The week of August 16-22 takes us from Lunar Day 9 to Day 15. This week we will highlight the moon crater Eratosthanes, viewable Monday night.

Eratosthenes on the moonEratosthenes: [NW/H8; L=11°W] A large (36-mi.) complex moon crater with terraced walls, central peaks, and a thick ejecta blanket. Halfway between Eratosthenes and Copernicus you will find conspicuous lines of secondary craters arcing around Copernicus (or where you estimate Eratosthenes crater on the moonCopernicus would be, since it is still just beyond the terminator tonight). These are some of the best secondary craters on the Moon 1.

1Eratosthenes: (275-195 BCE) Greek astronomer, mathematician, and chief librarian of the Great Library in Alexandria, a repository of knowledge that had not been rivaled until the creation of the Library of Congress!

At one point Eratosthenes ingeniously calculated the circumference of the Earth using deductive reasoning that is within the grasp of any clever high school student today. At the summer solstice in Alexandria, the Sun, at its highest point, cast a shadow that had an angle of 7° 12′. Eratosthenes had heard that at noon on the same day in Syene, a city that was due south of Alexandria, the Sun shown directly down a deep well without casting a shadow. He realized that these two facts placed him on the verge of a monumental discovery. He hired a man to pace off the distance between Alexandria and Syene, which came to be 5,000 stadia (about 500 miles). Since 7° 12′ is one-fiftieth of a circle, Eratosthenes calculated the circumference of the Earth to be 25,000 miles. He was only 100 miles off!

Eratosthenes greatest accomplishment was the library at Alexandria, which, at its peak, contained nearly one million books in the form of papyrus scrolls. He had a rather clever way of making sure he had the largest collection of books in the world: Whenever a ship came into port, Eratosthenes arranged with the King to have it detained until his army of scribes copied all the books that were on board! The books were then returned. The library was tragically burnt to the ground and Western civilization lost one of its greatest treasures.

There are several theories about who was responsible for the fire. Some say it was Julius Caesar who, on a campaign to Greece in 47 BCE, set fire to Greek ships in the harbor. The fire then inadvertently spread to the city and the library was destroyed. Another theory was that Theophilus, the Bishop of Alexandria, had become intolerant of the pagans’ growing influence and ordered a mob to destroy the Serapis temple (which also served as a sister library to the main one in Alexandria and contained 42,000 scrolls).

One of the more interesting theories is that Muslim armies attacked the city in 645 CE and Caliph Omar, when he heard that the library contained “all the knowledge of the world”, is reputed to have said of its holdings, “They will either contradict the Koran, in which case they are heresy, or they will agree with it, so they are superfluous.” As a result he ordered the library destroyed.


It is highly recommended that you get a copy of Sky and Telescope’s Field Map of the Moon, the very finest Moon map available for use at the telescope. It is available for $10.95 at and on Amazon. All features mentioned in this blog will be keyed to the grid on the Field Map and will look like this: Plato: [NW/D9]

Courtesy of Gray Photography of Corpus Christi, Texas
Lunar photos: NASA / USGS / BMDO / LROC / ASU / DLR / LOLA / Moon Globe. Used by permission

Andrew Planck
Eratosthenes – Complex Moon Crater

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