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The Question

(Submitted August 05, 1997)

A co-worker and myself have been talking about Mercury.I have a computer program that will show me the position in the sky, but I've yet to see it. When I looked for the position in the sky for him some time ago, I thought that it appeared that Mercury rose near the time of sunset and set some 45 minutes later. During our conversation, he said, "But Mercury is always near the Sun and should therefore be in the sky at approximately the same time." This makes sense to me, but I was still wondering about my previous suggestion. I speculated that perhaps at some time of the year Mercury would be behind the Sun, and at some day would peek out from behind, and if this occurred at or near sunset, Mercury would pop above the horizon for a brief period before setting again later. Is this possible? Or is there any other explanation that would have Mercury come above the horizon then drop below after a brief time?

I own binoculars and a f=1000mm D=114mm Newtonian telescope... will I ever be able to see Mercury with these or my naked eye?

The Answer

Your co-worker is right. Mercury, the innermost planet in our solar system, is never far away from the Sun as seen from Earth. The maximum separation is about 28 degrees. This is about 60 times the apparent size of the Sun, so it is rare for Mercury to be actually hidden behind the Sun. It is, however, very commonplace for Mercury to be hidden by the glare of the Sun (and of the daytime sky) --- it impossible to see Mercury unless it is more than about 10 degrees away from the Sun.

If your computer program shows (or can be set to show) which stars and planets are up in the sky during the day, you should almost always be able to find mercury very close to the Sun.

Once every 4 months or so, there is a period when Mercury can be seen shortly after sunset in the western sky; there is another period during which Mercury can be seen in the eastern sky shortly before sunrise. Your program should be able to help you figure out exactly when. At these times, Mercury is bright enough to be seen with your naked eyes, no binoculars or telescopes are necessary.

Best wishes,
Koji Mukai
for Ask an Astrophysicist

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