Dr. Terry L. Goforth
Associate Professor of Physics, SWOSU
June 2000

Look! In the sky! It's a star! It's a comet! It's Mercury!

Mercury appears low in the northwest sky below the Gemini twins Pollux and Castor just after sunset early in June of 2000.

Mercury is making an appearance in the skies near you. Look to the northwest shortly after sunset (around 9:30 p.m. is a good time). Just south of where the sun has set and about 10° to 15° above the horizon you can spot the planet Mercury, a dim star-like object in the evening sky. (For reference, your clenched fist held at arm's length is approximately 10° from thumb to little finger.) For further reference, look another 15° above Mercury. You'll see two stars in a roughly horizontal line. The northernmost of these stars is Castor, and the southern star is Pollux–the Gemini twins. These two stars and Mercury form a triangle pointing down with Mercury at the bottom tip.

Note that Castor and Pollux are (most likely) twinkling, while Mercury shines steadily. Stars, although actually very large, are also extremely distant and appear as pinpoints of light even in the most powerful telescopes. Because their apparent size is so small, slight disturbances in the atmosphere cause them to twinkle, getting brighter and dimmer and sometimes shifting in color. Planets, such as Mercury, are much smaller than stars and may look like pinpoints of light to the naked eye. However they are much closer to the Earth than stars and are easily resolved into disks even in small telescopes. Because they appear much larger, atmospheric disturbances are averaged out over their surfaces, and the twinkling is not observed.

The planet closest to the sun is aptly named for the speedy messenger of the gods, for it moves more quickly across the sky (relative to the "unmoving" stars) than any other planet. The other planets will play an extended engagement in the evening sky, remaining visible for months before disappearing into the sunlit sky only to reappear in the pre-dawn sky for several months. But quickly-moving Mercury makes its appearance for only a few weeks at a time. And even during those few weeks, Mercury is often overlooked because of its proximity to the sun.

Mercury's orbit is

Astronomers describe the relative position of objects in the sky by angular separations. If you draw a line from your eyes to the sun, and then draw a line from your eyes to the planet Mercury, you would find that the angle between those two lines is never greater than 28°. Thus Mercury, even at maximum separation from the sun, is never visible for more than two hours after sunset (or for two hours before sunrise). It is further limited by its dimness. Mercury is visible only by the light is reflects from the sun. It is a small planet–only Pluto is smaller–and has a very dark surface which only reflects about 10% of the light it receives from the sun. Thus Mercury never appears very bright and can only be seen when the surrounding sky is fairly dark–at least a half hour after sunset (or before sunrise).

So get out and enjoy the sight, but do it soon. Mercury is at maximum elongation (i.e., appears farthest from the sun) on Friday, June 9. It will slowly at first and then more rapidly appear closer to the sun, getting much dimmer within the next week and eventually disappearing into the twilight sky. Of course if you miss it, take heart. Fast-moving Mercury will return in just a few months, reaching its next maximum elongation in the evening sky on October 5.

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Last update:  June 5, 2000