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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
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