Astronomical Society of Coonabarabran

Aristoteles and Mitchell

by Harry Roberts

I guess everyone has heard of Greek thinker Aristotle, so this piece is really about the much smaller crater overshadowed by giant Aristoteles, the crater named after US astronomer Maria Mitchell. I had often checked the crater pair for good lighting and "seeing" hoping to sketch Mitchell, and when the right conditions occurred I ended up sketching both. It was a challenge as the whole site is very complex. The pair is found in the northern sea, Mare Frigoris.

Crater Mitchell is clearly older than Aristoteles, and has been partly buried by the impact of the bigger crater. Mitchell dates from the Imbrium period, 3.85 to 3.2 billion years old, while Aristoteles is younger, of the Eratosthenian period 3.2 to 1.1 billion years. Note the huge age range of the latter era, over two billion years! I often wonder what was the source of all the impactors that the period lasted for so long? So, being older, Mitchell is soft and degraded, and it's almost hexagonal in plan. Aristoteles, by contrast, looks fresh and is surrounded by a huge debris field of ejecta from the crater. It was a marvellous view with the terminator only 12º west of the site.

Aristoteles is noted for its very large, complex terraces and slippage features that show well on the west side. The floor is lava flooded with two low peaks offset to the south side, and an irregular ridge connecting to the north wall.

Mitchell too has a pair of softer central peaks, and Aristoteles' east wall seems to have obliterated Mitchell's western rim. The most interesting thing about Mitchell is the person the crater commemorates, for Mitchell is one of the few craters named after a female astronomer.

Maria Mitchell (pronounced Ma-rye-ah) was an American astronomer who, while starting out as an amateur, achieved considerable fame, mainly as a science educator. While most of the twenty or so craters named for women were created in the twentieth century, many posthumously, crater Mitchell was so named in her own lifetime by Beer and Lee in 1865; a great honour.

So what do we know of Mitchell's career? She was born in Nantucket in 1818 and gained an early interest in astronomy while assisting her father William in observing eclipses and other events, including comet Halley in 1835. She had precocious skills in science and maths and read the astronomical "greats" in French and Latin. She assisted her father in making a formal coast survey involving much transit timing of stars. Inevitably, in 1847, she discovered a comet, and thereby won an international medal, that brought her much fame as well as international travel, greatly enhancing her astronomical contacts. Ultimately links with the early US women's movement led to the founding of an observatory at the newly created Vasser Female College where Mitchell was engaged to teach. This observatory became renowned through its early stellar and asteroid photography, a detailed story too long to tell here. Consult S&T May 1992 for a detailed bio. Mitchell's home is now a popular historic site as well as a working observatory and centre of research (readers can apply on line for an internship).

So take a look at Mitchell, one of the larger craters named for women of science. Admittedly it's only 30 km in diameter, while nearby Aristoteles is three times larger, but remember that all the larger craters were named in the 17th century by Riccioli after (mostly ancient) men of science and letters, quite a few of whom were more legendary than real. When it came time to redress the gender imbalance there were precious few unnamed craters left, and even "scientific giants" like Einstein and Newton lie almost hidden from view in the moon's libration zones.

Add Mitchell to your crater tour when next exploring the moon.

aristoteles_jpg