Over the past couple of weeks, the Sun has been the star (haha! Ha! I kill me) of the sky, what with the eclipse and all. If you somehow missed this, then feel free to see some pix and video I posted, and read about the feelings I experienced leading up to and totality.
But the Sun’s eclipse understudy, and without which we’d have no eclipse, is the Moon. So, I thought it would be nice to feature it in an article...but I also like to shake things up a little, so I thought it would also be fun to show it in a way that’s pretty flippin’ weird.
That is not a photo of the Moon; at least, not really. It’s an image of the Moon made using radar! Astronomer Bruce Campbell (no, not that Bruce Campbell) used the huge 300-meter Arecibo radio telescope in Puerto Rico to bounce radar waves off the Moon, and used the also quite large 100-meter Green Bank Telescope in West Virginia to receive the reflected pulses.
Campbell has a video explaining how this works and why these sorts of radar maps of the Moon are made:
The resulting image is not quite what you’d see with your eyes. When we look at the Moon, we see sunlight reflected off it. Some parts of the Moon look dark (like the maria, large relatively flat lava-filled plains) and some bright (usually younger craters) depending on how much light from the Sun they absorb or reflect.
The radar map is similar, but has some striking differences. For one thing, this kind of radar tends to penetrate the lunar surface from a few centimeters to several meters. That means that some features obvious to the eye are completely absent from the radar image; rays — linear plumes of material ejected from craters after impact — don’t show up at all in the radar image, even though they are one of the most obvious features in visible light. The darker maria just fade away into the average dull grey of the surface, too, removing essentially all the easy-to-spot landmarks.
Fresh craters (well, ones that are less than a billion years old or so) tend to have lots of churned up material in and under them, so they reflect radar really well, and appear bright. You can see many of them in the image. But it’s funny: If you had handed me this shot and asked me to identify the features, I’d have had a really hard time! The caption in the press release mentioned that the bright crater in the upper right (at about the 1:00 position near the Moon’s edge) is Theophilus, and that helped immensely in me getting my bearings. As it was, it still took quite a bit of effort.
Using the Lunar Reconnaissance Orbiter interactive map at Arizona State University I was able to identify lots of other craters. The bright crater farthest to the right is Langrenus. Just to the left of it (and identifiable by the huge crack running inside it) is Petavius; the crack points to the crater Stevinus to its upper left. At the bottom right, half lit and half in darkness, is Humboldt. I am not 100% sure, but I’m fairly positive the fuzzy crater all by itself on the left side of the Moon is Tycho. That’s funny; when the Moon is full Tycho is supremely bright and obvious, with a ray system that extends for over 1500 km! But with radar eyes, it’s just another semi-bright feature.
Another strange thing about this image is the orientation. It looks like the half-lit Moon, right? But that’s deceiving. Remember, it was being illuminated by a radar beam sent from Earth. The data were then remapped so that the top edge of the image is actually the center of the Moon as seen from Earth! I know, that’s disorienting, and believe me it took a while for me to wrap my brain around this.
But I can prove it: See the craters near the top of the image, three in a row like an upside-down snowman, and another off to the side? Those are actually right near the center of the Moon as seen from Earth! The big one at the top is Ptolemaeus, then Alphonsus in the middle, then Arzachel (the one to the side is Albategnius). If you look on the LRO map you’ll see them just below the Moon’s mid-point.
It gets odder. At first glance, you might think the ragged line at the bottom of the radar image is the day/night line, called the terminator. But it’s not. It’s the edge of the Moon as seen from Earth! Everything below that line is physically around the limb of the Moon on the far side and so can’t be seen by the radar.
I know. It hurts the brain. But I promised you weird. Well, you got it.
Like the Sun, the Moon is such a fixture of the sky that we sometimes ignore it. But we should give it its due. If we don’t, well, then sometimes it can be a jerk about it.