Time Trouble

There's a great article in the May 6 Science news called To Leap or Not to Leap, which is about the problems in keeping clocks lined up with the Earth's rotation. Because of friction, the big blue ball is slowing down, but because of the melting icecaps it's also speeding up. It's therefore hard to predict how long a day will be in a few months. The really interesting part to me, though, is that the most precise clocks will soon be so good that they will have to take into account the gravitational field they're in very precisely. The altitude needs to be known within a few centimeters, and the tides could throw the clock off a tiny amount. That means that it will be very difficult to synchronize these to within their precision.

Gravity affects time because of the general theory of relativity. I recommend Lewis Carol Epstein's book for a very readable introduction.