The innermost planet is unique and odd in so many ways that it’s hard to find aspects that aren’t strange. The way Mercury moves, the way it looks, its features and newly discovered oddities all conspire to create a carnival of curiosities.
For centuries, astronomers assumed that one of its hemispheres always faces sunward, the way the Moon forever aims one side to Earth. With the nearby Sun pulling on it with ten times the gravitational force that we experience here, why shouldn’t its spin be locked in sync with its 88-day orbital period?
But radar pulses in the 1960s showed that it actually spins in 58.6 days. It has therefore achieved stability in a different way. Three rotations – three Mercury days – happen in the same exact interval as two of its years. This 3:2 resonance causes Mercury’s sunrises to happen 176 days apart. It’s a rather rare and special event there.
Then, too, Mercury has the most lopsided, out-of-round orbit of any planet. Its solar intensity varies from six to 14 times ours. Around the Mercurian perihelion, you want to be sure to use SPF two billion sunblock instead of your usual one 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 too, varying by an astonishing 7 ½ magnitudes. In the past year it went from magnitude +5.1 – fainter than the “Seven Sisters” – to -2.4, more than twice the brilliance of the Dog Star Sirius, the brightest star. And while Venus looks brightest when it’s near to us and appears as a crescent, Mercury shines most brightly when it’s farthest from us – because it then displays a nearly full phase.
As if jealously to resent Venus’ greater dazzle, Mercury may smash it to pieces. Thanks to perturbations caused by the Sun, and especially Jupiter, the Mercury orbit wildly changes shape. It goes from being a perfect circle to having an eccentricity more than twice as lopsided as it is at present – squashed enough actually to reach innocent Venus, the planet with the most perfectly circular orbit of all. Such a collision may happen in the next five billion years.
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 one-tenth 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’d never see the Sun at all. Result: permanently dark places, filled with ice. Strangely enough, then, the Sun’s nearest planet has ice deposits. They offer winter sports on a world badly needing it.
And even that isn’t the end of Mercurian strangeness. One of its two largest impact features, and its most striking by far, 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. Shock waves or else debris from the colossal meteor impact that formed Caloris traveled around the planet and then collided at the antipodal point to wreak havoc there.
The Messenger spacecraft will perhaps solve some of the tiny planet’s mysteries. Why does Mercury have a magnetic field, when larger Venus does not? What about the strange “tail” discovered in 2010, caused by sodium that oozes from its rocks and then is swept by the solar wind to point away from the Sun forever like a comet? Messenger recently arrived (March 2011) as the first-ever probe to orbit the peculiar innermost world. We await its revelations.
This weekend offers the year’s best chance to see that planet with just the naked eye. Get to a place with an unobstructed view of the west. The Hudson Valley Mall parking lots all work fine. Look low toward where the sunset happened. Watch between 6 and 6:20 p.m. Any star that you see low down is Mercury. It’s not very faint at all. But use binoculars if you want to cheat and make it super-easy. Optimum days begin this Thursday evening (Valentine’s Day) through Monday evening.
There’s an odd satisfaction in glimpsing this bizarre orange world.
Join the family! 
