The word
crater was adopted from the
Greek word for "vessel" (, a Greek vessel used to mix wine and water).
Galileo built his
first telescope in late 1609, and turned it to the Moon for the first time on November 30, 1609. He discovered that, contrary to general opinion at that time, the Moon was not a perfect sphere, but had both mountains and cup-like depressions. These were named craters by
Johann Hieronymus Schröter (1791), extending its previous use with
volcanoes.
Robert Hooke in
Micrographia (1665) proposed two hypotheses for lunar crater formation: one, that the craters were caused by projectile bombardment from space, the other, that they were the products of subterranean lunar
volcanism.
Scientific opinion as to the origin of craters swung back and forth over the ensuing centuries. The competing theories were: •
volcanic eruptions blasting holes in the Moon •
meteoric impact • a theory known as the
Welteislehre developed in Germany between the two
world wars which suggested
glacial motion creating the craters.
Grove Karl Gilbert suggested in 1893 that the Moon's craters were formed by large asteroid impacts.
Ralph Baldwin in 1949 wrote that the Moon's craters were mostly of impact origin. Around 1960,
Gene Shoemaker revived the idea. According to
David H. Levy, Shoemaker "saw the craters on the Moon as logical impact sites that were formed not gradually, in
eons, but explosively, in seconds." Evidence collected during the
Apollo Project and from uncrewed spacecraft of the same period proved conclusively that meteoric impact, or impact by
asteroids for larger craters, was the origin of almost all lunar craters, and by implication, most craters on other bodies as well. The formation of new craters is studied in the lunar impact monitoring program at
NASA. The biggest recorded crater was caused by an impact recorded on March 17, 2013. Visible to the
naked eye, the impact is believed to be from an approximately meteoroid striking the surface at a speed of . In March 2018, the discovery of around 7,000 formerly unidentified lunar craters via
convolutional neural network developed at the
University of Toronto Scarborough, Canada was announced. A similar study in December 2020 identified around 109,000 new craters using a
deep neural network. ==Characteristics==