Mercury on the mind

Posted by Rich Talcott
on Thursday, February 08, 2007
Mercury stands above the western horizon after
sunset in this view from 2005. Lee Coombs

Last evening was clear in Wisconsin and, with the temperature hovering in the single digits, relatively balmy compared with the past few nights. I took the opportunity to view Mercury. The solar system's most elusive bright planet, Mercury is never easy for us northerners to see. Luckily, the innermost planet was at greatest eastern elongation yesterday, so it was about as high in the sky as it could be.

And even more luckily, brilliant Venus was nearby to point the way. Venus is the brightest object now visible in the early evening sky, punctuating the twilight glow in the west-southwest. Last night, I went out about half an hour after sunset. Venus was dazzling, as usual. And there, to Venus' lower right and at two-thirds its altitude, stood Mercury.

At magnitude –0.7, Mercury's the third-brightest evening object, after Venus and the star Sirius. So why does it have a reputation for being hard to see? The trouble with spying Mercury stems from its proximity to the Sun. It lies at most 43 million miles from our star, while Earth orbits the Sun at a distance of 93 million miles. So, from our vantage point, Mercury never strays far from the Sun's glare.

But there's something more subtle going on here as well. Mercury lies only 18° from the Sun at greatest elongation, It appears relatively high in our sky now because the ecliptic - the apparent path of the Sun across the sky, which the planets follow closely — makes a nearly perpendicular angle to the western horizon after sunset in February. Mercury can appear as much as 28° from the Sun. Unfortunately, those times always coincide with the ecliptic making a shallow angle to the horizon from the Northern Hemisphere. Mercury may be farther from the Sun, but most of that translates into distance along the horizon, not to altitude above the horizon.

The situation is reversed in the Southern Hemisphere, where Mercury currently lies exceptionally low in the sky. But next month, at greatest western elongation March 21/22, Mercury will be 28° from the Sun. For us in the Northern Hemisphere, it barely will skim above the predawn horizon. But from the Southern Hemisphere, gleaming Mercury will lie 15° high a full hour before sunrise and easily will be the most conspicuous light in the eastern sky. No wonder our Southern Hemisphere compatriots have a hard time understanding why we think Mercury is elusive!

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