The calendar is a modified version of Keith Devens’s PHP code. The moon phase extensions started with some Perl code that’s in the public domain. To correct some errors, I updated this to use the algorithms in Meeus’s Astronomical Algorithms.
Times given in the calendar — hover your mouse over the symbols to see more information about the event — are either UTC or for your local timezone. “DST” after the time denotes Daylight Savings Time. jsTimezoneDetect is used to determine your current timezone, which can be saved by you (see settings). Red symbols for the new and full moons denote eclipses.
The Blue Moon is computed to agree with the (now) less common (and more difficult to determine) Maine Farmer's Almanac rule.
It’s worth understanding the difference between astronomical nomenclature and common usage for the moon phases and the season changes. In common usage, the moon is full (for example) on a particular night. The astronomical usage is much stricter, and is uniquely defined by the angular positions of the Moon, Earth, and Sun. Since the three are in constant motion relative to each other, a moon phase exists only for the instant that the angular definition applies. Thus, for astronomers, the full moon has a particular date and time associated with it. Depending from which timezone you observe on Earth, the date may be different for two independent observers; for this reason, the full moon symbol (and indeed any of the events shown on the calendar), may appear on different dates depending on the timezone used to construct the calendar.
The names of the equinoxes (when the sun crosses the Earth’s equatorial plane), and the solstices (when the sun reaches its limits of declination), are reversed between the Northern and Southern Hemisphere (since the seasons are opposite).
Calendar subscriptions are available from Eclipsareon.com.
Vernal Equinox. Once upon a time, this was the moment the sun entered the constellation Aries, but that hasn’t been true for a couple of ages now.
Summer Solstice: the noon sun is directly over the Tropic of Cancer. Of course, the sun isn’t actually in Cancer, but it’s fun to pretend.
Autumnal Equinox. The Ancients stole the claws from Scorpio, and refashioned them into the scale called Libra. Appropriately, at the moment of equinox, day and night cover equal areas of arc at any latitude on Earth.
Winter Solstice: the sun reaches the Tropic of Capricorn. In the North, this marks the least daylight of the year, and was traditionally a time of great festivities, as people celebrated the lengthening daylight.
I was born in the then-still-British colony of Uganda, and moved to Canada at a young age. Some time after graduating from college, I moved to “America.”
ed. May 2013