It is also an effective measure of the position of the Earth (or any other Sun-orbiting body) in its orbit around the Sun,[1] usually taken as zero at the moment of the vernal equinox.
[3] 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:[3] 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 Earth reaches a particular point in its own orbit, designated by the solar longitude.
The solar longitude for a given meteor shower would therefore not be constant if the current date were used as the epoch.