thumb|The crater [[Webb (crater)|Webb, as seen from Lunar Orbiter 1. Several smaller craters can be seen in and around Webb.]]
thumb|Side view of the crater [[Moltke (crater)|Moltke taken from Apollo 10.]]
Lunar craters are impact craters on Earth's Moon. The Moon's surface has many craters, all of which were formed by impacts. The International Astronomical Union currently recognizes 9,137 craters, of which 1,675 have been dated.
History
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."
alt=Lunar craters as captured through the backyard telescope of an amateur astronomer, partially illuminated by the sun on a waning crescent moon.|thumb|Lunar craters as captured through the backyard telescope of an amateur astronomer, partially illuminated by the sun on a waning crescent moon.
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. Smaller craters than this are being regularly formed, with a recent NELIOTA survey covering 283.5 hours of observation time discovering that at least 192 new craters of a size of were created during the observation period.
Lunar crater categorization
In 1978, Chuck Wood and Leif Andersson of the Lunar & Planetary Lab devised a system of categorization of lunar impact craters. Barlow is also creating a new lunar impact crater database similar to Wood and Andersson's, except hers will include all impact craters greater than or equal to five kilometers in diameter and is based on the Clementine spacecraft's images of the lunar surface.
The Moon Zoo project within the Zooniverse program aimed to use citizen scientists to map the size and shape of as many craters as possible using data from the NASA Lunar Reconnaissance Orbiter. However, it has since been retired.
Names
Craters constitute 95% of all named lunar features. Since 1919, assignment of these names is regulated by the International Astronomical Union.
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