Mercury

Mercury

Mercury is easily the most difficult of all the naked-eye planets to see. This is mainly because it is the closest planet to the sun and therefore tends to get lost in the bright glow of the morning and evening sky. In itself, and despite its small diameter of 3,010 miles, it can be quite bright, outshining Sirius and equalling even Jupiter on favorable occasions.

Mercury cannot stay bright for long. Its orbital period is only 88 days, so its motion in the sky is quite rapid. No sooner does it swing well clear of the sun than it moves sunward again. As it does so it changes in apparent size and phase and therefore in brightness. Sometimes it passes directly between the sun and the earth and then, seen through a telescope, looks like a small round black spot. Eleven crossings or transits took place in 20th century. The last five were on May 9, 1970; November 10, 1973; November 13, 1986; November 6, 1993’ November 15, 1999.

Mercury. Photograph in colors. Source : NASA, photograph of public domain

A Hostile World

Mercury’s orbit is decidedly elliptical in shape. Its distance from the sun ranges from 29 million miles 40 43 million miles. An average value is about 36 million. At these distances the sun will appear some nine times brighter and hotter than it does to us. For this reason alone Mercury must be extremely hostile world. Astronomers once thought the planet turned one and the same face perpetually toward the sun. In fact, one side was intensely hot and the other extremely cold. Recent observations, however, made with the giant 1,000-foot radio-radar telescope at Arecibo, Puerto Rico, indicate that the planet rotates once in about 59 days. It rotates in the same sense or direction as does the earth. Even so, Mercury must be a world of great extremes in surface temperature for, like the moon, its small mass and weak gravitational pull prevent it holding on to a protective blanket in the form of a dense atmosphere.