 According to the pattern of use in the "Farmers' Almanac", a "blue moon" is the third full moon in any period between either a solstice and an equinox, or between an equinox and a 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 the Sun.To compare, in 1983 the equal length seasons began at 1