Mercury

Mercury is the closest planet to the sun. The planet Mercury has a diameter of 3100 miles. At a distance of 36 million miles, it takes 88 days to circle the Sun. One day lasts 59 days of Earth time. Mariner 10 composite image of Mercury, 1974. PHOTO / NASA

Mercury is in the daytime sky every day of the year, but because of its close distance to the sun, and because it is usually washed out in the solar glow, it is one of the most difficult planets to observe.

Mercury can never get more than 28 degrees or two hours from the sun. It is small in angular size, shows no detail in an amateur telescope, and can be conveniently seen only within an hour after sunset or just before sunrise, depending on its position in its orbit around the sun. The best times to view Mercury are in the evening sky around the spring equinox, or in the morning sky, around the autumn equinox.

Mercury is brightest between greatest elongation and superior conjunction. It should be viewed at greatest elongation in order to obtain a high sky position. Even so, it is hard to see, since it is easily lost behind trees and rooftops which surround the average backyard telescope.

Facts about Mercury

A. Mercury is named after the Roman messenger of the gods. Mercury is intermediate in size between the Earth and the moon. It orbits very close to the sun and so is hard to see. In photographs, Mercury looks like the moon, in that it is heavily cratered and has no atmosphere. Although it is closest to the sun, Mercury is not the hottest planet. That distinction belongs to Venus. However, Mercury does have the largest temperature differences of any planet, varying from -170 degrees C to 430 degrees C.

B. There is a large impact crater on Mercury called the Caloris Basin. On exactly the opposite side of the planet, there is an area, where the shock waves converge known as the weird terrain.

C. Mercury is made mostly of iron. The percentage of iron in the core of the planet indicates that it may have been hit in its early developmental stages by a very large object. This impact could have dislodged much of the lighter materials constituting the planet, and left it as the small iron sphere it is today. The planet also has long curved ridges called lobate scarps. These cliffs are wrinkles on the surface caused by the slow cooling of the iron core.

D. Mercury has an odd rotation about the sun. It rotates on its axis one and a half times for each orbit around the sun. That means there are three days in every two years on Mercury. Furthermore, the orbit of Mercury about the sun is notable in that it is very elliptical.

It may come as a surprise, but many modern astronomers have never seen Mercury. It is one planet which can be spotted only if you know where to look. Moving swiftly through the sky, at maximum elongation it is never more than 28 degrees from the Sun as seen from Earth. Named after the Roman God of commerce, it is the closest planet to the Sun and second smallest planet in the Solar system. Its diameter is 40% smaller than Earth and 40% larger than Moon. It's even smaller than Jupiter's moon Ganymede and Saturn's moon Titan. Mercury is visible for weeks in the evening sky and later in the morning sky. When observed at dawn or dusk, Mercury is so low in the horizon that the light must pass through 10 times the amount of Earth's atmosphere than it would if Mercury was directly overhead. Like the moon and planet Venus, Mercury also has it phases, which can be observed through a standard scope or binoculars. The Hindu  god of business, "Booda", is one of the nine idols of the "Nav Grah" in most Hindu temples.

Manoj Pai, Ahmedabad, Gujurat, India

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