Do you gain seconds in a day?

Answer 1

This is a rather complex topic. Here are some very basic starting points: Greenwich Mean Time means just that: mean, or average. We arrived at the standard "second" by first determining the length of the year, and then figuring out how long the second would have to be so that the 24-hour day and the 365+ day year would reconcile over the course of a year. What many people may not realize is that the length of the "solar day" , the actual time elapsed between transits of the sun at any given location, is not a constant. One basic reason for this is that the earth's orbit is elliptical, while its rotational period is reasonably constant. But clocks based on Greenwich Mean Time do give us time periods of very precise length. You should check articles on "The Equation of Time", and the "Analemma". For parts of the year, the sun "lags" behind mean time, and at other parts of the year, the sun "precedes" mean time. Another interesting thing about seconds is that every few years we must add one to the year. This is called the "Leap Second", and is independent of the days that are added regularly on "Leap Years". Leap seconds are rarely if ever publicized, and often mis-understood. Some have very erroneously used the frequency of added leap years to calculate the "slowing down" of the earth. Leap seconds can't be used for this purpose, and attempting to do this shows a lack of understanding. Even though adding leap seconds happens now and then, the effect is not cumulative in nature. Discovering the explanation is too enjoyable to spoil here.