Solar longitude, commonly abbreviated as Ls, is the longitude of the Sun as seen from a given body, i.e. the position of the Sun on the celestial sphere along the orbital plane of that body. It is also an effective measure of the position of the Earth in its orbit around the Sun, usually taken as 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 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.