The Moon rises late this weekend as it approaches last quarter. As the shadows fall long across the central meridian of the Moon, this is an excellent time to see the ancient and striking crater Plato, which has tantalized astronomers with rare and unexplained mists and flashes of light on its dark, lava-covered floor. (Note: This article is adapted from our upcoming course Around the Moon in 28 Days: Lunar Observing for Beginners, available next week at Stargazer University.)
Plato lies in the northern reaches of the Moon’s face, on the edge of Mare Imbrium (the Sea of Rains), and just south Mare Frigoris (the Sea of Cold).
The crater Plato on the “shores” of Mare Frigoris and Mare Imbrium (click to enlarge)
It’s a big crater, spanning some 100 km. But unlike many large impact craters, it lacks a rough floor and jagged central peak. That’s because the crater is ancient, about 3.84 billion years old, and was long ago filled in by a huge flood of lava caused by volcanic activity or an impact event in the early days of the Moon.
Plato is a favorite for backyard astronomers. It’s easy to see in a telescope… even in binoculars. It’s beautiful, especially at first and last quarter Moon, when the rim of the crater casts long shadows at lunar sunrise and sunset. And it’s a good visual challenge to see the many tiny craters on its otherwise smooth floor (see image at top of page).
Despite its dark appearance, or perhaps because of it, Plato is well known as a home for so-called “lunar transient phenomena” such as flashes of light, mists, and unusual colors that have been seen over the years. Most such observations were made in the 19th century, though on October 9, 1945 an observer sketched and reported “a minute, but brilliant flash of light inside the western rim. Lunar Orbiter 4 photos later showed that a new impact may have occurred at this location.
No matter how many times you observe this crater, it is ever changing and worthy of your attention. Go have a look for yourself… Plato is a fun place to power up with a telescope and contemplate.