This image taken by the Messenger spacecraft in September 2009 shows large areas of Mercury’s surface that appear to have been flooded by lava. Photo courtesy of NASA
Living in the woods, we don’t see Mercury unless we go looking for it. It’s always close to the sun, and so it only appears sometimes in morning or evening twilight, and rarely gets high enough in the sky to clear the firs around the house. So in general, we have to wait for the right time and a certain clarity (in the sky) to see it with binoculars.
This week, Mercury is rising over the eastern horizon at about 6:30 a.m., nearly in tandem with the sun, so you can’t see it in the sun’s glare. By next week, it will be just visible an hour and a half or so before the sun rises.
When you spot it, it’s gorgeous. A silvery, luminous fleck of light over the glowing horizon. The Greeks called it Stilbon, “bright one,” and 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 “hermeneutics” 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 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 in every 88 Earth days. This means it peeps up over our sunset and sunrise horizons 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, Mercury made one rotation for every one revolution around the sun. This is called “one-to-one spin-orbit resonance”; the moon, for example, revolves in one-to-one spin-orbit resonance around the Earth, meaning it’s rotating at exactly the right speed to keep the same face to us all the time.
For Mercury, one-to-one resonance 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. This 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 1960s because Mercury is very hard to read, concealed there in the sun’s heat and blinding glare much of the time.
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. Mercury’s arid surface has been closely glimpsed only a few times. The first was in 1975, when the Mariner 10 spacecraft made several flybys of it. Then, more than 35 years later, the Messenger spacecraft arced over the planet three times, before crashing into it in 2015. The BepiColombo spacecraft, launched by the European Space Agency and the Japan Aerospace Exploration Agency, is on track to make several flybys of Mercury in the next few years.
Here on Earth, we catch glimpses of Mercury only 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. Like dreams that appear just before waking, then fade back into the glare of sleep.
Dana Wilde lives in Troy. You can contact him at dwilde.naturalist@gmail.com. His book “Winter: Notes and Numina from the Maine Woods” is available from North Country Press. Backyard Naturalist appears the second and fourth Thursdays each month.
Send questions/comments to the editors.
