Why is the seasonal Blue Moon the third full moon and not the fourth? That is, in one of the seasons, there will be four full moons instead of three, and one of them will be ‘blue’. But the calendar year lasts 365 days, so this difference of days accumulates, and, according to the Gregorian calendar, there are not 12 but 13 full moons approximately once every 2.7 years. Since our natural satellite reaches a full moon approximately every 29.5 days, there are three full moons for each season, and the duration of 12 lunar cycles is 354 days. And winter starts on December 22 with the winter solstice. Then on June 22, on the day of the summer solstice, summer begins and lasts until September 21 - the day of the autumn equinox. Thus, the countdown of the solar year does not begin on January 1 but on March 21 - on the day of the vernal equinox. This point in the sky is called the spring equinox or the spring solstice. The solar year begins when the Earth returns to the same point relative to the Sun that it occupied at the beginning of the year. The early start of the seasons was determined not by the Gregorian calendar year but by the solar year. The seasonal or traditional Blue Moon is the third full moon in that astronomical season, which has four full moons instead of three. Based on the second condition, traditional seasonal and modern monthly Blue Moons are distinguished. Why is it so rare? Due to the unevenness in the lengths of the lunar cycle and the calendar. The phenomenon occurs approximately once every 2.7 years. If two full moons occur in one calendar month, then the second one is called the Blue Moon. We are talking about the full moon - the lunar phase when the Earth’s satellite is in opposition to the Sun and is illuminated by it so that we can see its full disk. Credit: Coffman MU / Getty Images/iStockphoto
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