You can use this month’s focus on the Moon to learn more about lunar features. The most prominent lunar features, as seen from Earth, include impact craters, basins, and mountains.
Lunar Craters: Lunar craters result from the collisions of meteorites and comets with the Moon’s surface. There are many hundreds-of-thousands of craters on the lunar surface, ranging in sizes from mere millimeters to the largest, the South Pole-Aitken Basin, at 2,500 km. About three-hundred thousand craters larger than one-kilometer adorn the near-side of the Moon. The most prominent of those are Tycho and Copernicus.
Larger craters appear to have streaks, or rays, extending from their outer edges, like spokes surrounding the center of a bicycle wheel. These features are the result of lunar material ejected by an impact. Better-known ray systems include those surrounding Craters Tycho and Copernicus, as well as Aristarchus and Kepler.
Catenas, or crater chains, are another interesting lunar feature sometimes associated with craters. These occur when weaker incoming objects are fragmented by tidal forces before impacting. The comet ShoeMaker-Levy 9, which struck Jupiter in 1994, is a good example of an object being first fragmented by tidal forces and then impacting as several individual objects to form a chain of craters. The Moon’s Davy Crater Chain, is believed to be the result of such an event.
Interestingly, not only does the Moon’s near-lack of atmosphere contribute to its higher impact rates, those impacts ultimately contribute to what little lunar atmosphere there is, through a process known as sputtering. When atoms released by an impact are not fast enough to escape into Space, they become gravitationally bound within the lunar corona. This process is similar to that which contributes to the formation of planetary rings.
Lunar Plains/Planitia (Maria): This graphic names the better-known lunar maria, as seen from Earth.
Maria appear as darker splotches on the lunar surface. These are lower altitude regions, or basins, that are layered in solidified basaltic lava from ancient volcanic eruptions. Although the name maria means seas, these basins were likely never filled with water. Closer inspection will reveal many additional features within and along maria, including dorsa (wrinkle-ridges,) craters, and rilles (long grooves or canyons.) Measuring seventy-five miles long and up to nine-hundred-fifty feet deep, Hadley Rille is one of the largest on the moon and marks the landing site of Apollo 15.
Lunar Mountains (Mons): While Earth’s mountains are created by plate tectonics, lunar mountains result from impact events. These landforms peak as high as five kilometers, the largest of which typically border maria. The tallest lunar mountain is Mons Huygens, of the Appenninus Mountain Range and near the aforementioned Hadley Rille and the Apollo 15 landing site.
As the moon wanes from Full to New this month, take advantage to explore the lunar surface. To help you make the most of your observing sessions, I have some free lunar charts noted here (near the bottom of the post) and some information about lunar phases, here!