The Sciences

Mercury's Rising

By Bob BermanJun 1, 1994 5:00 AM


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No planet is more slippery than Mercury. Some astronomers (including such luminaries as Copernicus) went through life without ever seeing the elusive innermost planet. Its intimidating reputation keeps many beginners from even trying.

A shame. While it doesn’t blaze like the here-I-am beacon of Jupiter that’s now shining in the south, or possess the riveting fire of Venus that so dominates evening twilight, Mercury’s brightness makes it one of the top five stars. Were it not for its incurable habit of hugging the sun, it would quickly grab attention.

Still, once or twice each year Mercury rises like a phoenix from the ruddy embers of the sunset. That time is now, when it briefly swings to the edge of its orbit while slanting favorably above the western horizon.

May’s final week and the opening days of June offer the best chance for carving a Mercury notch into your belt. Making it easier still, Venus--the heavens’ brightest star--now serves as signpost. Miss this chance and the closest you’ll come is seeing the little planet’s namesake rising in summer’s thermometers.

You’ll need an unobstructed western horizon. If you live on the West Coast or near any open area like a lake or even a cemetery, you’re in luck--if a cemetery can be considered lucky. In cities, try a rooftop.

Forty minutes after sunset will do it. First you’ll notice dazzling Venus, the evening star. Mercury is the much dimmer star about halfway between Venus and the point of sunset. It’s the only thing there! If you spot any star to the lower right of Venus, you’ve found it. And then you’ll have joined an exclusive club.

Binoculars snap Mercury right out of the twilight. But you don’t need a telescope: higher power doesn’t greatly improve the experience. The tiny charbroiled world often displays moonlike phases when viewed telescopically, but rarely any additional details as it shimmers in the turbulent air close to the horizon.

Yet over the centuries occasional drawings recorded at the eyepieces of amateur instruments have depicted Mercury with bright white poles--ice caps! Such sightings were always dismissed as illusions akin to the canals of Mars. Mercury’s fiery, airless surface, hot enough to melt lead, would surely be the last place to host an Arctic panorama!

So astronomers were amazed when radio pulses were bounced off that cratered world in 1991: the best interpretation of those signals spells ICE. Until then we didn’t think Mercury had any water at all. Moreover, Mariner 10 had passed by in 1974 and 1975, and no caps were seen.

It turns out that the spacecraft’s cameras had missed the exact poles. Apparently, shallow depressions permanently shielded those regions from both the low sun and the passing craft. Mercury’s unique talent as the only planet to travel through space without any tilt to its axis has prevented the poles from ever tipping toward the raging nearby sun.

What a splendid reward for curious Earthly eyes, that this tiny world of fire and ice could not hide its mysteries forever. It’s another case (like the strange radial spokes on the rings of Saturn) of keen-eyed amateurs seeing esoteric detail, in moments of steady air, that multimillion-dollar observatories had completely missed.

It may look like a common star, but the mercurial vagabond grants special satisfaction to those who catch it hovering in the ambrosial twilight of spring.

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