May 27, 2024

Mercury a perfectly strange planet

Living in the woods, we don’t see Mercury very often. It’s always close to the sun, and so it only appears sometimes in morning or evening twilight and never gets high enough in the sky to clear the firs around the house. So in general you have to wait for the right time and a certain clarity (in the sky) to see it with binoculars.

When you do spot it, it’s gorgeous. A silvery, luminous fleck of light in the glare of the setting or rising sun. The Greeks called it Stilbon, “bright one,” and they associated it in the morning with Apollo, the god of light and knowledge, and in the evening with Hermes.

Hermes was the Greeks’ version of the Romans’ Mercury, who was said to have invented the lyre and was the god of travelers, treaties, commerce and thieves. More peculiarly, he was the gods’ messenger, who carried communications to humans in dreams. That made him an interpreter of sorts. Our word “hermeneutic” means interpretation. Hermes was a guide of dead souls and invented astronomy and the musical scale. In some ancient traditions he was understood to be the same as

the Egyptian god Thoth who invented writing. Thoth was said by some to be the author of ancient scripture – of words that reveal glimpses of unknown activities deep in the mind or elsewhere.

Anyway, planet Mercury has its own peculiarities. It’s the closest planet to the sun, averaging about 35 million miles away (Earth averages about 93 million miles), and whips around once every 88 Earth days. This means it peeps up over our sunset or sunrise horizons for a few weeks about four times a year, which might be described as “mercurial,” if you’re paying attention. It’s blistering hot on the side facing the sun – reaching 860 degrees Fahrenheit – and colder than the moon on its dark side – as low as minus 300 – because its wispy atmosphere can’t distribute heat around.

It used to be thought that, like many other solar system bodies such as the moon, Mercury made one rotation for every one revolution around the sun. This is called “one-to-one spin-orbit resonance”; it means the moon turns at exactly the right speed to keep the same face to us all the time. For Mercury it would mean its one side facing the sun would be perpetually broiled and the other side deep-frozen. But this turned out not to be the case. Mercury rotates in a different resonance, equally perfect, but stranger: It spins exactly three times for every two times it circles the sun, which would have fascinated the 16th century astronomer Johannes Kepler who was compiling evidence that harmonies in the structure of the universe reflect the mind of God.

The 3:2 resonance was not discovered until the mid-1960s, though, because Mercury is very hard to read – it’s concealed in the sun’s heat and blinding glare most of the time. We catch only glimpses of it when its orbit takes it far enough out to bounce sunlight off the surface and translate it into that silvery drop in our twilight times, when your clearest dreams occur and then fade quickly away.

The sun itself is better understood than Mercury. Spacecraft and telescopes have negotiated the heat and glare and been turning their readings into knowledge of nuclear fires deep in the sun for decades now. Lately Mercury’s arid surface is partly revealed because the Messenger spacecraft has arced over the planet twice in the past year and sent back the first close-up information in more than 30 years. How long it will take to interpret those data, no one knows.

Dana Wilde may be reached at Recent Naturalist columns can be seen at

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