After dominating the western evening sky for more than half a year, the planet Venus has now transitioned into the morning sky. But during April another planet will take its place in the western evening twilight.
Mercury is often cited as the most difficult of the five brightest naked-eye planets to see. It's also 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.
Since it's the planet closest to the sun, Mercury never strays too far from the sun's vicinity in our sky. Hubert J. Bernhard, who for many years was a lecturer at San Francisco's Morrison Planetarium, noted that Mercury "stays close to the sun like a child clinging to its mother's apron strings."
Relatively few people have set eyes on Mercury (there is even a rumor that Copernicus never saw it). While that is certainly true most times of the year, there are other intervals – and we're about to enter one of them – where making a sighting of Mercury requires almost no effort. You simply must know when and where to look, and find a clear west-northwest horizon.
Currently, Mercury is moving slowly pulling away from the sun's vicinity. By April 8, it will be visible about 40 minutes after sunset, very near to the west-northwest horizon. Yet, if your sky is clear 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. On that Wednesday evening, Mercury will be shining at magnitude –1.5, which means that it will rank as the brightest object in the early evening sky, surpassing even Sirius (the brightest of all stars).
Over the next couple of weeks, Mercury will slowly ebb in brightness, but it will also slowly gain altitude as it gradually moves away from the vicinity of the sun.
It reaches greatest eastern elongation – its greatest angular distance east of the sun (20-degrees) on April 26. Shining then at magnitude +0.4 (just a trifle dimmer than the star Procyon in Canis Minor), it will set about one hour and 50 minutes after the sun, making this Mercury's best evening apparition of 2009. On this night, using binoculars, you'll also be able to see the beautiful Pleiades Star Cluster hovering a few degrees directly above Mercury, while just above the Pleiades will be the slender sliver of a crescent moon just two days past New.
Mimicking the moon
Mercury, like Venus, appears to go through phases like the moon.
Currently in telescopes, Mercury is a nearly full disk (99-percent illuminated), which is why it starts out appearing so bright. But by April 22, it will appear only half-illuminated, and the amount of its surface illuminated by the sun will continue to decrease in the days to come. So when it begins to turn back toward the sun's vicinity after April 26, it will fade at a rather rapid pace.
In fact, by May 2, Mercury's brightness will have dropped to magnitude +1.4, about as bright as the star Regulus in Leo; only 1/15 as bright as it was on April 8. In telescopes it will appear as a narrow crescent only 20-percent illuminated. This, in all likelihood will be your last view of it, for during the following evenings the combination of its lowering altitude, plus its descent into the brighter sunset glow will finally render Mercury invisible.
Hot and cold
In old Roman legends, Mercury was the swift-footed messenger of the gods. The planet is well named for it is the closest planet to the sun and the swiftest of the sun's family, averaging about 30 miles per second; making its yearly journey in only 88 Earth days.
Interestingly, the time it takes Mercury to rotate once on its axis is 59 days, so that all parts of its surface experiences periods of intense heat and extreme cold. Although its mean distance from the sun is only 36 million miles, Mercury experiences by far the greatest range of temperatures: nearly 900-degrees F (482-degrees C) on its day side; -300-degrees F (-184-degrees C) on its night side.
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., solved the mystery when he pointed out that they were one and the same.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.