A year is 365 days long; everyone knows that. Of course, then there are leap years, which have February 29th, so they are 366 days long. But did you know that there are also leap seconds?

Leap years are put in place to keep the seasons and other events in sync with the calendar. If we didn’t have them, the days of the calendar would eventually be thrown off so that things like Christmas would happen in the middle of the summer, and nobody wants that (sorry, Australia).

While the leap year keeps the calendar pretty much spot on to where it needs to be, it isn’t precise down to the second. Some things, such as scientific experiments, GPS navigation, and financial transactions, needed to be perfect.

So, in order to make the tiny microadjustments that are needed to keep our clocks spot on, the world has agreed to use leap seconds where needed.

These are calculated based on atomic clocks, which are used to measure Coordinated Universal Time (UTC). Atomic clocks monitor the vibrations of specific atoms, usually cesium atoms.

Cesium atoms oscillate at 9,192,631,770 times per second. Exactly that number, always.

So, based on those oscillations, time keepers are able to determine when the Earth’s orbit is moving faster than can be accounted for by the normal leap year schedule.

When that happens, they add in a leap second either on June 30th or December 31st.

This was first done in 1972, and they actually did it on both of those days in order to get the clocks moved where they needed to be. That, combined with the fact that it was a leap year, meant that it was the longest calendar year in modern history.

Since 1972, there have been 30 more leap seconds. Scientists have found that the orbit of the Earth’s rotation has actually sped up ever so slightly, which means that fewer leap seconds will be needed going forward.

In fact, they may need to use a negative leap second at some point in the future.

If you thought that was interesting, you might like to read about why we should be worried about the leak in the bottom of the ocean.