Fast Facts on the Planet Mercury History Size Orbit Distance to the Sun and More

About the planet Mercury, fast facts including history, size, orbit, distance to the sun and more.

MERCURY

Five planets are visible to the naked eye.

Of these, Mercury, closest to the sun, is the most difficult to see. Nicholas Copernicus, who 1st recognized the sun as the center of the solar system, admitted he had never seen Mercury.

Being only 4/10 of the earth's distance from the sun, Mercury appears as an evening or morning star for only a few minutes. If you have a telescope, the best time to observe this elusive planet is in the daytime.

In 1899, Giovanni Schiaparelli announced that he had studied Mercury for 17 years. From his hundreds of observations, he deduced that Mercury spun on its axis at the same rate it revolved about the sun--once every 88 days. Obviously then, the side always facing the sun must be very hot--as high as 774deg.F.--and the shadow side must be extremely cold--as far down as 400deg.F. below zero. Alas for the paper and pencil astronomer, it is not so. Just as the unsuspecting motorist has his speed clocked by radar, so radar was also used to time Mercury's spinning in 1966. The result is 58.6 days. Mercury is hot all over.

Mercury's orbit is a strange one. It appears to move a slight bit, a few miles each century. But to the careful astronomer who wants perfection or an explanation this is as if a familiar freeway had been moved a few yards overnight.

In 1847, the French astronomer Urbain J. J. Leverrier assumed the cause of the deviation was the gravity pull of a yet unknown planet, which he prematurely named Vulcan. The search for Vulcan, supposedly a planet much smaller than Mercury but closer to the sun, held the attention of astronomers for decades until Albert Einstein explained the difficulty in 1907 in his General Theory of Relativity.

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