Saturday, May 26, 2007

Mercury spotting guide

Mercury is the only planet closer than Uranus never to be orbited by a spacecraft. Mariner 10 did a couple of flybys back in the 70’s, but those only managed to image about 45% of the surface, and the proximity of the planet to the sun means that space-based telescopes have been unable to take pictures. As a result, there is surprisingly little good remote sensing data for this planet, despite its being known since antiquity.

Luckily, this is scheduled to change. The NASA MESSENGER spacecraft is due to orbit Mercury in 2011. Like Pluto, Mercury has an eccentric orbit that is inclined from the ecliptic- the plane in which that the other 7 orbit. As a result, the spacecraft will require numerous gravity assists in order to match velocity closely enough to insert itself into orbit. Since the second Venus flyby is happening in just over a week, this is an opportune time to address some of the basics of this planet.

The most basic information about a planet is its location. It is difficult to study a planet without finding it first, so here is a handy, step-by-step guide for how to find Mercury for the next 2-3 weeks.

1. Find a clear, unobstructed view of the western horizon.
2. Just after sunset, watch the stars come out, admire the incredible brightness of Venus, and wait until the sky is dark enough for the main seven stars of Orion appear.
3. Draw an imaginary line from Rigel to Bellatrix. Continue along this line for one and a quarter times its length. As of tonight, Mercury lies at the end of that line. An annotated photo that I took this evening is below (click to enlarge):

Over the next two weeks, Mercury will get higher in the sky, so that by June 6 or so, you will need a line between Rigel and Betelgeuse, not Bellatrix. At this point, it will be at its highest point in the sky, with a visual magnitude similar to Betelgeuse.

After that, it will hang in the sky for a few days before quickly dimming and dropping away. By mid June, Orion will be getting difficult to spot before sunset as well.

Note that northern hemisphere viewers may find that Rigel sets before the sky gets dark enough to see it. Y’all may need a different spotting method.

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