|MadSci Network: Earth Sciences|
The moon is gently applying the brakes on the Earth's rotation. Check out an answer by Erika Gibb in the Astronomy archive for May 2001. She quotes the rate of slowing as 1.5 milliseconds per century, and sees the final state of that process as a situation where both a day and a month are 47 of our present days long. At the rate of 1.5 milliseconds per century, it would take 86400 * 1000/1.5 = 57,600,000 centuries to slow the Earth's rotation by 1 day to 2 days. this works out at 5.76 (American) billion years. The present age of the earth is about 4.5 billion years. If Erika's figures are correct, we are going to have a lot of more important things to worry about before the Earth stops rotating. If the Earth were to stop rotating, it would have no effect (or rather, less than 1%) on gravity. It could cause changes to the ways weather systems work, and make life mildly to severely uncomfortable in a number of other ways. But there are a lot of more important things to worry about. Let's get timescales in perspective: -- Major changes to the Earth's rotation will take billions of years. -- the projected lifetime of the sun (before it does something drastic) is less than about 10 billion years (check out other answers in our MadSci Astronomy archives). -- humans have been on the Earth for somewhere between about 200,000 and 2 million years. --"civilizations" have much shorter lifetimes than that. Ancient Egyptian civ lasted about 3000 years, and has the record for any that we know of. The average is about 800 years. Our present civ it is fairest to date from the European renaissance, and on that basis would work out at about 500 years old.
Try the links in the MadSci Library for more information on Earth Sciences.