Walk on the Wild Side(February 20, 1999)
Although by now you'd think I'd be jaded, I still manage to be surprised by where Bad Astronomy turns up. The PBS television show ``Nature'' has been in production a long time, and does an outstanding job scientifically describing the world around us in an easy to understand and visually dramatic way. But even as fine a production as this can occasionally stumble.
Bill Jefferys, fellow astronomer and teacher at the University of Texas, emailed me some time ago telling me that the show had made an unusual statement in one episode titled ``The Wild Side of New York''. This episode was about the wild life of New York City, and they didn't mean the kind you hear about on prime time TV. it was actually about the wild animals that manage to thrive in the concrete jungle. I didn't get a chance to see it, but I ran into Bill at a meeting of the American Astronomical Society in his hometown. Bless him, he brought me a copy he taped himself. I brought it home and watched it, and about 45 minutes into it I came across the scene he meant. I was able to transcribe the narration, and I present it in full below. Read it and see if you can make sense of it. Afterwards, my dear Bad Readers, I'll talk about just where this show goes astray. Here is the full transcription. I take responsibility for any errors in transcription, but not for any errors in the astronomy!
[The story begins talking about the yearly cycles of animals, and
segues into a discussion about the length of the year.]
The fresh new year seems just like the last. But it's subtly different: it will be 1/50000 of a second shorter than last year, which was that much shorter than the one before.
As with most confused passages about astronomy. It's difficult to know where to begin! So I'll simply start at the beginning and work my way through.
Bad: The fresh new year seems just like the last. But it's subtly different: it will be 1/50000 of a second shorter than last year, which was that much shorter than the one before.
Each year, as we get older, we claim that the years go by a little faster. Well, they actually do. Surprisingly, it's all because of the Moon.
Good: The opening statement is almost correct, but not quite. The year is defined as the time it takes for the Earth to revolve around the Sun. It might surprise you to learn that there are different kinds of years, depending on how you measure the Earth's position! You can measure it relative to the stars, or to the Sun's position in the sky. But that isn't so important here; what is important is that if you pick one kind of year, the time it takes for the Earth to go around the Sun is pretty much constant. The length of the year is almost exactly the same from year to year. However, the time it takes for the Earth to spin once on its axis is changing. I have a web page describing this in detail, but for now I'll just say that it is indeed the Moon causing this. We'll see this in a moment. But the number quoted is about right; after about a year the Earth is spinning one part in fifty thousand slower. That's where the first confusion is in the narration: it isn't the year that's getting shorter, it's the days that are getting longer! Since the day gets longer, fewer of them can fit in one year.
With that said, I'll now add that if you want to be really picky, the narration is even more incorrect because in fact the year is getting longer. The reasons are subtle, but it boils down to the same reasons the day is getting longer. The Sun is the culprit here; the tides from the Sun affect the Earth's orbit, moving it vvvveeeerrrryyyy slowly farther out from the Sun. Over time, this means the Earth orbits the Sun more slowly, increasing the length of the year. Even worse, there is another effect: the Sun is constantly blowing a solar wind. This wind is only a very tiny fraction of the Sun's mass, but that mass is lost forever from the Sun. It's the Sun's mass that controls how much gravity we feel from it, and since the Sun is losing mass, its gravity gets weaker too. That also moves the Earth very slowly away from the Sun. So if you think the years are flying by faster, it's an illusion! They're getting longer.
Bad: Four billion years ago, a collision with a rogue planet blasted the Moon from the Earth's surface. It's been slowly receding from us ever since. As the Moon retreats, the tides- caused by its pull on the Earth, and which slow the Earth's rotation by friction- are getting smaller. With smaller tides, the Earth spins a little faster, shaving a little time off the years.
Good: This passage starts off well, then ends simply wrong. The current theory of lunar formation is indeed that a Mars sized planet collided with the Earth shortly after the Earth itself formed. The explosion sent debris into orbit around the Earth, and eventually this coalesced into the Moon. The Moon probably formed quite close to the Earth, and began receding slowly due to tides. Tides depend very strongly on distance, so as the Moon receded, tides got smaller. This part of the passage is correct.
But like I said before, tides slow the Earth's spin, not speed it up! I think I understand where the writers got confused. Bear with me, this may be a bit confusing itself! When the Moon formed close to the Earth, tides were enormous. The rate at which the Earth slowed was very high. In other words, if you measured how long it took the Earth to spin once on its axis, and waited a year and remeasured, the Earth would have slowed quite a bit. However, as the Moon receded, tides got smaller, the so the braking action applied to the Earth diminished. Nowadays, billions of years later, the Earth's spin is slowing much slower. Think of it this way: you're driving a car, and you see something in front of you. You hit the brakes hard, then ease up a little. At first, you slow rapidly, then as you ease up on the brakes, you slow more slowly. The same thing happens to the Earth as the Moon moves farther away: it's easing up on the brakes!
That's where the writers got confused. I think that they thought that since tides get smaller as the Moon recedes, it was making the Earth spin faster. However, what's really happening is that the Earth's spin is braking more slowly. Those are two very different things!
Bad: A few hundred million years ago, the Moon was much closer, the Earth's orbit was much slower, and time has a way of adding up. A year then was 44 months long, and the Moon's gravity sent huge tides roaring across the globe.
Good: This final part, like the first part, is almost correct. Remember, a long time ago the year was roughly equal in length to what it is now (if anything, it was actually shorter in time because the Earth was a little bit closer to the Sun back then). The month, however, is usually defined as the time it takes for the Moon to go around the Earth. In reality, the month in our calendar isn't really that length of time; the Moon takes about 27 days to go around the Earth. This is inconvenient for the Western calendar, so the length of the month was changed to fit in better. The Hebrew calendar, however, is still based on lunar orbits. Anyway, if billions of years ago the Moon was much closer to the Earth, it must have orbited faster. The length of time it took to orbit the Earth once was much smaller. That's what the narrator meant in that passage! The month was shorter, so more of them fit into a year. Right now the Moon orbits the Earth roughly 12-13 times in a year; back just after it formed it orbited the Earth three or four times faster.
One thing is right in that passage: at the distance the Moon used to be from the Earth, the tides would have been huge. The Moon would have almost certainly been on a very elliptical orbit back then as well (besides slowing the Earth and making the Moon recede from the Earth, tides also tend to make an elliptical orbit circular over time). That means that sometimes the Moon was much closer than others, which in turn means that sometimes the tides were much larger than other times. What a thing that must have been, to see the Moon looming hugely in the sky! Then to see a huge wave of water following it around the globe; water locked in step with the clockwork dance of gravity, rotation and revolution. Of course, life was probably just getting a toehold on Earth at that time, so no one was around to appreciate it. Good thing for us that things happened the way they did, so that we were able to be around long enough to appreciate the Moon for ourselves!
Now, don't let this passage fool you. ``Nature'' is a wonderful show, and I highly recommend watching it. But remember, just because you heard it on TV, even on a science show, doesn't make it right!