Solar longitude, commonly abbreviated as Ls, is the ecliptic longitude of the Sun, i.e. the position of the Sun on the celestial sphere along the ecliptic. 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.[2] 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 (the year of seasons) the planet is in, but without the inaccuracies of a calendar date, which is perturbed by leap years and calendar imperfections. Its independence from a calendar also allows it to be used to tell the time of year on other planets, such as Mars.[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 | Date | Time (Earth days) |
|---|---|---|
| 0° | 11 April 1955 | 199 |
| 90° | 27 October 1955 | 183 |
| 180° | 27 April 1956 | 147 |
| 270° | 21 September 1956 | 158 |
| 0° | 26 February 1957 | Total: 687 |