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The Riddle of Mercury
The Riddle of Mercury

Here’s a riddle for you. Of the seven brightest objects in the sky, which one have most amateur astronomers never seen?

The answer is Mercury. At its brightest, it rivals Sirius and outshines Saturn, but very few people have ever seen it. The problem is that it never strays far from the Sun, and, because of the geometry of the ecliptic, it’s only well placed a couple of times in the year. This is once in the evening sky in the spring and again in the morning sky in the fall, times when the ecliptic makes a steep angle relative to the horizon.

February is one of those rare windows in which Mercury is well placed for observers in the Northern Hemisphere in the evening just after sunset. It will be best placed at the time of Eastern elongation on February 24, when it is farthest from the Sun, but it can usually be spotted a week or so before or after that date.

For viewers at about 45° north latitude, Mercury will be almost directly above the point where the Sun sets. You won’t be able to see it at first because of the glare from the sunset. But if you sweep the area with binoculars you will soon pick it up as a tiny speck of light in the twilight glow. (Do not look at the Sun through binoculars directly as it will cause permanent eye damage - sweep for Mercury only after you are sure the Sun is below the horizon). As the sky gets darker, Mercury will be easier to see, but will also be sinking lower and lower in the sky. Soon it will be visible to your naked eye, now that you know where to look.

The view of Mercury through a telescope may be disappointing. You should be able to see its tiny disk, smaller than Mars is currently, and make out its phase. Like Venus, Mercury shows phases because it is closer to the Sun than the Earth. However, because it is so close to the horizon, its image will usually be bubbling from the effects of our atmosphere, and smeared into a spectrum from atmospheric refraction.

If you note carefully the spot where Mercury appears relative to terrestrial landmarks, you should be able to pick it up earlier the next night, which will give you an opportunity to observe it higher in the sky. And you can congratulate yourself on seeing something that relatively few other people have ever seen!

February 2006

Geoff has been a life-long telescope addict, and is active in many areas of visual observation; he is a moderator of the Yahoo "Talking Telescopes" group.

Details
Date Taken: 05/26/2011
Author: Geoff Gaherty
Category: Archives

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