Mercury is now an evening star hovering at its most prominent position of 2020. It’s definitely worth taking a look. You don’t need a telescope or binoculars, and it’s not hard to find. Here are viewing tips plus a few surprising facts about planet Mercury.
Simply look toward sunset at around 6 PM any time for the the next couple weeks. As the fading light of dusk hugs the western sky, dazzling Venus pops out at you as the brightest thing in the sky. You don’t have to search for Venus – it will find you, because its brilliance will catch your eye on its own. Now look far to the lower right of Venus. Any “star” you see down there is Mercury. See? Easy!
Make sure tall hills, houses, or trees don’t obstruct that direction, so get to place with an open western vista. No need to get too crazy: you’ll need to look lowish, but not super-low. Mercury at that time will hover about ten degrees high – matching the top of a closed fist held at arm’s length, with the bottom of the fist aligned with the horizon, or at eye level.
The innermost planet is odd in so many ways, it’s hard to find aspects that aren’t strange. The way it moves, the way it looks, and its features all conspire to create a carnival of curiosities.
For centuries, observers with telescopes caught fleeting glimpses of blotchy surface markings, which led them to conclude that one of its hemispheres always faces sunward—the way the Moon forever aims one side to Earth. This made sense. With the nearby Sun pulling on it with ten times the gravitational force we experience here, why shouldn’t its spin be locked in sync to match its 88-day year around the Sun? But radar pulses in the 1960s showed that it actually spins in about 59 days. This means that three rotations, three Mercury days (59 x 3) happen in the same interval as two of its years (88 x 2). The consequences are dramatic. This 3:2 resonance between its day and its year lets us see the same face of Mercury every second time it orbits the sun. So those venerable observers weren’t quite wrong. They did observe repeating patterns – but on alternate orbits. No doubt they shrugged off the observations where the markings didn’t fit.
Then, too, Mercury has the most lopsided, out-of-round orbit of any planet in the solar system. Its Sun distance mutates from 30 to 40 million miles. This is huge: Around Mercury’s perihelion you want to be sure to use SPF 2 billion sunblock instead of your usual 1 billion.The eccentric orbit also makes that crater-covered planet speed up and slow down more than any other, a variation that would sometimes make a sunrise on Mercury stop in its tracks and reverse itself. The Sun comes up, goes back down, then rises a second time.
Mercury alters its brightness more than any other planet, varying by 300-fold. These evenings it’s not too far off its brightest, which is why you’ll spot it down there in fading twilight. And as if to jealously resent Venus’ greater dazzle, Mercury may smash it to pieces sometime in the next five billion years. Thanks to perturbations caused by the Sun and especially Jupiter, the Mercury orbit wildly changes shape. It goes from being a perfect circle to being twice as lopsided as it is at present—squashed enough to actually reach innocent Venus, the planet with the most perfectly circular orbit of all.
As Mercury spins, it displays not the least axial tilt. Earth, Mars, and Saturn are all tilted 20-something degrees, but Mercury alone rotates straight up and down, not even 1/10th of a degree offset from perfectly vertical. This means that at its poles, half the solar disk is always below the horizon. Standing within the slightest polar depression or crater, you never see the Sun at all. This results in permanently dark places filled with ice. Strangely enough, then, the Sun’s nearest planet has ice deposits extensive enough to be detectable from Earth. They offer winter sports on a world badly needing it.
And even that isn’t the end of Mercurian strangeness. Its largest impact feature is the enormous Caloris Basin. At its antipodal point – the precise opposite location on Mercury to Caloris – is the so-called Weird Terrain. This hilly region is unlike anything else. Apparently, shock waves or else debris from the colossal meteor impact that formed Caloris traveled around the planet and then collided with itself at the antipodal point to wreak havoc there.
With all this, don’t you want to join the ranks of the minority of humans through the centuries who have laid eyes on the speediest, strangest, and most elusive of all the bright worlds? This is your chance, the next clear evening.