# Scientist Shows There Are Two Ways to Measure a Day on Earth – We Use The Longer One

## 1 min

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.”

## Like it? Share with your friends!

hate
14
hate
confused
4
confused
fail
21
fail
fun
18
fun
geeky
16
geeky
love
9
love
lol
11
lol
omg
4
omg
win
21
win

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Choose A Format
Personality quiz
Series of questions that intends to reveal something about the personality
Trivia quiz
Series of questions with right and wrong answers that intends to check knowledge
Poll
Voting to make decisions or determine opinions
Story
Formatted Text with Embeds and Visuals
List
The Classic Internet Listicles
Countdown
The Classic Internet Countdowns
Open List
Submit your own item and vote up for the best submission
Ranked List
Upvote or downvote to decide the best list item
Meme