Thursday, May 08, 2008

Watch Mercury rise and fall all through May

Watch Mercury rise and fall all through May
by Dave Adalian

If you go outside about half an hour after sunset tonight and look just above the western horizon where the sunburned sky has yet to fade you’ll find bright pink Mercury, fastest of the planets, shining in the Sun’s wake.

Elusive Mercury--the smallest of the planets at 3,032 miles wide--is putting in its best appearance of 2008 during May. But, you’ll still need to be quick to see it.

Just 800 miles wider than Earth’s moon and never further than about 36 million miles from the Sun, Mercury is difficult to find until the sun is far enough down. By then, though, the time for viewing is short.

When you find it, you’ll notice the messenger of the gods carries a distinctly reddish hue, but it’s an illusion. Mercury, which is similar in size to our moon, is also about the same color, and Mercury, too, is covered in thousands of impact craters that bear witness to our Solar System’s violent past.

Mercury’s ruddy tint, however, is an artifact of the air, which distorts the planet’s color as the light from so far away travels through Earth’s dust-filled atmosphere.

If you look at Mercury through a telescope, you’ll notice it also shares another trait with the Moon: it goes through phases. Right now, the Iron Planet--so called because it has a massive iron core larger even than Earth’s--is about 60 percent lit. But, it’s waning, and by the middle of the month it will be just 25 percent illuminated.

At the same time, however, Mercury is also coming closer to Earth, and so while its dark side comes into view it will also seem to grow larger.

As you watch night to night, notice that Mercury trails a bit further behind Sol for the first few days. Around midmonth, Mercury will stop drifting away from the sun, then after a couple of days will start falling back sunward. A week later it will take the keenest of eyes to detect Mercury in the afterglow of twilight, and by the end of the month there’s no chance of seeing it at all.

While Mercury is invisible to us back on Earth, it will be rounding the sun on an inside track then emerging on the morning side. But, it won’t be visible to early risers until late June. It will be easy to find about half an hour before dawn on July 1 when it’s paired with the thinnest sliver of an almost new moon.


This column originally appeared in the Visalia Times-Delta and Tulare Advance-Register on May 8, 2008.

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