Hey, Sky Fans! Welcome back! It’s been a while, I know. Sorry to have left you out in the dark for so long. I’ve been working on a couple of other projects that have slowed me down a little, and this cold just won’t quit.
Earlier this month, I was out looking at the sky. It was cold, but not too cold, and windy, but not too windy. I had my binoculars with me so I could get a bit of a closer look at the Pleiades (Messier 45). The moon was up. It was right around first quarter, so the right-hand half as we see it from here on Earth was lit, while the left-hand side was invisible, pulled into the night. I like to see the moon’s phases and imagine what’s happening on the far side of the moon; what it would be like to be 240,000 miles or so on the other side. The first- and third-quarter phases are particularly easy to do this with, because it’s half the moon that’s lit. So, left becomes right, right becomes left and then you let your mind fill in the details.
That brings us to this week’s word of the week, terminator. No time-travelling robots here. The terminator is the line, the point, seen by a far-off viewer, on an object where day becomes night, where day terminates. If ever there was an astronomy thing that I was glad to learn there was a word for, terminator is it. It’s not a tough concept, no, but I like that there’s a word for it.
The terminator, while not the cause of the moon’s phases is sort of what gives them their… phasiness. After all, the moon’s phases are just differences in the amount of daylight we see on the moon’s nearside, the side facing us. As the Sun-Earth-Moon line changes, the amount of daylight we see from here changes, the terminator moves, and, over time, we have the pattern of phases. If you have young kids, mermaids are involved, too. Don’t forget the mermaids.
From here on Earth, day doesn’t just become night. It’s not like it’s bright and sunny in one spot, and then you can hop to left a bit and be in total darkness. Thanks to our atmosphere’s sunlight-scattering skills, the sky doesn’t just go dark the instant the sun is fully below the horizon. It slowly fades. An open sky darkens from blue. Different wavelengths of light scatter differently at different times, and different colors appear and disappear. It’s often still very bright, with plenty of light to see by, after the sun has set.
From a distance, though, as the Apollo astronauts saw, there was a hard line separating where it was day from where it was night on Earth. That line that they saw when they looked at Earth, and you see when you look at the moon, that’s the terminator. The other half disappears into the night.
Here’s Earth’s terminator, seen from the astronauts aboard Apollo 8 in 1968:
This isn’t just an Earth and Moon thing, though. Anywhere there’s day any night, that is, anywhere part of an object is facing toward a bright light source, and part of that same object is facing away, there’s a day side (facing toward) and a night side (away). The line at the end of the daytime side is the terminator.
I’m going to miss Cassini when its mission ends later this year. Here you can see Saturn moons Enceladus (above the rings) and Rhea (below), and their terminators:
And here’s this orange’s terminator:
Most of us don’t really think of an orange as having a day side and a night, but it does. My top notch photo skills prove it.
So, there you have it. The next time you see the moon or any of those gorgeous photos from Cassini, or an artistically shadowed orange, you’ll be looking at those thing’s terminator, where day becomes night.
Clear skies, everyone!