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November 2003: Mars in Aquarius

All this month the ruddy planet Mars travels eastward through the constellation Aquarius, not to reach the Aquarius-Pisces border till December 4. On November 22, the brightly shining planet just misses occulting (passing in front of) the fourth magnitude star 90-Phi Aquarii. If blessed with a dark country sky, you should have little trouble seeing the dim star above Mars without optics. Binoculars, however, come to your rescue whenever seeing conditions are less than stellar.

The gap between the planet and the star amounts to a scant 1/6 degree of arc, roughly the equivalent of 1/3 the Moon's diameter. Actually, the angular distance between the two orbs varies slightly, depending upon your position on the globe. In fact, if two observers happen to observe Mars far enough apart (like from Paris, France and Cayenne, South America), and have the optics and a micrometer capable of measuring to one second of arc (1/3600 of a degree), then it's possible for the observers to compare Mars' positional difference relative to the same star and to determine Mars' parallax. Parallax then enables a skilled mathematician to compute Mars' distance from Earth in miles or kilometers.

In 1672, when Mars was in the vicinity of the three Psi stars of Aquarius, the time was ripe for obtaining the first reliable parallax of Mars. To see these historic Psi stars for yourself, place Phi Aquarii toward the top of your binocular field. (If you have difficulty finding Phi Aquarii, remember that Mars will be near it a few days before and after November 22.) Chances are you'll see three little stars bunched together beneath Phi, perhaps in the middle or the lower middle part of your field of view. The two righthand Psi stars are of fourth magnitude (about the same brightness as Phi), whereas the lower left Psi star shines somewhat dimmer at fifth magnitude.

The English astronomer John Flamsteed (1646-1719) predicted that Mars would occult the middle Psi star on October 1, 1672. This announcement probably spurred the astronomical world into action, with the closely-knit Psi stars of Aquarius backdropping Mars and giving astronomers a stellar grid from which to map the planet's place in the sky.

Thanks to the pioneering efforts of Copernicus and Kepler, astronomers in the time of Flamsteed already had a very good idea of the distances of the planets relative to the astronomical unit (AU) -- the Earth's mean distance from the Sun. For example, it was known that the mean distance of Mars from the Sun was a bit more than 1.5 astronomical units. The only problem was that nobody knew how many miles were in an astronomical unit.

But if the mileage from Earth to Mars could be computed, then the astronomical unit was as good as figured. In 1672 the Italian-born astronomer Gian Domencio Cassini (1625-1712) and his partner Jean Richer (1630-1696) made the first good approximation of Mars' distance from Earth. With Cassini stationed at Paris and Richer at Cayenne, they both observed Mars at the same time and found Mars parallax somewhere around 24 seconds of arc.

From there, they found the astronomical unit to be some 87,000,000 miles, not far from the modern measure of 93,000,000.

copyright November 2003 by Bruce McClure


McCarthy Observatory Bulletin
Sky Astronomy Software
Star Names Their Lore and Meaning by Richard Hinckley Allen, page 51.

November 8th Eclipse

Year's Northernmost Moon