Wednesday, April 04, 2007

The Moon and Mars

The Rogueclassicist asks for a reference regarding his notice that on this day in 357 B.C. Aristotle observed the transit of the moon past Mars.

I'm not sure that the following passage from De Caelo will help with the date, but Aristotle does here seem to mention the event to which the RC is referring. This, then, is from De Caelo 2.12, translated by J.L. Stocks:

There are two difficulties, which may very reasonably here be raised, of which we must now attempt to state the probable solution: for we regard the zeal of one whose thirst after philosophy leads him to accept even slight indications where it is very difficult to see one’s way, as a proof rather of modesty than of overconfidence.

Of many such problems one of the strangest is the problem why we find the greatest number of movements in the intermediate bodies, and not, rather, in each successive body a variety of movement proportionate to its distance from the primary motion. For we should expect, since the primary body shows one motion only, that the body which is nearest to it should move with the fewest movements, say two, and the one next after that with three, or some similar arrangement. But the opposite is the case. The movements of the sun and moon are fewer than those of some of the planets. Yet these planets are farther from the centre and thus nearer to the primary body than they, as observation has itself revealed. For we have seen the moon, half-full, pass beneath the planet Mars, which vanished on its shadow side and came forth by the bright and shining part. Similar accounts of other stars are given by the Egyptians and Babylonians, whose observations have been kept for very many years past, and from whom much of our evidence about particular stars is derived. A second difficulty which may with equal justice be raised is this. Why is it that the primary motion includes such a multitude of stars that their whole array seems to defy counting, while of the other stars each one is separated off, and in no case do we find two or more attached to the same motion?

3 comments:

David said...

I've previously used Starry Night software to confirm (or, in a few cases, question) the dates of 6th century solar eclipses which appear in early medieval Irish chronicles. By setting one's location to modern Glasgow (as the annals were probably being produced at Iona during this period) or alternately Istanbul (some entries were copied from Marcellinus Comes), we can simulate the celestial events described by ancient chroniclers.

If I set my location to Athens, then, I see the half-moon transiting Mars exactly as Aristotle describes in the passage you selected: Mars enters the shadow, and emerges from the lit side. The transit takes place on Friday, April 6, 357 BC. What's odd is that it appears to take place in the middle of the day. According to the software, the moon rose in Athens at about 12:45pm that afternoon, with Mars already behind it. Mars begins to emerge from the lit side at about 2:30pm, but, even though Mars was fairly close (1.14 AU) to Earth and shining at magnitude 0.43, nobody would have seen it in the middle of the afternoon.

The software adjusts for the difference between the Gregorian and Julian calendars, and I feel fairly confident that its margin for error is no more than a few minutes either way. Is it possible that Aristotle didn't view the actual transit, but rather inferred it from the Moon's position on the nights of April 5 and April 6?

david said...

curiosity question from the rogueclassicist ... what version of starry night do you use? ages ago I tried to use the 'basic' one for this purpose and found it sorely wanting ...

David said...

I'm using v.5.03, a "Special Edition," released in 2005. I suspect that this is also the "basic" version, since it came free with my telescope.