October 20, 2017

AstroEvent: The Best Appearance of Mercury in 2011.

Mercury+Jupiter in the dusk. (Photo by Author).

If you’ve never seen the inner-most world of our solar system, this week is your chance. On Tuesday, March 22nd, the planet Mercury reaches a greatest elongation of 19 degrees east of the Sun.  Due to the varying angle of our ecliptic during different times of the year coupled with an eccentricity of 0.21 for Mercury’s orbit, not all apparitions of the innermost world are equal. Look for the tiny world low to the west about a half hour after local sunset. Mercury just passed a conjunction with the largest planet in our solar system Jupiter on March 16th and received its first permanent orbiting resident from Earth March 18th in the form of NASA’s Mercury Messenger spacecraft. Through a telescope, Mercury won’t show much for detail except for a tiny moon-like phase approaching half during greatest elongation…be sure to take the time to cross this inner-most world off of your naked eye bucket list!

The astro-word for this week is perihelion. This is a body’s closest approach to the sun during the course of an elliptical orbit. One of the great mysteries of the early 20th century was why the predicted perihelion of Mercury moved around its orbit by an observed 43” arc seconds per century. Hidden Vulcanoid worlds tugging on the tiny planet were evoked, but none were found. Finally, Einstein and general relativity did away with any need for mythical inner worlds, as the effect was explained by a slow spiraling of Mercury’s perihelion through the local curvature of space time caused by our Sun. The precession of Mercury’s perihelion still stands as one of the great proofs of relativity, and had to be worked into calculations by NASA engineers to assure Messenger would find its mark.      

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