Suppose you could speed up Earth’s rotation so that a day was only half as long? What would happen? Well, for starters we’d have to make new clocks that only have hours 1 to 6 for am and pm. If you had tickets to an 8 o’clock concert, you’d be out of luck: 8 o’clock no longer exists.
But maybe a more germane question is, why do physicists ask nutty questions like this? It’s never going to happen—just move on, right? Well, here’s the deal. Thinking about counterfactual scenarios gives us insight into how things work here in reality. Plus, it’s fun! Need I say more? OK, it might even help us build a working space elevator.
Oh, you don’t know what a space elevator is? It’s a sci-fi staple, a tether from Earth up to an orbiting space station in geosynchronous orbit. A cable-climbing car would ride up and down, just like a regular elevator. Basically it’s a way of getting out to space as easily and routinely as you ride an elevator to your office in the morning—no rockets required.
Let’s start off with some basic questions and build up to some more complicated physics.
What Is a Day?
You can’t get more basic than that. But the answer isn’t simple. If you said a day is 24 hours, you’re right—and wrong. If you’re standing outside, the time at which the sun is at its highest point in the sky is called local noon. If you stand there until the next local noon, the amount of elapsed time is defined as 24 hours. So an hour is 1/24th of the time between two local noons.
But wait! This isn’t the same as a complete revolution of the Earth. If you measured the time of a complete revolution, you’d find that it’s not exactly 24 hours. The reason is that the Earth is doing two things at once: It’s spinning on its axis, which causes the sun to appear to move across the sky. But it’s also orbiting the sun over the course of a year, which means a complete rotation will not result in the sun being in the same position in the sky.
There are actually two different types of days. The solar day is the one you are thinking of, and it’s the one described above. The other type is called a sidereal day. Here’s a totally not-to-scale diagram that will help you understand the difference:
