Earth Historic Rotation Rates ``` Name: Nikki Status: student Grade: 6-8 Location: AZ Country: USA Date: Spring 2012 ``` Question: Did Earth ever spin faster than it does now? Did Earth ever have 18 hour days instead of 24 hours? Replies: Nikki, Earth rotates on its axis and continues to slow down because of tidal braking. Yes, it is the tides that result in the ever-slowing rotation of Earth. The main cause of the tides is the moon. That is, the ocean is constantly being pulled toward the moon while Earth rotates under this bulge of water. In order for the sun to be overhead at noon we add one second to the clock about every eighteen months. Yes, that is true, Earth's rotation slows by aboutone second every eighteen months. About 600,000,000 years ago, there were 420 days in one year! About 300,000,000 years ago there were 395 days in one year. Today, there are about 365.25 days per year. Think about the future, think about 300,000,000 years in the future. How many days do you expect will be equal to one year at that time? Your question, however, was about the number of hours in one day, yet my answer was the number of days in one year. So, to answer your question, about 600,000,000 years ago each day was about 20.85 hours long (because 20.85 hours per day, times 420 days per year, equals 8757 hours per year -- which is roughly the number of hours in a year today). To have 18 hours per day, Earth must have been spinning very fast. Indeed, that would correspond to 487 days in one year. That would be equivalent to a time about 695,000,000 years ago! At that time in Earth history there were only bacteria, and I do not think bacteria would get dizzy from spinning so fast! --- Leslie Kanat, Ph.D. Professor of Geology Johnson State College Dr. S. W. Carey in New Zealand and his students did some work that showed the earth spun faster at one time. They counted the rings on a type of corral which gave the same information as tree rings. You can probably find more information about his work online or at the library. Hope this gets you started on an interesting search. Robert Avakian Oklahoma State Univ. Inst. of Technology Nikki I found this statement in Wikipedia at the following URL: http://en.wikipedia.org/wiki/Earth%27s_rotation#Rotation_period It says: Over millions of years, the rotation (of the earth) is significantly slowed by gravitational interactions with the Moon; both rotational energy and angular momentum are being slowly transferred to the Moon: see tidal acceleration. However some large scale events, such as the 2004 Indian Ocean Earthquake, have caused the rotation to speed up by around 3 microseconds by affecting the Earth's moment of inertia. Post-glacial rebound, ongoing since the last ice age, is also changing the distribution of the Earth's mass thus affecting the moment of inertia of Earth and, by the conservation of angular momentum, Earth's rotation period. I can only find references to Earth rotation speed back to the 19th Century (the 1800’s), the earliest time during which creditable measurements are available. So, Earth may have had 18 hour days a long time ago (meaning a fast rotational speed) but nobody was around at that time to make these complex measurements. I recall an experiment years ago where some folks were locked in a dark room to remove them from natural light to see what night/dark rhythm they would fall into. I recall that they fell into a 23 hr cycle suggesting that Earth’s day was shorter, that in our distant past it was spinning faster and is now spinning slower. But I cannot find any current references to that experiment. But at any rate, Earth’s rotation has been slowing since the 19th Century. Sincere regards, Mike Sewart Hi Nikki, The Earth does change speed, but only a little bit. Think of the Earth as a spinning top. If I blow on it or touch it, the spin slows. Now think of Earth as a top that has tape wrapped on it. I can spin it and time how long it spins before it tips over. Now I can move the tape around and spin, then time that. The times will be slightly different. We could also spin a top very close to a wall or far away from a wall… the times will be slightly different. The blowing, the tape moving and the wall experiments just show us that our Earth responds to outside or on Earth interference. We can think of the tape experiments as the effect of earthquakes. We can envision the wall and blowing experiments as the action of the tides from the Moon interfering. We can think of the touch experiment as the result of a comet or large asteroid striking the Earth’s surface. The Earth did spin faster, but probably not so fast as 18 hour days. But over the long, long time, we are slowing down. For the time being, the Earth is in a slight speed up period. Eventually we will slow down. This reference is for your teacher or parents to discuss with you: http://tycho.usno.navy.mil/leapsec.html Happy spinning! Peter E. Hughes, Ph.D. Milford, NH Nikki, I do not know about 18 hours, but there are fossil evidence that 600 million years ago, the days were only 22 hours long - we can see this from coral that form bans (similar to tree growth rings) as they go through a single day and from rock that preserve the deposits between high and low tides. The slowing of Earth's rotation is due to tidal friction. You already know that Moon's gravity has the effect of producing tides -we see this very clearly in the twice a day high and low water tides, but the rock of Earth also experiences this tidal effect. this bulging of Earth, the fact that it is in friction with the rest of Earth as it rotates, has the effect of slowing down the rotation of Earth (not by much, but continuously). Greg (Roberto Gregorius) Canisius College Click here to return to the Environmental and Earth Science Archives

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