See Mercury, the elusive planet

By Joe Rao

updated 3:54 p.m. ET April 18, 2008

The planet Mercury is often cited as the most difficult of the five brightest naked-eye planets to see. Because it's the planet closest to the sun, it never strays too far from the sun's vicinity in our sky. It is often referred to as "the elusive planet."

And there's even a rumor that Copernicus never saw it, yet it's not really hard to see. You simply must know when and where to look, and find a clear horizon. And for those living in the Northern Hemisphere, a great "window of opportunity" for viewing Mercury in the evening sky is about to open up.

Mercury is called an "inferior planet" because its orbit is nearer to the Sun than the Earth's. Therefore, it always appears from our vantage point to be in the same general direction as the sun. In the pre-Christian era, this planet actually had two names, as it was not realized it could alternately appear on one side of the sun and then the other.

Mercury was called Mercury when in the evening sky, but was known as Apollo when it appeared in the morning. It is said that Pythagoras, about the fifth century B.C., pointed out that they were one and the same.

Space.com/StarryNight.com On April 23 from midnorthern latitudes, start looking for Mercury half an hour after sunset.

A bright evening ‘star’
Mercury just passed superior conjunction on April 16, but in the days to come it will bolt out to become easily visible low in the west-northwest at dusk. On Wednesday evening, April 23, Mercury should be visible within about 30 minutes after sunset if your sky is quite clear. Mercury will be shining at magnitude –1.6, slightly brighter than Sirius (the brightest of all stars). In fact, at that particular hour of the day, Mercury will be the brightest object in the sky!

So, if your sky is free of any horizon haze and there are no tall obstructions to your view (like trees or buildings) you should have no trouble in seeing it as a very bright "star" shining with just a trace of a yellowish-orange tinge. By April 30, Mercury will be setting as late as 85 minutes after the sun. That evening, binoculars may show the Pleiades star cluster 4 degrees directly above it. (Your clenched fist held at arm's length measures about 10 degrees in width.)

In the evenings that follow, Mercury will slowly diminish in brightness, but it will also slowly gain altitude as it gradually moves away from the vicinity of the sun. This is just the start of Mercury's best apparition of the year for mid-northern viewers. On the evening of May 6, be sure to look for a delicately thin sliver of a 1.5-day old crescent moon sitting just a couple of degrees above and slightly to Mercury's right.

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