This chart shows why we need February 29th

Those born in February must decide: should they be sad that their birth month is shorter than all the others – or should they be happy that it is so special? This year, February once again has one extra day, and Leap Day children can exceptionally celebrate their birthday party on February 29, the correct date. But why is February so out of sync and why do we need a leap day at all?

Our calendar is a solar calendar. Its basis is a solar year (also called a “tropical year”), i.e. the period of time during which the Earth revolves around the Sun exactly once. In addition to this circular motion, another rotation of our planet also plays a role: the Earth revolves around itself in its orbit around the sun. This circular movement forms the basis for the calendar unit ‘day’.

Unfortunately, these two circular motions don’t add up nicely – in other words, a solar year doesn’t have exactly 365 whole days. The Earth needs an additional 5 hours, 48 ​​minutes and 46 seconds to complete one full revolution around the Sun. If this small difference is not corrected for, it will accumulate over time, causing seasons and astronomical events such as equinoxes to gradually shift in the calendar (see the second chart below).

The old Julian calendar already compensated for the difference mentioned by adding an extra day – a leap day – every four years. However, this correction was not precise enough: the year was now about 11 minutes too long. The Gregorian calendar we use today corrected this error. He calculates leap years using the following formula:

Over time, the calendar would reflect astronomical reality increasingly poorly. Astronomical events such as the equinoxes, and therefore the seasons, would gradually shift in the calendar. In just 400 years, the vernal equinox would fall in late June instead of late March. The longest day would be at the end of September, the shortest at the end of March.

This was the case in the ancient Roman calendar Februaryius – the one after the purification festival February was named at the end of the year – originally the last month. At the same time, it was the shortest at 28 days; the remaining months varied between 29 and 31 days. Together these formed the 355 days of the ancient Roman calendar, which used leap months to align with the solar year. This leap month was added in the last month – February – and not at the end, but on February 24. The remaining days of February followed the leap month.

The intercalary month continued to be added in February when the beginning of the year began in 153 BC. BC was postponed from March 1 to January 1. The month names September (the seventh), October (the eighth), November (the ninth) and December (the tenth) are still reminiscent of the previous arrangement. After Caesar’s calendar reform in 45 BC. In the 1st century BC, the Julian calendar named after him replaced the old Roman calendar. The leap month was deleted, but February was given a leap day (the 24th was counted twice in a leap year). The remaining months of the year, which now has 365 days, alternated between 30 and 31 days – February remained at 28 and 29 days.

At the stroke of midnight (UTC) on December 31, 2016, the International Service for Earth Rotation and Reference Systems (IERS) added the last leap second yet. Although the name suggests it, the leap second has nothing to do with the leap year. Leap seconds have to do with the definition of the second and the rotation of the Earth and affect time, but not the calendar.

Originally, a second was defined as 1:86,400 of the average day length, since a day is divided into 24 hours of 60 minutes of 60 seconds each (= 86,400 seconds). However, this universal time second fluctuates in duration because the rate of Earth’s rotation is subject to small fluctuations and decreases on a long-term average. The definition of the second, which has been valid in the International System of Units SI since 1967, is based on an atomic physical constant. These ‘atomic seconds’ remain virtually constant for millions of years.

The difference between the astronomical Universal Time, which is based on the universal second, and the International Atomic Time (TAI) therefore increases over time. To adjust the time to astronomical reality, the Coordinated Universal Time (UTC), whose second corresponds to the “atomic second”, must be sporadically adjusted by a leap second. This happens irregularly because the speed of the Earth’s rotation fluctuates. Normally, leap seconds are inserted at the end or middle of a year. They are then added to the last minute of June 30 or December 31 in the UTC timescale – 23:59:59 is then followed by 23:59:60 and only then 00:00:00. According to IERS, the next possible date is July 15, 2024.