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Mercury dances to gravity’s tune

Mercury resides only a third as far from the sun as Earth, so sunshine on Mercury is a good nine times brighter – the first of many factors that make Mercury a very strange planet.

Unfortunately, it’s also a very difficult planet to study. Because Mercury orbits so close to the sun, the sun is always close to it in the sky. We see mercury only at dawn or at dusk, rising just before the sun, or setting just after it.

The sky is never totally dark at dawn or dusk, making Mercury hard to find. Tonight, however, just after sunset, Mercury should be easy to find if you have a good view of the western horizon.



When we look at a star high above us, our view is filtered by some dozens of miles of air in constant motion. The moving air bends the star’s light, just as light from a spoon is bent in a half-glass of water. Bent starlight makes stars twinkle.

When we look at a star on the horizon, we’re looking sideways through many more miles of air. Bright stars don’t just twinkle; their light is broken into every color of the rainbow – very pretty, but a nuisance if you’re trying to do astronomy – trying, say, to study Mercury. The alternative is to study Mercury, not at twilight, when it lies near the horizon, but at midday, in a clear blue sky.




We can determine how fast a planet rotates by watching how surface patterns move. Working under tough conditions, astronomers of the late 19th century deduced that Mercury’s rotation – the length of its day – lasted exactly as long as the time it took to orbit the sun – the length of its year: 88 earth-days.

The moon, too, orbits in “locked rotation” around the Earth: one lunar day lasts a month. Thus, the moon always shows the same face to the Earth; similarly, locked rotation would cause Mercury to always keep one side facing the sun.

If the sun were to beat down on the same part of the Earth 24/7, that side of the Earth would become so hot as to be uninhabitable. The antipode – the opposite side – would be bitter cold and equally uninhabitable.

Fortunately, Earth rotates. Orbiting the sun in “rotisserie mode” allows all parts of the Earth to share the sun’s warmth.

Orbiting in locked rotation so close to the sun, temperatures on the sunside of Mercury would be hellishly hot; at the antipode, hellishly cold. Between these, however, would lie a narrow region of fairly moderate temperatures: the “twilight zone.”

Even as mid-1960s television was making “twilight zone” a household term, astronomers bounced radio waves off Mercury and found it to spin in less time than it takes to orbit the sun. Mercury completes one rotation – one day-night cycle – in 58.65 earth-days.

Calculator exercise: Multiplying the length of Mercury’s year (88 earth-days) by two, we find that two Mercury-years last 176 earth-days.

Multiplying the length of Mercury’s day (58.65 earth-days) by three, we find that three Mercury-days last exactly 176 earth-days.

Two Mercury-years equal three Mercury-days. Every time Mercury orbits the sun twice, it spins on its axis thrice.

Mercury is (slightly) football-shaped. Like an ocean buoy that rights itself after a wave tips it over, Mercury “rights itself” to point its long axis toward the sun when it feels the sun’s gravitation most strongly – when it’s closest to the sun.

After sunset tonight, find the very young, very thin crescent moon, just above the western horizon. Mercury hangs, star-like, just to its right.

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Alan Stahler trained as a biologist and is an amateur astronomer. He teaches private enrichment classes for students of all ages. His science programs can be heard at noon on alternate Tuesday on KVMR-FM (89.5).


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