Solar Longitude Tables

Solar longitude, often written L_s, 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 L_s = 0° (the vernal equinox), summer at L_s = 90° (the summer solstice), fall at L_s = 180° (the autumnal equinox), and winter at L_s = 270° (the winter solstice).

Why is Pluto's L_s 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.