365 Days of the Moon -- August 30, 2014

Proclus and bow tie rays

A bow tie on the Moon! The target of today's 365 Days of the Moon is not what you think. The broad unwinking black eye of Mare Crisium is simply a bystander to the real point of interest, the 17-mile wide crater Proclus. OK, there are a lot of small craters here. Which one is Proclus? Look for what looks unusual and out of place...like the strange ray system on the western rim of Mare Crisum. Part of the ray system is missing. This is what is called a bow tie ray system.

Ordinarily, meteor strikes create round craters regardless of what angle they strike the ground, and ray systems are more or less symmetrical around the crater. This is because the crater is not punched into the ground, but is created by the explosion of the meteor vaporizing from the heat of a hypervelocity impact. This process generally still holds true then the impactor arrives at an extremely shallow angle. The twist is oblique impacts alter the resulting crater ray system. The lower the impact angle, the larger the “zone of avoidance” in the direction the meteor came from. Thus highly oblique impacts create the bow tie effect where no crater rays extend in the direction the impactor arrived from.

Image taken with a Celestron 11 Edge HD and a Skyris 274M camera.

Image copyright Robert Reeves 2014

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