365 Days of the Moon -- August 09, 2014

Mare Imbrium wide view

      Mare Imbrium (Sea of Rains) is the classic prototype for maria that formed within an impact basin. It is obvious that circular Mare Imbrium is
      bordered by curved adjoining mountain chains. With hindsight, we now know these mountains are actually the rim of a impact basin. By definition, a
      lunar basin is any huge crater exceeding 300 kilometers across. Imbrium more than fits the bill. In the billion years following the creation of
      the Imbrium Basin some 3.8 billion years ago, the excavation was filled by hundreds of lava eruptions until it was smoothly paved over like we see it

      But this seemingly obvious explanation for this feature's origin was long in coming. It was 280 years after the perfection of the astronomical
      telescope before a geologist, not an astronomer, deduced the truth about the origin of Mare Imbrium. In the 1890's, Karl Grove Gilbert, chief
      geologist for the US Geological Survey, concluded that Imbrium formed by of a colossal collision on the Moon. But Gilbert's discovery was ignored
      for another half century until Ralph Baldwin independently came to the same conclusion in the late 1940's. Indeed, the universal acceptance of
      the impact origin of lunar features was not complete until the Apollo expeditions to the Moon. Sometimes the obvious takes a while to sink in.

This image is a montage of two images taken through a Celstron 11 Edge HD and Skyris 274M camera.

Image copyright Robert Reeves 2014

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