Hey, everyone! Thanks for spending some of your Wednesday here! And, before I forget, a happy 453rd birthday to Galileo Galilei. With his early telescope, he was the first person to see Jupiter’s four big, conveniently named, Galilean moons (Io, Ganymede, Callisto, Europa), and saw Venus’s phases. This opened the door for him to get behind heliocentrism: the idea that the earth isn’t the center of the universe. No, the sun is at the middle of the solar system and everything is orbiting it, not earth. Happy birthday!
Did you see all the astronomy news last week? There was a lot of it, right around last week’s full moon, lunar eclipse, and appearance of comet 45/P, wasn’t there? Lots of people and, presumably fake, news outlets were talking about what was advertised as a triple threat, which seems like sort of an aggressive way to talk about far-off piles of rocks that couldn’t possibly have any less interest in the people looking at them.
It might sound like sacrilege, but I kind of wonder if sometimes these news stories are counterproductive. You’re right, you’re right, you’re right. “Anything to get people to look up” is usually my big philosophy , right after “socks first, then shoes.” This time, though, the news was about a full moon, which happens every month, a lunar eclipse, but a minor one, and a comet that would be hard to find, tough to see without binoculars, and at its best very late at night. I wonder if having to deal with the inherent obstacles of seeing the eclipse and comet would actually turn people away from the sky. To quote the Who, “Won’t be fooled again!”
It’s too bad, though. With the exception of the real big announcements, things like gravitational waves, astronomy, at least this version of it, is a pursuit for the mellow and patient. Things happen slowly and subtly. It’d be a shame for people to look away because they couldn’t find this big thing or that, when there’s often another one on its way before too long.
The other day, at the supermarket with the good beer selection, I got into a conversation with a guy I know and his son about what was, at the time, that night’s eclipse, and they had two big questions for me about them. I’m always happy to talk about astronomy, but talking about astronomy in the beer department? Hot dog!
First, more easily, why do lunar eclipses only happen at full moon?
That’s because Earth’s shadow is cast off into space into a reasonably small area that’s directly behind it (from the sun’s perspective, or the perspective of someone on the day side of the earth). So, the line goes Sun -> Earth -> Earth’s shadow. In order for the moon to be eclipsed, it needs to fall into that shadow, right? So, with a little bit of leeway, it needs to be directly behind the earth, too… where the shadow is. The “little bit of leeway” is where the penumbra is, as in this month’s penumbral eclipse.
The second question is a bit harder and brings us to our word of the week! Hoo boy!
The second question they asked about how eclipses work is also a really common one, one I’ve been asked before, “Okay, Mr. Smarty Pants, why isn’t there an eclipse every month?”
In order for an eclipse to happen, the moon needs to orbit its way into earth’s shadow. Right, we just covered that. Space is three dimensional, though. It’s not just left and right or forward and back. Sometimes things are up there, and other times they’re down…other there. As it turns out, the moon’s orbit around the earth is tilted relative to the Earth’s orbit around the sun by about 5 degrees.
You ever do the thing where you take two hula hoops, or two bracelets, and you nest one of them inside the other? Imagine one of those tilted by 5 degrees relative to the the other. If you hold one parallel to the ground and put a giant glowing ball of superheated gas in the middle, you’ve got a good model of the Sun-Earth-Moon system. The level one represents Earth’s orbit, the inclined one the moon’s.
A node is a point in one object’s orbit around another where its orbit crosses the plane of that second object’s orbit. So, the moon’s nodes are the points in its orbit where it crosses the plane of Earth’s orbit around the sun. In our hula hoop model, the nodes are two spots where the angled hoop crosses and touches the level hoop. What’s more, each node has its own name. The ascending node is the node at which, in our example, the moon is moving from south to north, upward over the earth’s orbital plane. The descending node, then, is the node at which the moon is moving downward, from north to south, below Earth’s orbital plane.
As seen from Earth, the plane of the earth’s orbit shows itself as the path the sun takes across the sky. The path is called the ecliptic, which got its name because eclipses can only happen when the moon crosses it during the full phase (lunar eclipses) or new phase (solar eclipses). More often than not, the moon misses; it’s not full or new at one of the nodes. If it were, we’d have 24 eclipses a year: 12 lunar, 12 solar. When the timing is just right, though, ka-pow, eclipse.
So, there you have it. Now, let’s see if we can use node in conversations three times today. I’ll catch up with you later. Clear skies, everyone!