- An animation by planetary scientist James O'Donoghue reveals that Earth has two types of day: sidereal days and solar days.
- The sidereal day happens each time Earth completes a 360-degree rotation. That takes 23 hours and 56 minutes.
- The solar day — the one humans count in the calendar — happens when Earth spins just a little further, and the sun is at the same point in the sky as it was 24 hours ago.
- We count 365 days in a year even though Earth spins 366 times.
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How long does it take Earth to complete a 360-degree rotation? Not quite 24 hours, it turns out — it's precisely 23 hours and 56 minutes.
But because Earth is constantly moving along its orbit around the sun, a different point on the planet faces the sun directly at the end of that 360-degree spin. For the sun to reach the exact same position in the sky, Earth has to rotate 1 degree further.
That's how humans have chosen to measure days: not by the Earth's exact rotation, but the position of the sun in the sky.
Technically, these are two different types of day. A day measured by the completion of a 360-degree rotation is called the sidereal day. A day based on the position of the sun, however, is a solar day. The latter is four minutes longer than the former, making the even 24 hours we're used to.
"It's only because we move around the sun in an orbit that the solar day takes 24 hours," James O'Donoghue, a planetary scientist at the Japanese space agency (JAXA), told Business Insider. "If we didn't orbit the sun, both days would be the same."
He made the below animation to show how this works.
Because we go by solar days in our calendars, we count 365 days in a year. But Earth actually completes a full rotation (a sidereal day) 366 times per year.
O'Donoghue describes the difference between these two types of day as a matter of choosing which background object we use as a basis of comparison for Earth's rotation. A full rotation relative to the position of the sun is a solar day. A full rotation relative to all the other stars we see is a sidereal day.
If we used the sidereal day instead, "the sun would rise about four minutes earlier every day," O'Donoghue said. "After six months of doing this, the sun would be rising 12 hours earlier."
He added: "We've decided to tie our daily rhythm to the sun, not the stars. In fact, the stars rise about four minutes earlier every day because of our choice."
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