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 the Sun. To compare, in 1983 the equal length seasons began at 1. 48 am on 23 March, 9. 15 am on 22 June, 4. 42 pm on 21 September and 12. 10 am on 22 December, while the astronomical seasons began at 4. 39 am on 21 March, 11. 09 pm on 21 June, 2. 42 pm on 23 September and 10. 30 am on 22 December (all times GMT). Due to a misinterpretation of this definition in the March 1946 Sky & Telescope magazine, "blue moon" has also been used in the sense of "the second full moon in any month which contains two full moons (this usage has been noted as "erroneous" by Sky & Telescope in 1999). According to either definition, "blue moons" occur with the average frequency of intercalary months, seven times in 19 years, the Farmers' Almanac system of "full moon names" effectively defining a lunisolar calendar.