Friday, January 27, 2006

March 2, 2000

Watch the western skies this month for a lovely pas de quatre among the Moon, Mars, Jupiter, and Saturn.

The dance begins about one hour after sunset. The three planets are arrayed in a diagonal line plunging towards the western horizon, with Mars the lowest and Saturn the highest. Between them is Jupiter, which, at the start of the month, is the brightest object in the whole sky.

On the 7th, the waxing crescent Moon joins the planetary trio. At first it appears on the horizon below them all, but on succeeding evenings it glides by each planet in turn as the Moon pursues its easterly orbit around yet another planet, Earth. By the 10th, the Moon has become the highest of the four members of the lunar-planetary alignment.

Meanwhile, the planets are converging at a rapid clip, and actually display four types of motion simultaneously. First, each night all three planets set in the west because of the easterly rotation of the Earth. Second, all three planets are lower each night due to the Earth's revolving around the Sun more rapidly than they. Third, they are moving eastward against the background stars in their orbits around the Sun. Fourth, they appear to be approaching one another because of their own relative orbital speeds.

The vernal equinox returns at 2:35 a.m. on the 20th. On this date the Sun rises due east and sets due west. By coincidence, the Moon will also rise due east that same evening. This is a good month to get oriented!

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