Like a middle-aged couch potato the moon’s been developing a paunch, leaving behind its delicate crescent figure and swells into a bulgy 8-day orb. Ah, but there’s beauty in that bulge. It’s filled with thousands of large, overlapping craters many of which you can see in a pair of binoculars magnifying from 7x to 10x. Just point, focus and marvel.
The top half of the moon displays fewer craters but more lunar seas or “maria” (MAH-ree-uh). These large, dark splotches are actually basins excavated by good-sized asteroids that filled with magma from below 3-3.5 billion years ago. The white areas of the moon, called the highlands, compose the original crust that formed nearly 4.5 billion years when the mostly-molten moon began to cool. It bears the record of billions of years of impacts by meteoroids of all sizes. That’s what’s on display tonight through Thursday.
You can see this battering in binoculars best right now. There are two reasons why. A large section of the highlands faces us squarely at the same time the sun is rising there. The low sun angle makes every crater, craterlet, mountain and cliff cast long shadows that to create a dramatic moonscape of shadow and light. The view in a small telescope will blow your socks off. Closer to full moon, the sun shines on the same craters but at a much higher angle. Shadows are so short, there’s little contrast between high and low spots, so everything looks flat and pasty. Lunar sunrise and sunset are the best times to view surface detail on the moon.
So keep an eye on the terminator — that’s the expanding curve along the left (eastern) side of the moon. During the moon’s waxing phases it marks the line of lunar sunrise. After full moon, the terminator defines the line of advancing sunset.
Meanwhile, on the far side of the the Chinese Chang’e 4 lander and Yutu-2 rover are preparing for another long lunar night. Nights last about two weeks and get very cold — the rover has recorded temperatures as low as –310° F (–190° C). News of the mission comes in dribs and drabs, so there’s been little about its progress since May 28. At that time, the equipment had made it through five lunar nights since landing in the farside Von Kármán crater on Jan. 3, 2019 and was ready to begin its 6th lunar day. Future updates can be found at #ChangE4.
The crater lies within the South Pole-Aitken Basin, the largest, deepest and oldest impact basin on the moon and one of the biggest in the solar system. SP-AB is about 1,600 miles (2,500 km) across and 8.1 miles (13 km) deep and one of the best places on the moon to sample rocks from the lunar mantle, the deep layer of rock that lies beneath the crust. None of the Apollo missions collected mantle material, but the Yutu-2 rover identified rocks earlier this year with a composition different from the crust. Light reflected from the minerals revealed signs of olivine and pyroxene, materials expected to compose the mantle. As of late April, the Yutu-2 rover had traveled 587 feet (179 meters) from the lander.
Lunar phases on the near side and far side are complimentary. When we see a new moon, the far side is full. When we see a half-moon in the evening sky, the opposite half of the far side is lit up. And when we see a full moon, the far side is in new moon phase. Since the lander-rover lie along the mid-point of the lunar far side, the sun sets there about the same time we see a half-moon. Tonight, the moon will be 8-days-old with the rover and lander sliding into shadow at the start of the next long lunar night. Like two bears, they’ll remain in hibernation before waking up two weeks hence to start another day.