NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: September Equinox computation
From: Herbert Prinz
Date: 2002 Sep 24, 17:48 +0000
From: Herbert Prinz
Date: 2002 Sep 24, 17:48 +0000
Pierre Boucher wrote: > Which method would you use to PRECISELY compute (hh-mm-ss) the September > equinox? > Pierre, If this were an astronomy list, I would say that according to the definition of equinox, you compute the ecliptic longitude of the Sun from a sufficiently accurate ephemeris for two reasonable guesses t1 and t2 and then solve for t such that L(t) = 180deg, either by interpolation or by iteration, dependent on whether you do it manually or with a computer. But I assume that you are asking how to do do it with the means that the modern average celestial navigator has at his disposal. The answer is that you can't do it to the required precision. For starters, modern nautical almanacs don't tabulate ecliptic longitude anymore. (Thanks God!). The next best thing is to solve for SHA = 180deg (or RA = 12 hours) and the worst thing you can do is to solve for Dec = 0deg. Neither is strictly correct, but using the SHA will get you THEORETICALLY within a few seconds of the correct time whereas using Dec will get you there within a minute, or so. In practice, however, you must compute SHA from the difference of GHA Sun and GHA Aries from your Nautical Almanac, which means that you have to interpolate a value that changes only 2.5' per hour from two values that are burdened by two rounding errors each of up to 0.05'. On top of this, the entries for GHA Sun in the Nautical Almanac are shifted on purpose by as much as 0.1' from their correct value. (This has nothing to do with "Selective Availability"; it facilitates the use of the interpolation table without a need for v-correction.) In late September, the entries are too high by 0.1' on average. In short: You can't even rely on getting within a minute of the correct time of equinox with the Nautical Almanac. Herbert Prinz