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In the 19th and early 20th centuries, the "Maine Farmers' Almanac" listed blue moon dates for farmers. These correspond to the third full moon in a quarter of the year when there were four full moons (normally a quarter year has three full moons). Full moon names were given to each lunation in a season. The seasons used were those of the mean tropical year, equal in length, as opposed to the astronomical seasons which vary in length because the earth's speed in its orbit round the sun is not uniform.