Mercury rising: Early April offers rare glimpses

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Because April's night skies offer a sparkling gem for those in northern latitudes: clear views of Mercury for about half an hour after sunset. During the first two weeks of the month, just look west-northwest no more than 10 to 18 degrees above the horizon (see diagram.)

Mercury's orbit keeps it so close to the sun that it's rarely visible to the naked eye. The sun's rays overwhelm what relatively little light Mercury reflects.

To be visible, Mercury must be situated as if north of the sun. Then for approximately 30 to 60 minutes after the sun drops below the horizon, we can see the planet. The reciprocal holds true during the last two weeks of September when we can see Mercury half an hour before the sun rises above the horizon. But it's a lot easier to take a look a half-hour after sunset than a half-hour before sunrise. That's why April offers such a gift.

An extra treat on the nights of April 2, 3, and 4, will be the proximity of Mercury to the new moon. The moon will appear in silhouette.

If you watch for Mercury on each of these nights, you will easily see how astronomical objects are in constant motion as, from night to night, there will be a discernable distance between the two. Perhaps this is why the ancient Greeks described Mercury as the fleet-footed messenger of the gods.

One special note: National Dark Sky Week is April 1 to 8. Its purpose is to raise awareness of light pollution and the wonders of the night sky. For more information and how to participate go to: www.darksky. org/newsroom/pres-rel/pr030-108.html.

MERCURY: The closest planet to the sun is rarely visible because of the blazing orb next door. But for the first two weeks of April, the tiny planet can be seen just above the horizon about a half an hour after sunset.