Mercury
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Late February and Early March 1999 Provides Outstanding Opportunity for Viewing Elusive Planet Mercury

+ Von Del Chamberlain +

February 24, 1999

Of all the planets known way back into antiquity, the one probably least well-known by observation to most people is Mercury. It is not a matter of being too dim, for at times it is one of the brighter objects of the night. The problem for viewing it is that when it is visible in our sky it remains for such a short time. It is always closely on the heels of the Sun, either following the Sun down in waning evening light or preceding the Sun in growing light of dawn. One must be watching for the short time when the sky is dark enough and when Mercury is high enough above the horizon to be seen. When it is not completely lost in the glow of sunlight, the planet is always playing hide-and-seek in either our morning or evening sky.

Right now we have an excellent opportunity for viewing elusive Mercury. You can easily find it as the sky begins to darken by first locating brilliant Venus just south of west. Below Venus is another bright planet, Jupiter. Tonight the magnificent pair will be less than two degrees apart and the separation will grow by about one degree each evening. Mercury is about 10 degrees below and slightly right of Jupiter. It will be the third brightest object in that part of the sky, easy enough to see if you are looking at the right time. Just wait until the sky is dark enough to pick it out, but you must look before Mercury sets below the horizon as darkness comes.

Over the coming days, keep watching Jupiter as the best guide for locating Mercury. By end of February the three planets, Venus highest, Jupiter in the middle, and Mercury closest to the horizon, will be about evenly spaced apart. Then, going into March, Jupiter and Mercury will get closer and closer together as both move lower and set earlier. March 5th will mark their closest pairing, then Mercury will rapidly dim and disappear in the evening twilight. It will be gone before the middle of March.

Watching Mercury leads to understanding of the intriguing ideas people have had about this planet that always hovers around the Sun. The Greek name for the planet was Hermes, swift messenger of Olympian gods, called Mercury by the Romans. Indeed, this is the swiftest of all the planets, for it orbits the Sun in only 88 days, more than 4 times in one Earth year. It has a most unusual period of spin upon its axis; one day on Mercury is equal to about 59 Earth days. Thus a day lasts most of a "year" on Mercury with about one and one-half days (rotations) per year (orbits).

Mercury is second smallest of the nine planets orbiting the Sun. Isn't it interesting to note that both the outermost planet, Pluto, and the innermost, Mercury, are the smallest of the major objects in the Sun's family. Mercury is also similar to Pluto in having an unusual orbit: not as circular as the orbits of other planets; and straying farther out of the plane of Earth's orbit.

Mariner 10

In 1974 when images of Mercury were sent back by the spacecraft Mariner 10 we learned that Mercury looks amazingly like Earth's Moon. Mercury is similar to our Moon in another way; neither body has a significant atmosphere. Without gases to shield the surface from intense solar radiation or to convey heat around to the dark side, the temperature range on Mercury is the greatest of any solar system planet: more than 400 degrees Centigrade at high noon and about 200 degrees below zero centigrade after sunset--a range of 600 degrees. This makes the planet at once one of the hottest and coldest surfaces in the solar system. In 1992 we learned that ice exists just beneath the surface at the Mercurial poles.

This mosaic of Mercury was taken by the Mariner 10 spacecraft during its approach on 29 March 1974. The mosaic consists of 18 images taken when the spacecraft was 200,000 km from the planet.

Another interesting fact that has become recently known about Mercury is that it has a high average density suggesting that it lost some of its lighter outer surface at some time during its history, leaving the denser layers containing lots of metals. This might be due to large impacts during its early history. Detailed studies of its surface indicate that it was heavily bombarded followed by slow cooling for about 4 billion years.

This mosaic, also by the Mariner 10 spacecraft on the 29 March 1974, was taken while it retreated from the planet. Again consisting of 18 images, these images show somewhat more of the illuminated surface.

We can summarize human observation of Mercury as a mysterious world of surprises. Indeed, we might think of it as the arts and humanities planet. Like subject matter treated by skilled artisans it dashes from stage left (evening sky) to stage right (morning sky), making only brief, yet frequent entrances and exits onto the stage of the sky, much of the time lurking at the edge of perception. In spite of how quickly it dashes in and out of view, it has revealed itself to us gradually, requiring our finest analysis to begin to learn its violent and intriguing character and history. Appropriately, scientists have chosen to name its features, only recently known to us, after great writers, artists and composers: craters bear names like Shakespeare, Tolstoy, Bach, Goethe and Mozart.

As you go out to look around, be sure to watch the swift movement of Mercury in our evening sky for the last days of February and first week of March. Keep watching as Venus continues to move farther out into the evening sky while Saturn glides toward the sunset. On March 19 the pair will be only about 2.5 degrees apart with the delicate crescent Moon very near to the left. Then, during the latter part of March, Saturn will disappear into the glow of sunset, ending the evening planetary drama of 1999 that has lured us into spring.

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