Full moon is called a blue moon. The term "blue moon" used in this sense may date to as early as the 16th century, but it became well known in the United States due to the "Farmers' Almanac" (published since 1818). According to the "Farmers' Almanac", a "blue moon" is the third full moon in any period between either solstice and equinox, or between equinox and solstice, (calculated using the mean tropical year), which contains four full moons. These seasons are equal in length, unlike the astronomical ones, which vary in length depending on the Earth's speed in its elliptical orbit round
September equinox end of summer and the beginning of autumn (autumnal equinox) in the Northern Hemisphere, while marking the end of winter and the start of spring (vernal equinox) in the Southern Hemisphere. The September equinox is one point in time commonly used to determine the length of the tropical year. Date and time of the September equinox that occurred from the year 2010 to 2018 (UT) are listed as follows: The point where the Sun crosses the celestial equator southwards is called the first point of Libra. However, due to the precession of the equinoxes, this point is no longer in