On June 30 some years a leap second is added to the most precise clocks the human species has yet succeeded in devising. Why? Think of leap year, when we add an extra day to keep our human calendars aligned with the solar year. A leap second involves adding an extra second — or possibly subtracting one — to keep our human clocks aligned with the solar day.

Neither of these fine-tunings was necessary during the earliest period of our evolution when we, like the plants and animals around us, responded directly to the sun. The problems arose when our ancestors began thinking it was important to tell, measure, and keep time.

Early sundials were good at telling time, and hourglasses could measure short periods of it, but neither kept track of its continuous passage. So scientists invented clocks. While calendars required only that the number days agree with the solar year, clocks introduced smaller time units: hours, minutes, and seconds. And as the time units became smaller, precision became more important.

Clocks have been refined over the years, resulting in today's extremely precise atomic clocks. The problem now is that these atomic clocks are too precise. The Earth wobbles and fluctuates as it rotates on its axis and is in fact slowing down. So atomic time can differ from the Earth's rotation time by what can accumulate toward a second.

Because we can't adjust the Earth's rotation, we have to adjust our atomic clocks. A group of extremely attentive observers working at the International Earth Rotation Service decide exactly when we need to add — or subtract — a leap second.

Since June 30, 1972, we’ve added 22 leap seconds, nine of them to June 30 and thirteen of them to December 31. A June 30 leap second is added right after what we in Vermont experience as 7:59 p.m. Eastern Daylight Time. At the Greenwich Observatory it is 23:59:59, then 23:59:60, then 00:00:00 of July 1. A December 31 leap second works the same way, only it’s a tad more exciting because it happens on New Year’s Eve.

June and December are both such busy months for me that I don’t really have time to celebrate a leap second. But I’m grateful to the Earth for making my clocks give me those extra seconds every once in a while to do with what I will.