What algorithms or formulas are available for computing the equinoxes and solstices? I found one of these a few years ago and implemented it, but the precision was not great: the time of day seemed to be assumed at 00:00, 06:00, 12:00, and 18:00 UTC depending on which equinox or solstice was computed. Wikipedia gives these computed out to the minute, so something more exact must be possible. Libraries for my favorite programming language also come out to those hardcoded times, so I assume they are using the same or a similar algorithm as the one I implemented.
I also once tried using a library that gave me the solar longitude and implementing a search routine to zero in on the exact moments of 0, 90, 180, and 270 degrees; this worked down to the second but did not agree with the times in Wikipedia, so I assume there was something wrong with this approach. I am, however, pleasantly surprised to discover that Maimonides (medieval Jewish scholar) proposed an algorithm using the exact same idea a millenium ago.