See Dazzling Aristarchus, The Moon’s ‘Searchlight’

This simulation shows how the moon will look tonight (March 10) when Aristarchus comes into view along its upper left (northeastern) edge. The crater is located not far from the terminator, the border between lunar day and night. Binoculars will easily show it, one of the brightest spots on the moon. Credit: Virtual Lunar Atlas

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.

View of the Aristarchus and its neighbor crater, Herodotus, photographed from lunar orbit through the window of the Apollo 15 command module. Herodotus is more eroded and more ancient than Aristarchus with a lava and debris-filled floor. Schroeter’s Valley (at right) is a feature called a sinuous rill. Hot lavas from beneath the lunar crust once coursed between its banks. All these features sit atop a large dome-like land feature called the Aristarchus Plateau. Credit: NASA

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.

Overhead view of Aristarchus photographed by NASA’s Lunar Reconnaissance Orbiter (LRO) reveals beautifully terraced walls caused by slumping or collapse of the material after the impact. The white arrows show the corners of the panorama pictured below. Credit: NASA/GSFC/Arizona State University

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.

We’re looking from the side in this panoramic view of the west wall of Aristarchus taken by the LRO. It shows melted rock deposits, bright exposures of the mineral anorthosite (part of the moon’s original crust) and dark streamers of volcanic ash. Anorthosite also helps to brighten the crater. Full width of panorama is about 15.5 miles (25 km). Credit: NASA/GSFC/Arizona State University

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 will appear bright tonight but even more brilliant — almost beacon-like — at full moon. The olive-brown colored of the surrounding region, known as the Aristarchus Plateau, is also best seen near full moon, which this month happens on the 12th. Also seen in the photo are Herodotus (right of Aristarchus) and Schroeter’s Valley. Credit: Ole Nielsen

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.