Today is February 29th, a date that comes once every four years. Why do we have that extra day? Let’s take a look.

The Earth in Orbit

A leap year is a calendar year with an added extra day, to keep the calendar year synchronized with the astronomical year. A calendar year has 365 days divided in 12 months, with 7 months (January, March, May, July, August, October and December) having 31 days, 4 months (April, June, September) having 30 days and the odd one out, February, having 28 or 29 days.

A standard calendar year has 365 days, with 8.760 hours (525.600 minutes, 31.536.000 seconds). This standard year is the one when we talk about light years as a measurement of distance, or yearly interests on mortgage payments.

A solar, or a tropical year is the time between successive vernal equinoxes (a point at which the sun appears to cross the celestial equator from south to north). This occurs on or about March 21st, marking the beginning of the spring in the Northern Hemisphere. Length of a tropical year is 365 days, 5 hours, 48 minutes and 46 seconds. This time period is used as the basis of the Gregorian calendar.

But a sidereal (also sometimes called astronomical) year, the time that Earth takes to circle once around the Sun, as measured against a fixed frame of reference (such as fixed stars, or sidera in Latin), is not so precise. On January 1, 2000, a sidereal year was exactly 365 days, 6 hours, 9 minutes and 9,76 seconds long).

That creates a problem, not so noticeable on shorter timescale, in that some events do not occur at the designated date (solstices or equinoxes for example).

The Julian calendar, with a length of a year of 365 days, and a leap year every 4th year, was proposed in 46 BC, with the aid of Greek mathematicians and astronomers, such as Sosigenes of Alexandria. The idea behind it was to create a calendar that remained aligned to the Sun without human intervention. It was known at that time that a year was 365 and 1/4 days long. Before that, Roman pontifexes arbitrarily added days to the year when needed to keep it in sync with the Sun.

But, by 1582 AD, it was also out of sync, by 11 days! Though a good approximation was made with the Julian calendar, it also wasn’t precise. So with the help of Christopher Clavius, a German Jesuit mathematician and astronomer, a new calendar was created by Pope Gregory XIII. It leaped from October the 4th to October the 15th in the year 1582. Also, a new rule for leap years was created: every year that is exactly divisible by four is a leap year, but centurial (those divisible by 100) years are not leap years, unless divisible by 400. so, years 1700, 1800 and 1900 were not leap years, but 1600 and 2000 were leap years.

That reform brought a good system that actually works, and won’t need adjustment for a long time. How will we know exactly when we need a tweak to it all?

“Is it … Atomic?”
Very Atomic!”

dialog from “The 5000 Fingers of Dr. T”

Precisely Measuring Time

The measurement of all this time is managed by the most precise clocks in existence, like the NIST-F2 caesium fountain clock in the National Institute of Standards and Technology. It will not lose a second in at least 300 million years. It works on the principle of the atomic clock. It measures the electromagnetic signal that electrons in atoms emit when they change energy levels. Early atomic clocks were based on masers (not the sci fi weapons, it is an acronym for Microwave Amplification by Stimulated Emission of Radiation, a glorified microwave in essence) at room temperature, but since 2004, more accurate atomic clocks first cool the atoms to near absolute zero by slowing them down with lasers and probing them in atomic fountains in a microwave filled cavity.

Now, why is such a precise clock needed? Because of the internet, navigational satellites (GPS, GLONASS, BDS, Galileo), banking and other technologies that depend critically on frequency and time standards.

Just remember all that the next time you’re late for something!


Ivan Majstorovic
Ivan Majstorovic

A Sci-Fi and Fantasy fan, gamer (computer, tabletop, pen and paper) and dad from Zagreb, Croatia.
Love doing interviews, reading and cooking.

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