Tonight the sun rises over Copernicus, one of the most magnificent craters on the moon. Unlike the gaping potholes waiting to jar your car this season, a smooth ride is guaranteed. At 58 miles (93 km) across and set off by itself like an old oak tree in a big field, it’s easy to see even in binoculars as a bright ring alongside the terminator. The terminator marks the division between day and night on the moon. From new moon to full, as phase of the moon increases, it’s the advancing line of sunrise. After full moon, when the moon’s phase wanes, it’s the advancing line of night.
It’s that low, slanting sunrise light that makes Copernicus stand out so wonderfully against the semi-dark moonscape. Long-shadows and starkly-lit rock formations like crater walls and peaks create a scene of dramatic contrasts. Copernicus is a relatively fresh crater that formed when a small asteroid struck the moon about 800 million years ago. The heat from the impact melted the local rock, covering much of the crater’s floor in “impact melt” visible to this day. Through a small telescope, look at the floor’s northwest quadrant, and you’ll see right away that it’s smoother — almost paved over — compared to the rest of the interior.
Several peaks poke up from the center of the crater that formed when crust compressed by the monster impact relaxed and rebounded afterword. The tallest reach stand some 2,950 feet (900 meters) high. But the real eye-catcher at least for me are its wonderful, stair-like terraced walls especially when viewed around sunrise like tonight. When larger craters like Copernicus forms, they have steep rims. Over time, the force of gravity causes the rock to slump and avalanche, forming a series of terraces and giant slump-blocks.
Even in binoculars you can tell that Copernicus is surrounded by feathery streaks of lighter material. These are its rays, and they extend for 500 miles (800 km) around the crater. Rays form from secondary impacts, when rocks blasted from the impact rain back down on the moon and punch out smaller craters. These tens of thousands of mini-craters excavate “fresh” lunar dust and soil. Cosmic rays and the solar wind gradually darken lunar soil, so the fresh material is much brighter in contrast to the older soils covering the area there before Copernicus formed.
There are many landforms near Copernicus to explore including Erastothenes Crater and two mountain ranges — the Apennines and Carpathians. The moon’s up from twilight till past midnight tonight, so anytime you feel like looking up, enjoy! If it’s cloudy, you’ll still see Copernicus well into next week though the lighting won’t be as pretty as tonight.