The planet Mercury is often cited as the most difficult of the five brightest naked-eye planets to see. Because its the planet closest to the Sun, it never strays too far from the Suns vicinity in our sky. Hubert J. Bernhard, who for many years was a lecturer at San Franciscos Morrison Planetarium once noted that Mercury "stays close to the Sun like a child clinging to its mothers apron strings."

Mercury is called an "inferior planet" because its orbit is nearer to the Sun than the Earths. Therefore, it always appears from our vantagepoint to be in the same general direction as the Sun. Thus relatively few people have set eyes on it; there is even a rumor that Copernicus, living in misty northern Poland never saw it.

For those Northern Hemisphere skywatchers, a great window of opportunity for viewing Mercury in the evening sky has just opened up. The window remains open through April 22, provide an ample number of chances to see this so-called "elusive planet" with your own eyes.

Currently, Mercury is visible about 45 minutes after sunset, very near to the horizon, slightly to the north of due west. If your sky is clear and there are no tall obstructions -- 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 the evening of April 4, Mercury will be shining at magnitude 1.1 on a scale used by astronomers to denote brightness that reserves negative numbers for the very brightest objects. Only three other objects in the sky will be brighter: the Moon, the planet Jupiter and Sirius (the brightest of all stars).

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.

Its at greatest elongation, nearly 20 degrees to the east of the Sun, on April 16. Shining at magnitude +0.2 (just a trifle dimmer than the star Rigel in Orion), Mercury sets nearly two hours after the Sun on this night, making this the innermost planets best evening apparition of 2003.

Mercury, like Venus and the Moon, appears to go through phases. When April began, Mercury was a nearly full disk, which is why it starts out the month appearing so bright. By the time it arrives at its greatest elongation, it will appear roughly half-illuminated, and the amount of its surface illuminated by the Sun will continue to decrease in the days to thereafter. When the planet begins to turn back toward the Suns vicinity after April 16, it will fade at a rather rapid pace.

In fact, on the evening of April 22, Mercurys brightness will have dropped to magnitude +1.4, about as bright as the star Regulus in Leo; only 1/10 as bright as it was on April 4. In telescopes it will appear as a narrowing crescent phase. This, in all likelihood will be the last view of it for most folks, for the combination of its lowering altitude, plus its descent into the brighter sunset glow will finally render Mercury invisible by the final week of April.

In old Roman legends, Mercury was the swift-footed messenger of the gods. The planet is well named for its inner orbit dictates it be the swiftest of the Suns family, averaging about 30 miles per second. Mercury makes its yearly journey in only 88 Earth days.

Mercury rotates once on its axis every 59 days, so all parts of its surface experience 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 Fahrenheit (482 C) on its day side and -300 degrees Fahrenheit (-184 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., pointed out that they were one and the same.

Looking ahead: We noted that Mercury will fade into invisibility in late April as it approaches the vicinity of the Sun. Normally we would have to wait until it emerges into the morning sky during late May in order to see it again, but a most unusual event on May 7 will allow some to see it from a very different perspective. Return to SPACE.com April 18 for more details.