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Mercury in astronomy



  Mercury's Caloris Basin


Mercury’s largest surface feature Caloris Basin is 700 miles wide and resulted from a collision with an asteroid during the solar system’s early history. In it, this crater with strange, emanating troughs is unique in the solar system. APOD March 5, 2015.


Mercury, the closest planet to the Sun, rotates three times on its axis for every two of its orbits around the Sun; that is, three of its days last two of its years. This whole-number relationship between its day and its year ("orbital resonance") is unique among the nine planets. That it is not a 1:1 relationship (like our Moon’s day equaling its year, which results in the same side always facing us) is unique in the solar system. Like our Moon, Mercury has no atmosphere and its surface is heavily cratered. It has the most extreme temperatures in the solar system, from -300° to +800° Fahrenheit - hot enough to melt lead!

For almost two centuries astronomers had been aware of a small but inexplicable discrepancy between Mercury’s actual very elliptical orbit around the Sun and what Newton’s laws predicted it should be. Scientists even postulated the existence of an unknown planet, Vulcan, or an Asteroid belt within Mercury’s orbit pulling on it to account for this difference. Einstein finally explained why Mercury behaved as it did. He reasoned as follows: according to Newton, the Sun’s pull on Mercury increases as Mercury gets closer to the Sun. And Newton’s laws also predict that, as the Sun’s pull increases, Mercury’s speed increase as well. He then added something totally new: that as Mercury’s speed increases, it literally gets heavier - or more precisely, that its mass increases. And if Mercury’s mass increases as it gets nearer the Sun, the Sun pulls on it all the more. With the publishing of his General Theory of Relativity in 1915, Einstein calculated what Mercury’s extra, relativistic mass should be and how it would effect Mercury’s orbit - and it exactly accounted for what was being observed! This successful prediction helped the early acceptance of his work.

The planets Mercury and Venus are the innermost planets in the solar system, never getting more than 28 and 48 degrees respectively from the Sun. They are always found in the same or adjacent signs as the Sun, except in rare instances when the Sun is in the second half of a sign and Venus is near its "maximum elongation" of 48 degrees. Mercury is so close to the Sun that it can only occasionally be glimpsed in the twilight or dawn sky just before or after the Sun or the horizon has swallowed it up.

This Planet-Mercury-in-astronomy page and much of this 600-page website are excerpted from the personalized Fine Art Book You and the Universe.

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© Carl Woebcke: The Planet Mercury in astronomy, 1991-2017. All rights reserved.