Hermetic Astrology on other planets

[Inky]

I'm not sure about any of the following.

The Martian 'year' is 687 days, so it would be 57.25 days for each sign and, as Fred Brackin said, 19.11 days for each Decan.

I haven't got the book the GURPS Hermetic Astrology rules are from, whichever one that is, I'm just going by my knowledge of astrology, so some of this may be different if there's something different about the way Hermetic Astrology works.

Mars's effect would probably be constant, whether it's high, low or average compared to what it was on Earth would depend on what was useful to your plot. Earth, on the other hand, might have an effect, and it's anybody's guess what that would be. It might be difficult to work out, because it would always be in effectively the same place in the sky as the Moon, so it would be difficult to distinguish the two.
The Moon might not have an effect, as it's the moon of a planet you're not on - you don't expect Titan, for instance, to have an effect in normal astrology. On the other hand, the contrast between the Sun and the Moon is really, really fundamental in Hermetic alchemy and such like, and taking the Moon out of the system would really confuse things. So what takes the Moon's place in the scheme of things?
Saying that it's the moon of whatever planet you're on might get weird - Venus and Mercury haven't got moons, and Mars has two and some planets have a lot more than that, although you could use the largest one.
A neat solution might be that Earth, or rather the Earth-Moon combination, takes the place of the Moon. After all, among the things the Moon represents in astrology are home, mothers, and the people, and Earth is our home planet and in most settings will have most of the people.

So you have a six-planet system -
Saturn
Jupiter
The Sun
Earth/Moon
Venus
Mercury.

You might also count Deimos and Phobos, in which case that'd definitely make things different from normal astrology - their meanings could be anything, of course, also Deimos orbits every 30 hours and Phobos every seven hours, which is so fast that it rises in the west because it moves faster than Mars turns.

If going to Mars is something that mages can do at all regularly in the setting, they'll probably have a research base there trying to work out how the heck it all works.

Originally Posted by Fred Brackin

If Hermetic Astrology is more astronomically accurate than popular astrology. This would be so. However, I believe the stars in popular astrology "froze" in place when the traditional charts were drawn up and I've heard that astronomical factors since then make the actual constellation of any Sign abot a month off.

Since Mars' year is 687 days (presumably Earth-days, Mars days are 43 minutes longer) a Decan that's 10.14 days long on Earth would be 19.11 days on Mars. Which Decan would probably be trickier and brings up that "literal stars position"question.

This is a common misconception. The signs of the Zodiac don't have anything to do with the constellations and aren't supposed to - they're defined by the seasons/the Sun's path along the ecliptic (tropical zodiac), as 12 equal divisions of the ecliptic starting from the Sun's position at the spring equinox, the constellation names are just convenient labels. In Indian astrology, on the other hand, the signs of the Zodiac really are defined by the constellations (sidereal zodiac), though I'm not sure that they assign quite the same meanings to them so it may work out the same.

Of course, if we're talking about a chart drawn up for Mars, that raises the question of whose spring equinox, Earth's or Mars's. Does Mars have an entirely different zodiac? When you start talking about charts drawn for other planets, the tropical zodiac - defined by Earth's orbit - starts to look like it doesn't make sense.

One thing's for sure, unless the mage is from a TL where there are computers and has some rather specialised software, or unless that mage research base has been drawing up tables of the planets' movements as seen from Mars, he or she will be completely flummoxed to know how to actually calculate the chart and would basically have to resort to observing the celestial bodies directly, and furthermore observing them over a full Martian day and night to get the ones that are below the horizon - a fair bit of Astronomy skill would be required.

Planetary dates, now that's an interesting one. The association of planets with days of the week is actually based on an association of planets with hours, with the days being named after the planet that rules the first hour of that day, counting from sunrise - that's why the days of the week are in such an odd order https://en.wikipedia.org/wiki/Planetary_hours . This means that you might get a completely different set of 'planetary dates' if you're using a different set of planets. The 'Hermetic Astrological week' might not even be the same length, depending what you get when you divide 24 by the number of planets you're using - in fact, if the number of planets isn't coprime with 24, some planets won't even have a day.
Of course, from an astronomical point of view, planetary hours and planetary days don't make any sense, since there's no logical reason not to count Uranus and Neptune and if you do count them you get a completely different answer, so it might be best not to look at the planetary hours/planetary days question too closely.