Right now is a good time for seeing the planet Mercury, especially if you’re on the Outer Cape, with its wonderful unobstructed views to east on the ocean side. Astronomy is very much a carpe diem thing: when something is up in the sky, you need to get out there and see it before it’s gone. Here is some background you’ll want before Mercury-watching.
Mercury is a scrappy little denizen of the inner solar system, the most elusive of the five planets visible to the unaided eye. It is a small, rocky planet, a little more than one third of Earth’s size. It is also the planet closest to the Sun, orbiting at an average distance of 36 million miles (as opposed to Earth’s 93-million-mile distance from the Sun).
The Sun dominates Mercury’s sky, appearing three times as big there as it does here. And Mercury is hot — very hot; the daytime temperature averages 800 degrees Fahrenheit. But because of its very slow rotation (one day on Mercury lasts 58 Earth days) and because there is no atmosphere to circulate heat, the nightside temperature plunges as low as -280 degrees F. For a visit, you’d have to pack your winter and summer clothes, and lots of sunscreen.
Mercury’s proximity to the Sun also means that it moves through our sky faster than the other planets do. The Greeks called the planet Hermes, after the swiftest of the gods (the Romans later gave it the name Mercury, whom they identified with Hermes). In our sky, Mercury precedes or follows the Sun closely and can be seen for only a short time either just before sunrise or just before sunset. It appears for about two weeks at a time, disappears for a month or so, then re-emerges from the Sun’s glare as another two-week apparition.
This is very different from the other planets, which are more widely separated from the Sun; Venus is often visible well after dark or before sunrise, while Mars, Jupiter, and Saturn are sometimes visible most of the night, and most of the year.
That good Mercury-viewing I mentioned will last from now through around March 10. “Good” is a relative term. If you’re a morning person — and I mean a serious morning person — then I invite you to step outside 30 or 40 minutes before sunrise and find a spot with a clear view to the east. An ocean beach would be perfect. Look low above the horizon a little south of east and you’ll see two stars, which are actually the planets Jupiter and Saturn. The twilight will dim their usual glory a little, but you should be able to find them. Then look carefully for a fainter star near them. That’s Mercury, not as bright as its two enormous siblings, but a cheerful pink-white. In late February you’ll find it between Jupiter and Saturn. As we move into March, it will wander east (left) of bright Jupiter.
The big challenge when it comes to observing the stars from the Outer Cape is the weather. It’s capricious, for one thing. And strong ocean breezes can cause even the stoutest of telescopes to shake, ruining the view in the eyepiece. But for observing Mercury with the naked eye, looking east onto the vast expanse of ocean just before sunrise, there is no better place on Earth to be.