2018 June 05
originally posted on facebook

Until radar observations in 1965, Mercury was believed to be in 1:1 tidal locking with the Sun instead of 3:2 because Mercury could be only be observed optically every other Mercury year.

Thanks to the high eccentricity of Mercury, the hottest temperatures on Mercury can be found only at 0N, 0W and 0N, 180W, on alternating years (there are two Mercury years to a Mercury day). At those points, the Sun goes directly overhead about an Earth week before Solar noon, then goes overhead backwards at Solar noon, then goes overhead forwards again after Solar noon. Effectively, the Sun stops overhead for several weeks when Mercury is at perihelion.

In 2012 NASA confirmed the existence of permanent ice on Mercury; this is possible because Mercury has the least axial tilt of any planet. (This is the same reason the coldest observed temperatures in the solar system are on the Moon.)

John Bevis is the only person known to have observed a planetary occultation, seeing Venus occult Mercury for a few seconds in 1737 between gaps in the clouds.

The next visible occultation will be in 2079. There will be one in July 2067, but it is only visible from the North Pole, which will (A) be in sunlight and (B) be underwater. The previous occultation was in 1818.

More trivia: asteroids 13579 and 24680 are respectively named “Allodd” and “Alleven”. Asteroid 2309 “Mr. Spock” led to the IAU explicitly disallowing names after pets… Mr. Spock being the name of the discoverer’s cat.

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