Mars stands on the moon, dances backward

COFFEE WITH WARREN, with Warren Harbeck
Cochrane Eagle, July 16, 2003

With Mars still much in the news as it continues toward its Aug. 27 closest approach to Earth in recorded history, I thought I'd connect once more with Bruce McCurdy for a briefing on two related heavenly events that most of us can enjoy from our own backyards.

"What's this about Mars standing on top of the moon?" I asked our stargazing coffee companion.

It's true, Bruce said. In the wee hours of Thurs. July 17, if the sky is clear to the southeast, you'll be able to watch the drama unfold.

For Alberta viewers, "the waning gibbous moon will pass less than one moon diameter beneath Mars between 1 and 3 a.m. MDT," he said. "Both objects will have risen around midnight and will be low above the SE horizon around the time of closest approach (2 a.m.)."

Mars will be the brightest point of light in the sky that night, "and the two will make a very striking pair, though Mars will still be 71.4 million km distant, 185 times as far as the moon," he said. "Mars should appear noticeably redder than the white and grey moon."

I might add that any of us with the patience to spend an hour or two watching all this happen will have a great opportunity to see the moon actually moving in its orbit around Earth. In the space of one hour, we'll see the moon pass the distance of its diameter from west to east (right to left) beneath Mars.

This is so because the moon in its monthly revolution around the Earth is racing in an easterly direction at approximately one moon diameter per hour (about 3500 km/h or 2200 mph) as observed against the background stars and planets.

Now back to Mars and my concluding statement last week that the warrior planet will dance backward for a while against the starry sky.

Here is how Bruce explained this illusion, in astronomical terms (a star chart, available over the Internet or at your local science or book store, may prove useful):

Much less obviously than the moon, and as observed over many nights instead of minutes, "Mars is currently making progress eastward against the background stars," Bruce said. "However, as Earth passes Mars, Mars will appear to slow and temporarily hit reverse gear. This so-called retrograde motion will be a particularly quick, tight swerve, covering some 10 degrees of sky (a closed fist at arm's length) between July 30 and Sep. 29.

"This will be difficult to track due to the paucity of bright stars in the backdrop constellation of Aquarius," Bruce noted. "So look at the bigger picture to find observational anchors. As Mars slows to a stop in late July, it will be approximately along a line formed by the two stars on the right side of the Great Square of Pegasus rising high in the southeast, and carrying down through Mars to the late-rising lonely luminary Fomalhaut low in the south. Over the next two months it will appear to back away to the right of this line. Binoculars will allow the observer to observe Mars motion from night to night against fainter stars in its immediate vicinity. By October Mars will resume its normal eastward motion."

To illustrate what Bruce is getting at, I like to draw on the analogy of the merry-go-round. As a rider makes a round, the rider sees a nearby pedestrian straight ahead strolling from right to left in the same direction as the merry-go-round is turning. As the faster rider passes the slower pedestrian, the pedestrian appears briefly to reverse direction and move from left to right against the background trees and buildings – "retrograde motion." But as the rider continues around further and looks back, the pedestrian is seen indeed to be walking forward, from right to left against the background.

You and I are like the rider on the merry-go-round and Earth is our horse as we make our 365-day revolution around the sun, the centre of this merry-go-round. Mars is like the slower pedestrian moving in the same direction in its 687-day stroll around the sun.

A concluding note to Adele Dyall in the Arctic: Bruce says the Mars events will be very near your horizon, but if it's not too light, you just might catch part of them.

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