From: "Shaun Moss" <email@example.com>
Subject: RE: [Mars_Time-sig] Calendars for other celestial bodies (was Jupiter Calendar)
Date: Fri, 19 Jul 2002 15:33:00 +1000
[ to Mars Society Time list, from "Shaun Moss" <firstname.lastname@example.org> ]
I have thought a bit about a calendar for Luna. The Lunar day, being equal to a Lunar month, is 29.5 Earth days long. One possible approach would be to divide this by 30 and operate using artificial days of 0.9833 Earth days long. But I think a much better approach is to use UTC over the whole Moon. Seeing as how an artificial day is required anyway, there wouldn't be much difference. Admittedly this would not by synchronous with the orbit of the Moon, but considering the dependence Luna will have on the Earth, it would be more convenient and practical. We will probably end up with the odd situation of living on the Moon, whose cycle is the original source of the month, yet using Earth months which are about a day longer than the Lunar month on average.
The most practical solution is probably to use a lunisolar calendar for the whole Earth-Moon system, but it would be hard to introduce this on Earth. Unless the Chinese take over.
I have also designed a calendar for Mercury, but I also haven't put this one on my website yet. It is very simple.
A few things about Mercury. It has a sidereal day of 58.646 days, equal to two-thirds of its orbital period which is 87.969 days. Because of this pattern, Mercury's solar day is actually equal to two whole orbits.
In the calendar I've designed, a Mercury month is defined as half of Mercury's sidereal day (= 29.323 Earth days). Therefore the stars (not the Sun) move across the sky in a 2-monthly cycle; a completed orbit every 3 months; and the Sun rises every 6 months. A solar day on Mercury is 3 months (one orbit) light, 3 months dark.
Using this system, this would be the only planet discussed so far with an exact, constant month length. Rather than use Earth days, I am using 30 M-days (not to be confused with a Martian day, more commonly known as a sol) per month. This means an M-day would be slightly shorter than an Earth day, roughly 23hrs, 27.5mins. With this system Mercury has one of the tidiest calendars in the solar system. Every 30 M-days is a new month; every 60 M-days the stars complete their journey around the planet (or so it would appear); every 90 M-days Mercury completes an orbit; and every 180 M-days the Sun comes up.
Rather than use Mercury's orbital period to count human years, an "M-year" (or whatever) can be defined as equal to 12 months (and therefore equal to 6 rotations, 4 orbits, or 2 solar days). Each M-year would therefore have exactly 360 M-days.