We’ll have an awesome moon standing up in the south at twilight this evening. It’s in waxing gibbous phase between half and full and spending the night in Sagittarius the archer. If you’ve got a few minutes and pair of binoculars, some really nice craters and seas will be in view. Even 7x binoculars will show the larger craters along the lunar terminator. This is the line dividing lunar day from night. Tonight’s terminator defines the curved edge (left side) of the moon and represents the advancing line of lunar sunrise.
As the moon’s phase waxes or increases, the terminator moves to the left (east), exposing more and more of the shiny disk until we see the complete circle at full moon. After full moon, the terminator becomes the advancing line of lunar sunset and moves to the right (west). Along and near the terminator, craters and mountains cast shadows in the sun’s slanting light, revealing crisp outlines and other features in detail. This is where you’ll want to focus your gaze as you examine the moon with binoculars.
Toward the bottom or southern portion of the moon, you’ll see lots of craters but one will stand out as larger than the rest — Clavius. Clavius, named for Jesuit priest Christopher Clavius, a 16th-century German mathematician and astronomer, is 140 miles (225 km) in diameter and 2.2 miles (3.5 km) deep. At freeway speeds, it would take about 2½ hours to drive from one end of the crater to the other.
Clavius has lots of company. The southern half of the moon is jammed with craters but the northern half not so much. Why? Happenstance. Larger asteroid-sized impacts excavated huge basins across the central and northern parts of the moon which later filled with dark-hued lavas, erasing much of the older lunar crust. We see these giant impacts as the dark spots, better known as the lunar “seas”, that make up the face of the man-in-the-moon.
The southern section remains as the much-bombarded, original crust of the moon aged 4.5 billion years. Astronomers call these regions, which appear white to the naked eye, the lunar highlands. The seas by contrast are about a billion years younger.
Working our way up from Clavius we encounter Mare (MAH-ray) Nubium (Sea of Clouds) and then the magnificent crater Copernicus, astride the terminator. Copernicus, named for the Polish astronomer who shattered the old Earth-centered universe paradigm in 1543, is 58 miles (93 km) across and 2.4 miles (3.8 km) deep.
You’ll notice a pale white coloration to the right (west) of Copernicus — these are rays and represent material expelled by the impact that excavated the crater. All that spinning, tumbling rock fell back to the surface moments later and dug out thousands of smaller craters in a ray-like pattern. Splat!
The lunar topsoil or regolith is dark from bombardment by the solar wind, cosmic rays and micrometeorites. These cause iron in lunar soil to melt and vaporize and produce a dark coating around the surrounding minerals. When all those hunks of lunar crust came raining down, they broke through the surface and exposed lighter, less weathered dust and rock, the reason the rays are so much brighter than their surroundings.
Keep going up from Copernicus and you’ll soon arrive at the moon’s outdoor swimming pool, Plato. Of course, there’s no liquid water on the lunar surface, but the crater’s oval shape and dark color resemble a swimming pool or lake seen from above. Plato is 68 miles (109 km) across and just 0.6 miles (1 km) deep. Why so much shallower than our other featured craters? Similar to what occurred to create the lunar seas, lava flooded up from below to partially fill Plato. That also accounts for its dark color is caused by a high iron content in the lava. Exactly like the lunar seas.
There’s much we can learn about the moon’s evolution with just a pair of binoculars. I encourage you to follow our satellite through its phases and identify additional craters and seas. If you don’t have a map, no problem. Just download a copy of the Virtual Lunar Atlas by Christian Legrand and Patrick Chevalley and you can see what to look for any night of the year.