our solar system formed about 4.5 billion

Our solar system formed about 4.5 billion years ago, when a dense cloud of interstellar dust and gas collapsed in on itself and began to spin. There are vestiges of this original movement in our planet’s current rotation, thanks to angular momentum — essentially, “the tendency of the body that's rotating, to carry on rotating until something actively tries to stop it,” explains Peter Whibberley, a senior research scientist at the UK’s National Physical Laboratory.

Thanks to that angular momentum, our planet has been spinning for billions of years and we experience night and day. But it hasn’t always spun at the same rate.

Hundreds of millions of years ago, Earth made about 420 rotations in the time it took to orbit the Sun; we can see evidence of how each year was jam-packed with extra days by examining the growth lines on fossil corals. Although days have gradually grown longer over time (in part because of how the moon pulls at Earth’s oceans, which slows us down a bit), during humanity’s watch, we’ve been holding steady at about 24 hours for a full rotation — which translates to about 365 rotations per trip ’round the Sun.

As scientists have improved at observing Earth’s rotation and keeping track of time, however, they've realized that we experience little fluctuations in how long it takes to make a full rotation.