In just under three hours, another leap second will be added to the atomic clocks that keep the official time. The reason is because the Earth’s rotation is slowing, ever so slightly. But there’s no reason to panic. You see, they used to just say that a second was 1/86,400th of a day. (24 hours times 60 minutes per hour times 60 seconds per minute.) But they eventually learned that not all days are the same length, so they came up with a new measurement, called Ephemeris Time. Ephemeris Time (ET) was defined as “the measure of time that brings the observed positions of the celestial bodies into accord with the Newtonian dynamical theory of motion.” As the Naval Observatory’s site on leap seconds says (first link above),
Confusion sometimes arises over the misconception that the regular insertion of leap seconds every few years indicates that the Earth should stop rotating within a few millennia. The confusion arises because some mistake leap seconds for a measure of the rate at which the Earth is slowing. The 1 second increments are, however, indications of the accumulated difference in time between the two systems. NOTE: The other system being the Atomic Clock] (Also, it is important to note that the current difference in the length of day from 86,400 seconds is the accumulation over nearly two centuries, not just the previous year.) As an example, the situation is similar to what would happen if a person owned a watch that lost 2 seconds per day. If it were set to a perfect clock today, the watch would be found to be slow by 2 seconds tomorrow. At the end of a month, the watch will be roughly a 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.
This scenario is analogous to that encountered with the leap second. The difference is that instead of setting the clock that is running slow, we choose to set the clock that is keeping a uniform, precise time. The reason for this is that we can change the time on an atomic clock, while it is not possible to alter the Earth’s rotational speed to match the atomic clocks! Currently the Earth runs slow at roughly 2 milliseconds per day. After 500 days, the difference between the Earth rotation time and the atomic time would be 1 second. Instead of allowing this to happen, a leap second is inserted to bring the two times closer together.
So, if I understand them correctly, Newtonian Physics is right, but since our observation of the stars based on out measurement of time does not match the theory, we’re going to change how we measure time. Well. There it is.
So, there will be an extra second this year. It will be inserted at 7 PM EST (4 PM PST). What will you do with your extra second? I’ll probably be eating.