If the Earth's rotation period is less than 24 hours, why don't our clocks fall out of sync with the Sun?
We have leap years because the revolution on the Earth around the Sun is not exactly 365 days but about 6 hours off from that figure. If we did not have leap years, our calender would fall out of sync with the sky and we would eventually have Christmas in July.
My question is this: The Earth's rotation is actually about 4 minutes shy of 24 hours. Why doesn't our clock fall out of sync with the sun, that is, without having "leap minutes," would not the sun eventually be directly overhead at midnight?
That's a good question. The day is defined to be the time between one dawn (or noon) and the next' i.e., the day is defined with respect to the position of the Sun in the sky. Now, the earth revolves around the sun in the sky and so the time taken for the Sun to come to the same position in the sky is longer than the time taken for the Earth to rotate once around itself. You can convince yourself about this by drawing a picture of the Earth in orbit around the Sun and rotating around itself at the same time.
So, the Earth's rotation period is actually 4 minutes less than what we call as one day. As a result of this, the Sun's position in the sky at noon is roughly fixed, but the stars slowly drift apart. So, the stars which are overhead at midnight today will slowly move in the sky until they will be overhead at noon 6 months apart.
If instead we defined a day to be the time taken by the Earth to rotate round itself exactly once, then as you mention, the Sun will be overhead on some day at midnight.
Get More 'Curious?' with Our New PODCAST:
- Podcast? Subscribe? Tell me about the Ask an Astronomer Podcast
- Subscribe to our Podcast | Listen to our current Episode
- Cool! But I can't now. Send me a quick reminder now for later.
How to ask a question:
If you have a follow-up question concerning the above subject, submit it here. If you have a question about another area of astronomy, find the topic you're interested in from the archive on our site menu, or go here for help.
This page has been accessed 46177 times since April 29, 2002.
Last modified: February 3, 2006 11:58:54 AM
Ask an Astronomer is hosted by the Astronomy Department at Cornell University and is produced with PHP and MySQL.
Warning: Your browser is misbehaving! This page might look ugly. (Details)