Solar Longitude Tables

Description of Solar Longitude

Solar longitude, often written Ls, is a kind of calendar to measure the time of year for solar system bodies other than the Earth. Since the Gregorian Calendar is based on Earth's 365.25 day orbital period, it isn't as useful for other planetary bodies that may have much shorter or longer seasons. Instead, solar longitude denotes the time of year as an angle measured from vernal equinox in the northern hemisphere, which occurs when the Sun crosses the equator into the northern hemisphere.

One month on Earth corresponds to a solar longitude interval of approximately 30° and each season starts at a multiple of 90°. Spring in the northern hemisphere begins at Ls = 0° (the vernal equinox), summer at Ls = 90° (the summer solstice), fall at Ls = 180° (the autumnal equinox), and winter at Ls = 270° (the winter solstice).

Why is Pluto's Ls changing so fast compared to Uranus and Neptune? Charon, Pluto's largest moon, is so large compared with Pluto itself that the two bodies form a binary system centered on a point in space between them. Whenever Charon is ahead of Pluto in its orbit, the moon pulls Pluto along and makes it speed up, and when Charon is trailing behind, it slows Pluto so much that the planet fully changes direction and orbits backwards about 49.6% of the time. So while it may appear from the rapidly changing solar longitude that Pluto is orbiting too fast, as much as 0.13° per Earth day, its frequent backward motion negates the rapid changes so that on average Pluto only orbits about 1.45° every Earth year, less than 0.004° per Earth day.

Current Ls*
*Interpolation of timeseries obtained from the JPL/Horizons portal, calculated using JPL's DE440 and DE441 ephemerides.

Solar longitude tables for solar system objects

Click a solar system body for a table of solar longitude between 1960 and 2070.
PlanetsSmall bodies