First there were two and now there are three. While the evening sky’s depopulated of bright planets, the morning’s full of ’em. Alongside Jupiter and Mars, Mercury recently joined the scene shortly after its Dec. 13 conjunction with the sun. Though not terribly high, Mercury’s attainable so long as you have a clear view to the southeast at dawn. It’s fairly bright at magnitude +0.6 (a little brighter than Antares) and will be in good view through the first week of January.
Mercury always looks like a star with the naked eye, but a small scope will show its phases which are identical to those of Venus and the moon. Right now, the planet appears a little fuller than a last quarter moon, but it’s quickly filling out. As Mercury waxes to full, it moves further from Earth and shrinks in size but because more of its surface is illuminated by sunlight, slowly brightens.
Because the planet is the closest one to the sun and inside Earth’s orbit, it always appears near the sun in the sky. Dawn and dusk are the only times it’s visible with the naked eye. Mercury is also the smallest planet with a diameter of 3,032 miles (4,881 km) which is just 872 miles larger than the moon. Both Jupiter’s moon Ganymede and Saturn’s Titan are larger but because they orbit planets instead of the sun, they’re considered moons.
Larger telescopes under superb conditions might reveal a few fleeting markings on the planet, but the best views have come from spacecraft, especially NASA’s MESSENGER probe which orbited Mercury from 2011 to April 2015 and sent back thousands of photographs of the surface. One of the planet’s most amazing features is the Caloris Basin, which at 948 miles (1,525 km) across, is one of the largest impact structures in the entire solar system. And it all it took to do the job was a 62-mile-wide (100 km) asteroid.
I’m sure you’ll have no trouble believing that Mercury’s sunny side roasts at 801° F (427° C). But did you know that the temperature drops to –279° F (–173° C) on the side facing away from the sun? That’s a spread of nearly 1,100 degrees! That’s what happens on a planet with no atmosphere and long nights. It takes about 59 days for Mercury to spin once, which allows it’s night side to dip to frightening lows. Recently, astronomers have detected water ice in permanently shadowed craters at the planet’s poles. No doubt it was delivered by comets which have a habit of sling-shotting through the inner solar system from their refrigerated hideout in the Oort Cloud and — over time — running into planets.