Leap Second

Everyone knows the leap day. Every four years (except century years not divisible by 100), an extra day is added to the year in February to keep the calendar in synch with the seasons.


However, there is also a leap second. This is used every couple of years to keep the Coordinated Universal Time close to the mean solar time. The reason why it is needed is that the Earth's rotation is slowing down, but not by a lot.


The second is added just before midnight on either June 30th or December 31st. The time would go from 23:59:59 to 23:59:60 to 00:00:00. The last time a leap second was added to the UTC was last June 30th.

Leap Day

Under Julius Caesar, the idea of a leap day/leap year was added to help keep the seasons in alignment with the calendar. Leap days were added every four years to account for the orbit of the Earth around the Sun, which was approximately 365.25 days (it is actually 365.2425 days).


This calendar worked fine until 1582, when Pope Gregory XIII wanted to make sure that the Christian holy days fell around the same time every year. By 1582, the Julian calendar was out of sync with the seasons by 10 days. The calendar developed under Gregory was able to account for the difference between 365.25 days and 365.2425 days. We still have a leap day every four years, but to help account for above difference, this calendar did not have a leap year if the years was divisible by 100 but not by 400. For example, 2000 was a leap year since it is divisible by 400. But 1900 was not and 2100 will not be leap years. This will help keep the calendar in sync with the seasons until 9282, meaning an error of only 1 day every 7700 years. The Julian calendar had an error of 1 days every 128 years.

February

February is named for the Latin word februum which means purification. It is the shortest month, always lasting less than 30 days. It did not actually join the calendar until 713 BC, and was actually the last month (following January) until about 450 BC when January and February were moved to the beginning of the calendar.

The number of days in February varied between 23 and 27 days to account for the precession of the equinox and did not have the length of 28 days until the Julian calendar was established under the rule of Julius Caesar. Leap days were added every four years to account for the orbit of the Earth around the Sun, which was approximately 365.25 days (it is actually 365.2425 days and created problems in the 1580s which we will discuss later).