With the moon turning full tomorrow, let’s take a look at one of its brightest craters, Aristarchus (Aris-TAR-kess). Although only 25 miles in diameter, it’s bright enough to see with the naked eye and positively dazzling in a telescope. The name suits its appearance perfectly. Aristarchus was the Greek philosopher who first proposed the concept of a sun rather than Earth-centered solar system. To this day the crater celebrates this brilliant and ultimately correct idea.
The reason for its radiance has to do with its relative youth. Aristarchus was excavated by a huge meteorite impact “only” 450 million years ago. You’d almost call that recent in lunar years especially considering that most of the moon’s craters are some nine times older, having formed nearly 4 billion years ago. Sunlight and solar radiation darken the lunar surface over time; Aristarchus hasn’t been around long enough for that to happen.
You can see the crater with you unaided eye as a small bright spot in the large dark region that covers much of the eastern half (left side) of the moon called Oceanus Procellarum or Ocean of Storms. It helps that Aristarchus contrasts so well against the dark moonscape. If you have any difficulty seeing it, whip out a pair of binoculars for a second look. Even 5x will bring the shimmering spot into view.
Aristarchus sits on a 125-mile-wide plateau that rises a little more than a mile (at maximum) above the vast “oceanic” plain. The rise is riddles with cracks called rills which long ago carried lava from beneath the moon’s crust into the basin that is now Oceanus Procellarum.
The biggest rill and easiest to see in a small telescope is named Schroeter’s Valley. It begins near the crater Herodotus and winds through the plateau before fading out at the edge of the great “ocean”. If you catch the lighting right, the rill’s as spectacular as the crater. Within its shadowy curves, it’s not hard to imagine flowing lavas cutting a path through the region and emptying into the vast depression surrounding the plateau. As for the plateau itself, it was lifted to its present height by magmas that welled up from beneath it.
While quiet now, the moon once grumbled and roared with vulcanism.
Before we leave Aristarchus, we have to talk about color. Drab grays, whites and the palest of browns tint most of the lunar surface. No so with the plateau. Through a small telescope it has a distinct olive or yellow-brown tinge caused by a 4-12 inch covering of volcanic ash – more evidence of the wild and woolly moon of long ago. See it best around full moon phase.
I like to imagine standing on the Earth 3 billion years ago, watching the flash of crater impacts and glowing lava flows on the lunar nightside through my little telescope. I’d never be able to take my eyes off our companion world. Frankly, I still can’t.