Mercury’s morning show

SkyCaramba weekly astronomy blog for the week ending April 14, 2012

Mercury makes a fairly good appearance in the morning sky this month for people in the southern hemisphere. A thin waning crescent moon helps you spot the messenger planet at that time. 

The distance, measured in degrees, between the sun and a planet is called the planet’s elongation. The planets farther from the sun than Earth reach elongations as low as 0° when they’re on the other side of the sun and as high as 180° when they’re opposite the sun. The planets closer to the sun than Earth never reach close to 180°. 

Mercury’s greatest elongation is about 27° and it will be about that far from the sun on the morning of April 18. Its closeness to the sun means you never get to see it more than about an hour and 45 minutes before sunrise or after sunset. Even then, you have to be in the right place to get a fairly good view. 

The best places to be for this greatest elongation are around 32° south of the equator. Uruguay, northern New Zealand, and southern South Africa are ideally situated. If you’re in one of those places or close to their latitude, be outside about an hour and a half before sunrise on the 18th. You’ll see a thin crescent moon to the east. A bit to the right of it is Mercury. 

Without other bright objects nearby, Mercury is hard to identify. When you can only see some of the stars in a constellation and you’re not used to looking at them close to the horizon morning after morning, you’re not so sure which object is which. This time the moon helps. 

If you’re looking for Mercury from points farther north, the moon will be up and to the left until you get to about 40° north. There, it’s above Mercury and you may have a great deal of trouble finding the planet in the dawn’s brightness. 

If you do get to see Mercury at this elongation, you’re seeing it from about 79 million miles (127 million km) away from us. It’s about 42 million miles (68 million km) away from the sun. It’s so close to the sun, it takes just 88 of our days for Mercury to orbit the sun once. 

While Mercury is hard to see and a few cloudy mornings around greatest elongation time will ruin the view, the good news is you don’t have to wait long for another greatest elongation. They alternate from morning to evening and are sometimes just six weeks apart. The next evening one will be July 1. The next morning one will be August 16. 

Mercury is mostly gray and mostly airless. Day side temperatures reach up to 845°F (452°C). Night side temperatures drop to as low as –300°F (-184°C). The planet rotates so slowly, an observer who stayed at one spot on Mercury would wait 58 Earth days to see any given distant star rise twice. Orbital motion on top of that puts 176 Earth days between sunrises. 

There’s a little about Mercury. ¡SkyCaramba!