This week and next Mercury will be brightest and highest in the evening sky. Not until May will skywatchers in mid-northern latitudes have as good an opportunity to spy the planet that spins closest to the sun. That’s what makes it so tricky to see in the first place. Mercury never gets far enough from the sun to appear in a dark sky, forever lurking in the twilight zone.
Still, I was surprised how easy it was to see last night. Higher up than expected too. I bundled up and went out to look 45 minutes after sunset. Nothing. Where was Mercury? It turned out I was looking too low. Once I raised my gaze a bit, a solitary “star” popped into view about a fist above the southwestern horizon.
While the planet shines tonight at magnitude -0.5 (brighter than Vega and Arcturus) the hazier, thicker air near the horizon robs it of 1.2 magnitudes, dimming Mercury to magnitude +0.7 or about as bright as Altair in the Summer Triangle. Still plenty easy to see with the naked eye.
I kept the planet in view until around 6:20 p.m. or more than an hour past sunset before subzero temps and 20 mph winds forced a retreat back into the car. If you’re in a mercurial mood, start looking about 45 minutes after sunset to the upper left of the brightest part of the lingering solar glow in the southwest. The planet hovers about 10-12 degrees (a little more than one fist held at arm’s length) high. Since Mercury has no bright company, if you see a single star in that direction, you’ve nailed it.
To be on the safe side, you might consider toting along a pair of binoculars. I guarantee that once you find it with optical aid, you’ll quickly see Mercury with the naked eye.
Once you’ve fixed in your mind where Mercury is located along your local horizon, get ready for a really fine event. This Friday the 31st, an incredibly thin one-day-old moon will sidle up some 5 degrees to the lower right of the planet. Are you thinking pictures? So I am.
Place your camera on a tripod – or at least wedge it firmly against something – compose a scene including moon and planet and take a series of photos with your lens set anywhere from f/2.8 to 4.5. ISO 400 speed should be fine with exposures ranging from 2 seconds to 1/15 second. While you’ll get a decent photo with a standard lens, a 100-200mm telephoto lens will make for a tighter, more dramatic image.
As you stand in the cold clicking away or simply admiring Mercury, here are some interesting facts about the planet to warm your brain cells:
* At 3,032 miles (4,880 km) across, Mercury is smaller than Jupiter’s moon Ganymede and Saturn’s moon Titan.
* Mercury’s orbit is the most eccentric or least circular of all the planets. Its distance from the sun ranges from 29 to 44 million miles (46-70 million km).
* While Venus is slightly hotter, Mercury has one of the most extreme temperature ranges of any body in the solar system. With virtually no atmosphere to capture and distribute the sun’s heat, the sun-facing dayside of the planet tops out around 800 degrees Fahrenheit while the nightside dips to -297 F.
All the dayside heat leaks right back into space during the long night. And it is a L-O-N-G night. The sun’s enormous gravity has locked the little planet in a 3:2 rhythm or “resonance”. For every two orbits around the sun, Mercury rotates three times on its axis.
Since the planet completes an orbit in 88 days (one Mercury year), its day is twice as long as its year or 176 Earth days. Mercury’s sunny side bakes for nearly six months and then chills for another six. No wonder it experiences such extremes of hot and cold.
* Mercury’s slight axial tilt of just 0.03 degree means that craters at its poles are steeped in perpetual shadow and never heated by the sun, making them perfect places to trap volatile materials like ice and keep them frozen for a long, long time. New data from the MESSENGER spacecraft now gives strong evidence for ice holed up in some of these craters.
* Mercury has a large (by volume) partially molten iron core and a planet-wide magnetic field, a feature lacking on Venus and Mars.
* Mercury is the most cratered planet in the solar system. Unlike Earth, Mars and Venus, which have been extensively resurfaced through volcanic and tectonic processes, Mercury’s retains much of its ancient battered surface.