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The ‘Why?’ behind leap years

We’re all familiar with leap years when that mysterious extra day in February arrives. But have you ever wondered why?

February 27, 2024

Leap years are like the calendar's way of playing catch-up with the Earth's journey around the sun. We all know there are 365 days in a year, but – plot twist - it's not exactly 365 days. It's about 365.25 days (a solar year).

That might not seem like much, but over a long period (we’re talking centuries here) our calendar would slowly drift out of sync with the seasons.

Enter the concept of the leap year.

The Julian Calendar

The idea is credited to the ancient Egyptians but was refined by Julius Caesar in 45 BC.

A new calendar was introduced (the Julian Calendar) that added an extra day to February, every four years. This was their way of absorbing the extra time each year.

But the Julian system wasn’t perfect because - plot twist number two - the solar year is more like 365.2425 days, not exactly 365.25.

Fast forward to 1582, and the Julian Calendar had caused the seasons to drift by about 10 days.

The Gregorian Calendar

This drift was corrected by Pope Gregory XIII with the introduction of the Gregorian Calendar, which is the system most of the world uses today.

The Gregorian Calendar tweaked the leap year concept by adding a rule:

  • A year is a leap year if it is divisible by 4.

  • But there's a catch - plot twist number three - if the year can be evenly divided by 100, it is NOT a leap year, unless the year is also evenly divisible by 400.

  • So, 1600 and 2000 were leap years, but 1700, 1800, and 1900 were not.

Keeping the seasons on track

This nifty adjustment means the Gregorian calendar is much more aligned with Earth's orbit, and our seasons stay in the same place year after year. Without leap years, our calendar would lose almost six hours every year, and over centuries, summer, autumn, winter, and spring would completely shift their positions in the calendar.

So, the next time February 29 rolls around, you'll know it's not just an extra day on the calendar – it's a clever fix to a celestial challenge, centuries in the making!

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