4. Does Earth rotate once a day?

You could go to the Moon to measure the rotation rate of the Earth.  But there are easier ways.  Image by NASA/Bill Anders via Wikimedia Commons.  

My last two posts have been on what exactly this thing is that we call a day.  Did you notice that I never mentioned the rotation of the Earth?  If I had asked you earlier to define a day, wouldn’t you have said it’s one rotation of the Earth?

Today nearly all of us think of the Earth as an object spinning in space.  We can measure how long it takes to spin once if there is a fixed reference point out in space.  Actually, there probably is no such thing because everything in the universe appears to move but that’s okay.  Stars and galaxies are so far away from us that we can ignore how much they actually move in a day.  We can treat them as fixed objects.

If the sky is clear tonight, you can go out and look for a bright star that just rose above a building or a tree.  Then you can go out again tomorrow night and see if the star is at the same spot at the same time.

You should find that the star took slightly less than 24 hours to get back to the same spot in the sky.  If a star rises at 10 pm tonight, it should rise around 9:56 pm tomorrow night.

A 360 degree rotation of the Earth does not take exactly 24 hours.  It takes 23 hours and 56 minutes.

Why the four minute discrepancy?  As the Earth spins, it does something else as well.  It moves slightly in its orbit around the Sun.  After completing one full rotation, the Earth has to turn for another four minutes before the Sun returns to the same location in the sky.  This is shown in the figure below.  Astronomers refer to our 24 hour day as a solar day.  One rotation of the Earth is known as a sidereal day.

sidereal_day_28prograde29
Sidereal and Solar Day.  From Time 1 to Time 2 is one sidereal day.  From Time 1 to Time 3 is one solar day.  Objects and distances are not to scale.  By Gdr via Wikimedia Commons.

A few thousand years ago, there were a few people that claimed that the Earth rotates.  But the majority seemed to believe it doesn’t.  And you can’t blame them.  We don’t feel the Earth move.  We see objects moving around us.

Those who paid close attention noticed that the Sun appears to move slightly slower than the stars.  The Moon moves even slower.  It takes 24 hours and 50 minutes to go around the sky once.

If everything goes around us and we could define the day based on any of them, the obvious choice is the Sun.  After all, we wouldn’t have daytime without it.

Even given what we know now, it makes more sense to define the day based on the Sun’s apparent motion than on the Earth’s actual motion.  Otherwise the hours of the day would have nothing to do with daylight.  The Sun would rise around 6 am one month, 8 am the following month, 10 am the month after that and so on until the time came when it rose at midnight.  How strange would that be?

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Author: Bhasker Moorthy

I'm a Professor of Astronomy at Harper College in Palatine, IL, USA. My email address is bmoorthy [at] harpercollege.edu.

7 thoughts on “4. Does Earth rotate once a day?”

  1. This slight discrepancy in not having an exact 24 hour day lead to Pope Gregory to send a missionary to retrieve the Mayan Calendar to create a calendar that actually worked. The Gregorian Calendar is a mixture of the old Roman calendar and the Mayan’s star data. Have you heard of the 13 month Calendar? Each day would be numbered the same for each day and would cover 364 days. On the leap year, an additional day would be added every 4 years and would be called “Year day” or “Day out of Time”, and would be added after December 28th.

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  2. That doesn’t sound right. The Mayan Haab calendar was 365 days long and so didn’t have leap years at all. The Julian calendar was based on a 365.25 day year while the Gregorian is based on a 365.2425 day year. In other words, the Gregorian calendar was a minor reform of the Julian calendar and very different from the Mayan Haab calendar. The Mayans had another calendar (Tzolk’kn) which was 260 days long and therefore had nothing to do with a year as we understand it.

    I don’t understand the 13 month calendar you’ve described. Is it 364 plus leap day or 365 plus leap day?

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    1. The 13 month calendar has a certain elegance to it. It would have 13 months, each month lasting exactly 28 days, or four 7-day weeks. Since 13 x 28 = 364, that still leaves an extra day to get to 365, and an extra 2 days during leap years. The annual extra day could be a “year day” (as Tom suggested) or “world day” that existed outside of any regular month/week. It’s just it’s own special day – a calendrical excuse for a long weekend, if you will. Likewise for the second extra day every 4th year.

      Of course, there is a certain inelegance to having 13 months. For example, it doesn’t neatly divide into 4 seasons – although, as I’ve come to learn since moving to the US East Coast, not all seasons are created equal long!

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  3. If we could define the day based on any of the options, the moon, the earth, or the sun, why the obvious choice is the Sun? I understand that we wouldn’t have daytime without it, but technically we could define our days in other ways. This is also confusing because you mention that given what we know now, it makes more sense to define the day based on the Sun’s apparent motion than on the Earth’s actual motion. Otherwise the hours of the day would have nothing to do with daylight because the Sun would rise around 6 am one month, 8 am the following month, and so on because of how the Earth rotates in relation to the Sun. However I feel like this proves your point of how we should define our days by something different. Regardless, this is a very thought-provoking post.

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