Lunatic’s Corner: Crater and Rima Calippus
June 2023 :
Calippus was a Greek mathematician and astronomer who is believed to have lived about 370-310 BC. He was a student of the School of Eudoxus and the foremost mathematician of his time. Though originally from the city of Cyzicus (modern Turkey), he is known to have been working with Aristotle in Athens by around 330 BC.
One of his greatest accomplishments was the development of the Calippic Period that has been used by astronomers ever since. The Calippic Period is a 76-year, 940-month cycle that brought the solar and lunar years into alignment and introduced the 365.25-day year. The Calippic Period became a standard for correlating observations over many centuries and therefore has contributed to the precision of later astronomical theories.
He also refined and improved the accuracy of Eudoxus’s theory of explaining the motion of celestial bodies using spheres. Calippus added six additional spheres to the system. By adding two more spheres to the Sun, Calippus was able to account for the inequality of the seasons owing to the Sun’s variation in velocity during the year (what we now know as the Earth’s variation in velocity according to Kepler’s Laws of Planetary Motion).
Crater Calippus can be found in the northeast quadrant of the Moon, on the eastern edge of Montes Caucasus (Caucasus Mountains). The Montes Caucasus themselves are located on the northwest rim of Mare Serenitatis (Sea of Serenity) and the eastern rim of Mare Imbrium (Sea of Showers), close to the center line of the Moon.
Just to the southwest of the crater, on the floor of Mare Serenitatis, is Rima Calippus, a sinuous rille that travels for about 40km from southwest to northeast. Sinuous rilles are thought to be the result of lava flows either flowing on the surface or from collapsed lava tubes.
As a small, simple crater, one would expect the 20-mile wide Calippus to have a bowl shape with a smooth floor. However, Calippus has some very irregular features. It is shaped somewhat like a polygon, with a protrusion and slumping material on the western wall. The walls are steep and surround a very rough floor, but it does not contain the central peaks and terraced floor characteristic of larger, complex craters. The irregular features may be attributed to its impact location on the eastern slopes of the mountains and not on a flat plain.
Both features are visible around Lunar day 5 and Lunar day 19. This is an interesting part of the Moon’s surface where mare meets montes and many interesting features are observable even with small telescopes. Put crater and rima Calippus on your list of lunar targets and you won’t regret it.
Michael Corvese is a confirmed lunatic of many years regardless of his recent interest in lunar observing.
