Friday, December 23, 2011

Dark Winter Nights

Some of you may have noticed how dark it is at night these days. One reason is that the Winter Solstice was on Dec 22 at 5:30:00 UT. The US Naval Observatory's Multiyear Almanac gives the times of the Winter Solstice for when the Sun crosses Right Ascension 18 hours. Another reason is that the New Moon also occurs this week on Christmas Eve at 18:06 UT. I used the Almanac to compute the angular distance between the New Moon and the Sun at the time of the Winter Equinox to see how often they occur together between the years 2000 and 2050.

The plot above shows that this is a rather rare event astronomically but in 2014 they will come rather close together, less than 1.5° in RA apart. The probability of this happening is about once in 240 years. The average number of days in a month is the Moon's synodic period of 29.5 days. So the probability that the New Moon will occur on the same day as the Winter Solstice is about once every 30 years.

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