What would happen if leap year didn't exist? See animation

Yesterday was February 29th, a day that only happens in what is called a leap year. Every four years, an extra day is added to the Gregorian calendar, making February 29 days instead of the usual 28.

Leap days and months are there to compensate for our calendars not exactly matching the solar year. The main consequence of not having a leap year is that the days start to deviate.

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This animation by astronomer Dr. James O'Donoghue clearly shows how this deviation occurs. It is worth noting that the video presents the seasons of the Northern Hemisphere. Here in the Southern Hemisphere, winter lasts from June to September, spring from September to December, summer from December to March and autumn from March to June.

This wouldn't have many real-world consequences, although it could be a hassle for those who rely on the seasons for work (like farmers) rather than arbitrarily setting dates for projects. It could also, as happened before the introduction of the Gregorian calendar, actually affect religious festivals.

The Gregorian calendar was introduced after Easter began to move away from the vernal equinox. Enough time without leap years, and it could end up being very cold during Carnival, or who knows (finally) we'll see snow at Christmas in southern Brazil. Have you thought?

Adjustments made over time

  • Calendars have changed a lot over the years and between cultures.
  • Before Julius Caesar introduced the Julian calendar in 45 BC, and with it leap days every four years, the Roman calendar had just 354 days.
  • To keep the calendar aligned with the Earth's passage around the Sun, this calendar had to introduce an entirely new month every four years.
leap year
Statue of the Roman emperor Julius Caesar. (Image: Jule_Berlin /

[O lendário rei de Roma, Numa Pompílio] In, [calculou] the difference between the lunar and solar year in eleven days, as the moon completed its annual course in three hundred and fifty-four days, and the sun in three hundred and sixty-five.

To remedy this incongruity [Numa] he doubled the eleven days, and every two years he added an intercalary month, to follow February, consisting of twenty-two days, and called by the Romans the month Mercedinus. This amendment, however, over time came to require other amendments.

Plutarch in The Life of Numa, around the beginning of the second century AD

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Sculpture of Pope Gregory XIII in Bologna, Italy. (Image: Kizel Cotiw-an /
  • The Julian calendar was a vast improvement over adding an extra month every four years, as it changed to just adding one day. However, it wasn't perfect.
  • To try to align the calendar with the solar year (the time it takes for the Earth to revolve around the Sun), the Julian calendar had a leap year every four years, in which an extra day was added.
  • The problem was that because the actual solar year has 365.24219 days, the Julian calendar quickly (and when we say quickly, we mean incredibly slowly) gains a day every 314 years.
  • While this may not seem like much, by the time the Gregorian calendar was introduced in 1582, the year had deviated by 13 days.
  • The adopters of the calendar tried to compensate for this, but due to a miscalculation, they only skipped 10 days instead of 13.
  • To avoid further deviations, the Gregorian calendar continued with leap years every four years, but now skips them when the year is divisible by 100 and not divisible by 400.

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