Multiply the sine of ecliptic longitude (reckoned forward or backwards from the nearest equinox) by the sine of 23.438 or whatever the current obliquity’s exact value is).
Take the inverse sine of the result. On Fri, Oct 14, 2022 at 4:57 PM Steve Lelievre < steve.lelievre.can...@gmail.com> wrote: > Hi, > > For a little project I did today, I needed the day's solar declination > for the start, one third gone, and two-thirds gone, of each zodiacal > month (i.e. approximately the 1st, 11th and 21st days of the zodiacal > months). > > I treated each of the required dates as a multiple of 10 degrees of > ecliptic longitude, took the sine and multiplied it by 23.44 (for > solstitial solar declination). At first glance, the calculation seems to > have produced results that are adequate for my purposes, but I've got a > suspicion that it's not quite right (because Earth's orbit is an > ellipse, velocity varies, etc.) > > My questions: How good or bad was my approximation? Is there a better > approximation/empirical formula, short of doing a complex calculation? > > Cheers, > > Steve > > > > > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > >
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