Mercurial Mercury

¡SkyCaramba! Weekly astronomy blog for the week ending January 31, 2015

Thousands of years ago, one of the planets wandering the sky earned a distinct reputation for the way it behaved. It was a lot faster than the others, it never got far from the sun, and it quickly moved from the evening sky to the morning sky and back again. To some observers, this planet was quick and sneaky like a thief. To others, it was dutifully quick like a messenger. Mercury became known as the messenger to the gods.

NASA image of the Caloris Basin on MercuryMercury is less than 5,000 km wide, making it the smallest planet in the solar system. It’s smaller than some of Jupiter’s moons. It’s the closest planet to the sun and needs just 88 days to complete one orbit. It rotates once every 176 days. And Mercury’s gravity is 38% of Earth’s.

The planet is heavily cratered and of various shades of gray like Earth’s moon. There’s evidence of ancient lava flows on the planet. Much of the planet remains unmapped because only a few spacecraft have visited and they couldn’t make an image of the entire surface. Mercury’s most distinct feature is called the Caloris Basin. It’s where an asteroid is believed to have struck the planet perhaps billions of years ago. Other features on Mercury appear to have been caused by the planet shrinking and warping its surface over billions of years.

With virtually no atmosphere (just a few particles can be detected sometimes), the sunlit side of Mercury warms up to 450° Celcius. The dark side drops to -275°. The Messenger spacecraft scanned Mercury in 2012 and discovered water ice in some craters at one of the planet’s poles. The craters are so deep, sunlight never gets into them.

Because Mercury orbits closer to the sun than Earth, it’s never up all night. It moves away from the sun, rising a litte earlier each morning or setting a little later each night. But before it gets very far, Mercury moves back toward the sun. The point where it turns around is called greatest elongation. When Mercury appears to be very close to the sun, it’s said to be in conjunction. When Mercury is between Earth and the sun, that’s an inferior conjunction. When Mercury’s on the other side of the sun, that’s a superior conjunction. Sometimes, Mercury even passes right in front of the sun in an event called a transit.

Mercury’s orbit is highly ellipitical. At perihelion, the planet is about 46 million kilometers from the sun. At aphelion, it’s around 70 million kilometers away. For this reason, its greatest elongations vary a lot in angular distance from the sun. It can get up to about 27° from the sun. On January 14, 2015, Mercury was about 19° from the sun in the evening sky. Because it orbits so fast, it will be at inferior conjunction on January 30. Then on February 24, it will be 27° away from the sun in the morning sky.

So if you missed Mercury earlier this month when it was close to Venus in the evening sky, you get another chance next month in the morning sky. It won’t appear near any other planets then. But the moon will pass by on the 17th and make it easy to spot.