Solar longitude


Solar longitude in effect describes the position of the earth in its orbit, being zero at the moment of the vernal equinox. Since it is based on how far the earth has moved in its orbit since the equinox, it is a measure of what time of the tropical year we are in, but without the inaccuracies of a calendar date, which is perturbed by leap years and calendar imperfections.
Solar longitude is especially used in the field of meteor showers, because a particular meteor shower is caused by a stream of small particles very close to the elliptical orbit of a comet, or former comet. This means that the shower occurs when the earth reaches a particular point in its own orbit, designated by the solar longitude.
The value of the solar longitude, like any ecliptic longitude, depends on the epoch being used. The solar longitude for a given meteor shower would therefore not be constant if the current date were used as the epoch. For this reason, a standard epoch is used, usually J2000.
Solar longitude can also be used for other planets, such as Mars. This gives a way of saying what time of year it is on that planet.
Solar longitude does not increase linearly with time, the deviation being larger the greater the eccentricity of the orbit. For instance, here are the dates for multiples of 90° solar longitude on Mars in the mid 1950s:
solar longitudedatetime
0°11 April 1955199
90°27 October 1955183
180°27 April 1956147
270°21 September 1956158
0°26 February 1957687

The Martian year is divided into 12 Martian months of unequal duration, with the breakpoints being at solar longitudes that are multiples of 30°.