Once every 24 hours, Earth completes one rotation on its axis, marking one day on our planet. This reliable rotational period is what allowed humans to develop systems to tell time and what signals to humans, animals and plants when it is time to rest.
But Earth's rotation hasn't always been so consistent. In fact, a long, long time ago, Earth's day was much shorter, said Sarah Millholland, an assistant professor of physics at MIT.
