 1° per day, or 1 arcsecond per 24 seconds of time. Thus it yields a more definite time than you'd get from declination stand-still. Similarly, an equinox is when the Sun's longitude is exactly 0° or 180°. Its declination would be zero at these times if the Sun were exactly on the ecliptic. But in general it's not, so normally the Sun is slightly displaced from the equator at an equinox. For example, I calculate the winter solstice at December 21 23:04:09 Terrestrial Time, or 23:03:02 UTC. JPL HORIZONS agrees within about 36 milliseconds of arc. Possibly the discrepancy is due to different ephemerides or precession / nutation models. Or I blundered a little with the JPL DE422 ephemeris and IAU 2006/00A model. At that time, longitude and R.A. are equal. I believe this is always the case at a solstice, because the applicable longitude and R.A. meridians coincide. It's not the case at the equinoxes, where the meridians are skewed by the obliquity of the ecliptic (ca. 23.5°). This would make no difference if the Sun were exactly on the ecliptic, but as I said above, generally it's not. For instance, I put the autumnal equinox at Sep 23 02:30:12 TT. At that time, Sun geocentric apparent R.A. was a couple tenths of an arcsecond short of 180°, and declination about .4 arcsecond south of the equator. The reason the Sun doesn't exactly follow the ecliptic is suggested by definition #1 of "ecliptic" in the glossary above: "1. The mean plane of the orbit of the Earth-Moon barycenter [center of gravity] around the solar system barycenter." Since Earth and Moon orbit once per month about their barycenter, in a plane not parallel to the ecliptic, an imaginary observer at the geocenter travels north and south of the ecliptic on a monthly cycle. Due to parallax he sees a corresponding wobble in the Sun's declination. It's a few tenths of an arcsecond.