Moonwatch - crater Eratosthenes
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NASA/GSFC/Arizona State University TYPE: Crater
SIZE: 58km (36 miles)
AGE: 3.2 billion years old
LOCATION: Latitude 145°N, longitude 11.3°W
RECOMMENDED EQUIPMENT: 4-inch telescope
Eratosthenes is one of the most perfect craters on the Moon. It is circular and extremely deep, with bright terraced walls. It is named after the Greek astronomer Eratosthenes of Cyrene (c. 276-195 BC), who carried out important mathematical work but is best remembered as having made a remarkably accurate measurement of the circumference of the Earth.
Crater Eratosthenes lies at the southern end of the Apennines, the most conspicuous mountain range on the Moon, with Mare Imbrium to the north and Sinus Aestuum to
the south. The shape of the outer rim is almost perfectly circular and beautifully terraced, and there is an outer rampart of ejecta. The central mountain group is lofty and complex.
the south. The shape of the outer rim is almost perfectly circular and beautifully terraced, and there is an outer rampart of ejecta. The central mountain group is lofty and complex.
When seen anywhere near the terminator Eratosthenes is a magnificent sight, but it is surprisingly elusive at full Moon, because it is swamped by rays from the relatively nearby Copernicus, and has no ray system of its own. By lunar standards it is young, and its formation 3.2 thousand million years ago marks the beginning of the ‘Eratosthenian era’.
There are no well-marked craters on its floor, but there are darkish patches that are easy to see even with a small telescope. Between 1919 and 1924 these patches were carefully studied by the American astronomer William Henry Pickering, who drew some unexpected conclusions. He believed that tracts of low-type vegetation existed, but found that the patches in Eratosthenes moved around: “While this suggestion of a round of lunar life may sound fanciful… it is based on the analogy of the migration of the fur-bearing seals of the Pribiloff Islands. The distance involved is about 20 miles, and is completed in 12 days. This involves an average speed of about 6 feet a minute which, as we have seen, implies small animals.”
"He believed that tracts of low-type vegetation existed, but found that the patches in Eratosthenes moved around"
It seems strange now to realise that less than a century ago, one of America’s most famous astronomers could write in this vein! Yet the patches in Eratosthenes do seem to change with the changing angle of solar illumination. Follow them through a complete lunation (lunar cycle) and you will see what I mean.
Perhaps Eratosthenes is at its very best at the time of sunrise, when the Apennines have come into view. Then Eratosthenes makes its entry, slowly and majestically; at first there is nothing inside but the tip of the central mountain complex, but the shadows creep back until the whole of the floor is sunlit. Not until later will the full glory of Eratosthenes be dimmed by the brightening of the rays from Copernicus, an even younger crater formed perhaps a thousand million years ago – the end of the Eratosthenian era and the start of the Copernican.
The area surrounding the highland upon which Eratosthenes stands is smooth, with only a few small craters, but there is one notable feature. Southwest of Eratosthenes, east of Copernicus, lies Stadius, one of the Moon’s most celebrated ‘ghosts’. It is about the same size as Eratosthenes, and must have once been a most imposing crater, but it has been so flooded with lava that its outline is now barely traceable, and only a few hills rise above the mare surface. There are plenty of ghost craters on the Moon, but not many are as distinctive as Stadius.
Under reasonably high illumination it is interesting to compare Eratosthenes with Copernicus. Copernicus is the larger of the two, and has a ray system, but otherwise they are remarkably alike.
This article appeared in the October 2009 issue of Sky at Night Magazine
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