A CIESE Realtime Data Project

Long Day? - Student Activities - Catch A Wave

Student Activities

Long Day?

Does the tidal movement of such massive amounts of water have an effect on the Earth?

Have you ever tried running through water? Is it easier or more difficult than running on land? Think about the Earth rotating in space. With all the movement of the water around it through tidal cycles, do you think it is easier or more difficult for the Earth to rotate?

The long-term change in the Earth's rotation, or braking, is caused primarily by friction in the oceans, where "friction" may refer to any number things including, bottom friction, various kinds of wave breaking, and scattering of tidal waves are all thought to play a role.

Due to tidal braking, after 1000 days our earth clock loses about 2.3 seconds, falling further behind the atomic clock. This long-term slowing of the rotation is a primary reason for periodically inserting leap seconds into the timekeeping process.

1. 1,000 days equals how many years?

2. 500 days equals how many years?

3. By the year 2020, how many leap seconds will have to be added to correct our Earth clock?

4. What would happen by the year 5000?

So why doesn't the Earth stop rotating?
Confusion sometimes arises over the misconception that the addition of leap seconds to the timekeeping process every few years indicates that the Earth should stop rotating within a few millennia.

The confusion arises because people sometimes mistake leap seconds for a measure of the rate at which the Earth is slowing. The 1 second (leap second) increments are actually indications of the accumulated difference in time between the two systems, atomic clocks and the Earth's rotational time, not the rate at which the Earth is slowing.

As an example, if a person owned a watch that lost 2 seconds per day, at the end of a month, the watch will be roughly one minute in error (30 days of 2 second error accumulated each day). The person would then find it convenient to reset the watch by one minute to have the correct time again. The person did not "lose" one minute, the time keeping device was simply adjusted to reflect the actual time.

Since we cannot change the rotation of the Earth, we simply change the atomic clock to reflect the overall difference between the two systems.

Currently the actual long-term change in the rotation rate increases the length of day by some 2.3 milliseconds per century.

Official U.S. Time