Mr. Harley: Your impatience is quite understandable.
Klaatu: I’m impatient with stupidity. My people have learned to live without it.
Mr. Harley: I’m afraid my people haven’t. I’m very sorry… I wish it were otherwise.
–The Day The Earth Stood Still, 1951
People love to talk about the constancy of the natural phenomena. Every 24 hours, from anywhere not within either the Arctic or Antarctic circles, for instance, there’s one sunrise and one sunset.
And, of course, there are two high tides and two low tides every day.
The Earth rotates on its axis, and revolves around the Sun. And this, pretty much, is the way things are, and always were, and always will be.
Well, kind of.
What if, instead of spinning a sphere in space, like the Earth, you were to spin a top, but in a completely frictionless environment?
It would continue spinning forever, like in that movie.
But what if I put just a tiny little force on it? Just enough, maybe, to add a little bit of friction? Even the air is sufficient to do it.
Well, of course, in the case of a top, it will lose speed and fall over. But the big deal for me is this: it slows down! So if something exerts the right force on the Earth, perhaps it will slow down, too. Any candidates?
The Moon! The Moon — gravitationally — exerts a greater force on the side of the Earth closer to it than the side farther away.
This differential force results in a tidal acceleration that slowly pushes the Moon farther away, and also very, very slightly slows the Earth down!
Think about it a minute. Every day contains 24 hours, which in turn contain 60 minutes, which in turn contain 60 seconds apiece, for a total of 86,400 seconds in a day. But if the Earth’s rotation is slowing down, that would mean that in the future, we’ll need more seconds to add up to a day, and that we needed fewer in the past.
How can we test this?
Look at tidal rhythmites, or the daily patterns left by the changes in the tides! Amazingly, we find patterns left behind from 620 million years ago, and find that a day back then lasted under 22 hours!
If you extrapolate back to when the Earth was first formed, you find that a day was only around 23,000 seconds, or six-and-a-half hours! And it continues to slow down! Every 18 months or so, because of the difference between 86,400 seconds and an actual day, we add an extra leap second to our clocks.
As we continue into the future, the day will continue to slow down, there will eventually be fewer days in a year, and after another 4 million years or so, we can totally eliminate the need for leap years!
So although the laws of physics are solid, the communication between the Earth and Moon changes enough that the tides, the days, and even the years are altered over time. And we’ve been around long enough that this really matters! Hope you learned something new, because it’s really amazing that we can measure time and changes this precisely, and find that the Earth is really slowing down!