February 29 only appears every four years on the calendar because it is a leap year. This system was put in place to adjust the solar calendar from 365 days to the tropical year of 365.2422 days.

In detail, for those interested!
## The leap years

## Calculation of the leap year

## The rules of the Gregorian calendar

A leap year is a year with one more day than ordinary years. This extra day is added to the month of February and occurs on February 29. On average, a leap year occurs every four years. This practice aims to compensate for the difference between the civil year of 365 days and the actual duration of the solar year, which is about 365.2425 days. This adjustment helps to more accurately align the civil calendar with the seasons. Leap years have an impact on many areas of society, including agricultural cycles, religious calendars, and long-term planning systems.

A leap year is a year with an additional day compared to normal years. This additional day is added to the month of February and corresponds to February 29th. The calculation to determine if a year is a leap year or not is based on well-established rules.

For a year to be considered a leap year in the Gregorian calendar, it must meet one of the following two conditions:

1. The year is divisible by 4, but not divisible by 100, unless it is also divisible by 400. For example, 2000 was a leap year because it is divisible by 400, while 1900 was not because although divisible by 4, it is also divisible by 100 but not by 400.

This means that the years 1600, 2000, 2400 are leap years, but 1700, 1800, 1900, 2100 are not. This rule was established to correct the difference between the too short year of 365 days and the too long year of 365 days and a quarter.

Following this calculation, December 25, 2020 was a leap year, the next February 29 will occur in 2024, then in 2028, and so on. This regularity in adding an extra day every four years helps maintain consistency between civil years and tropical years, which are slightly different in duration.

A year is a leap year in the Gregorian calendar if the year is divisible by 4, but years divisible by 100 are not leap years unless they are also divisible by 400. For example, 1900 was not a leap year because it is divisible by 100 but not by 400. However, 2000 was a leap year because it is divisible by 400. This system corrects the error accumulated by the Julian calendar, which added an extra day every four years without taking into account the centennial years. The rules of the Gregorian calendar therefore ensure better synchronization between the calendar year and the tropical year, thus reducing seasonal variations.

The Gregorian calendar was introduced in 1582 by Pope Gregory XIII to correct a accumulated time drift in the Julian calendar and ensure that the dates of Easter remain synchronized with the seasons.

Apart from February 29th, New Zealand has another time-related peculiarity: Chatham Island, located east of the main New Zealand island, is 45 minutes ahead of the mainland New Zealand time zone.

The zodiac that we know today has 12 signs, each corresponding to a month of the calendar. However, in antiquity, there were 13 signs, taking into account the sign of Ophiuchus located between Scorpio and Sagittarius.

Good to know
## Frequently Asked Questions (FAQ)

1
### Why do we need a leap year?

We need a leap year to adjust the civil calendar to the tropical year, which is the time it takes for the Earth to complete one orbit around the Sun.

2
### How do we know when a year is a leap year?

A year is a leap year if it is divisible by 4. However, century years (divisible by 100) are not leap years unless they are divisible by 400.

3
### What is the origin of the Gregorian calendar?

The Gregorian calendar was introduced by Pope Gregory XIII in 1582 to correct the accumulated discrepancy between the Julian calendar then in use and the spring equinox.

4
### What was the issue with the Julian calendar?

The Julian calendar had a leap year every four years, but this led to a gradual shift with the seasons.

5
### Which countries use the Gregorian calendar?

The Gregorian calendar is the most widely used civil calendar in the world, adopted by most Western countries and many others across the globe.

6
### Do all calendars use a leap year?

No, not all calendars use a leap year. Some calendars, like the lunar calendar, are based on lunar cycles and do not require adjustments related to leap years.

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