2024: a leap year is upon us

15 Jan 2024

Another leap year is upon us again. Here’s what you need to know about it.

The Gregorian calendar, which is used by almost all countries of the world, repeats itself in a cycle of 400 years. So, after 400 years there is a period when the calendar is repeated for each year with the same arrangement of dates and days of the week as in the calendar of 400 years ago.

So, the calendar for the year 2000 is equal to the calendar for the year 2400; the calendar for 2001 is equal to the calendar for 2401; the 2002 calendar to the 2402 calendar, etc. There are 97 leap years in that one cycle.

A leap year is a year that has 366 days instead of 365. This extra day is added to February so that it has 29 days instead of 28 that year.

But why are there leap years at all?

Leap years are needed to adjust the calendar to the movement of the Earth around the Sun. The Earth does not go around the Sun in exactly one round number, but in about 365.2422 days.

This means that one calendar year has an error of 0.2422 days, which is almost six hours. After 100 years, that error accumulated to 24 days, so if we were to stick to that calendar, it would turn out that after 100 years we would be in summer according to the calendar, but still in spring in reality.

In the next hundred years, the error would double, so it would happen that our distant descendants would be covered in snow in the middle of the calendar summer!

Fun fact: February 29 most often falls on Monday or Wednesday, 15 times in 400 years, then on Friday or Saturday, 14 times each, and on Sunday, Tuesday or Thursday 13 times each.

How is a leap year determined?

In the Gregorian calendar, the following rule applies for determining the leap year.

Every year divisible by four is a leap year, except years that are divisible by 100, but with the exception of those that are divisible by 400. This means that the years 1800, 1900, 2100, 2200, 2300 and 2500 are not leap years (because they are divisible by 100), but the years 2000 and 2400 are because they are divisible by 400.

Because of this, sometimes more than the usual four years pass between two leap years. Let’s say in the period from 1896 to 1904, a full eight years and not a single year was a leap year. The same case will be in the period between 2096 and 2104.

Something to look forward to

Halley’s comet is the most famous of all the comets that have been entered in astronomical catalogs. It was named after the English astronomer Edmund Halley, who observed it in 1682 and, based on records of comet appearances from 1531 to 1607, realized that it appeared periodically in our sky.

Halley determined the year of the return of this comet in 1759, so the astronomers, as well as the general public, eagerly awaited the announced arrival. Halley himself did not experience that arrival because he died in 1742.

The last time the comet was in our vicinity was in February 1986, and after it circled the Sun, it rushed further and further into the distant space of the Solar System, and that was the case until December 8, when it reached aphelion, the farthest point in a body’s orbit around the Sun.

Of course, according to Kepler’s second law of the movement of celestial bodies, it will now accelerate its movement as it approaches the Sun, and its speed will increase from some 0.909 kilometers per second at the next perihelion to more than 50 times.

It will reach perihelion, the closest point in the orbit to the Sun, and thus reach us in 38 years, 2061.

How about a tribute to the ones you love? Did you know that here at OSR you can name stars?

Name a star!