As the moon waxes toward full this Sunday, the advancing line of lunar sunrise, called the terminator, reveals one of the moon’s brightest features, the crater Aristarchus (air-iss-TAR-kuss). This 25-mile-wide (40 km) hole was blasted out about 175 million years ago. That makes Aristarchus a relatively fresh crater compared to many we see through our binoculars and telescopes.
On the moon, bright splashes represent fresh material that’s been excavated by impact. Over time, radiation from the sun along with cosmic rays gradually darken lunar features. I know 175 million years sounds like a long time, but it wasn’t that long ago when it comes to the moon’s 4.5 billion year history. Aristarchus is named for the Greek astronomer Aristarchus of Samos who lived from circa 310-230 BC.
He was the first person we know of who suggested that the motions of the planets would make a lot more sense if the sun were at the center of the solar system. From ancient times up through the Middle Ages, most took as gospel that Earth occupied that exalted position. So it seems appropriate that Aristarchus’s name would be pinned to the shiniest place on the moon.
The crater is surrounded by a nimbus of bright rays, which formed when chunks of the moon crust shot skyward by the impact plummeted back to the moon to create a halo of secondary craters, all of which dug up fresh dust and rock from below that hadn’t seen sunlight in billions of years. Altogether, rays and crater are big and bright enough to see with the naked eye as a bright splotch from full moon through about last quarter phase. Binoculars make spotting Aristarchus a snap.
If you have a telescope, you can explore further. Not far from the crater, you’ll see a long, snaking gully called a sinuous rill called Schroeter’s Valley. The rill once acted as a giant conduit (it’s 6 miles – 10 km – wide at its widest point) for lava early in the moon’s history when it bubbled with small volcanoes, lava flows and lava fountains. One of my favorite features lies at the head of Schroeter’s Valley called the Cobra Head. No kidding, it really looks like one!
Aristarchus and friends sit atop a large rise called the Aristarchus Plateau, literally one of the few places on the moon where you can see a color other than the shades of grey. To my eye, this volcanic region has a peculiar olive-brown hue. It’s subtle but quite different from the surrounding landscape and due in part to the greenish mineral olivine, also called peridot and a common rock of Earth’s upper mantle.
There’s always something fascinating to look at on the moon. Aristarchus is one of those craters that rewards no matter the instrument.