What do the years 2020, 2024, 2028, and 2032 have in common? Besides being US presidential election years and years for the Summer Olympics, they are also leap years. This means February gets an extra day, extending it to 29 days long. But Why Do We Have Leap Years in the first place? The reason is a bit more complex than simply “every four years.”
The Earth’s Orbit and the Need for Adjustment
We often think of a calendar year as being exactly 365 days long. These “common years” are our way of marking the time it takes for the Earth to orbit the Sun. However, 365 days is actually a rounded figure. The precise time it takes Earth to complete one full orbit around the Sun, known as a sidereal year, is approximately 365.242190 days, or more accurately, 365 days, 5 hours, 48 minutes, and 56 seconds.
This seemingly small difference of 5 hours, 48 minutes, and 56 seconds each year accumulates over time. If we didn’t account for this extra time, our calendar would gradually drift out of sync with the Earth’s actual position in its orbit around the sun. Imagine the seasons slowly shifting! Over roughly 700 years, the summers we expect in June in the Northern Hemisphere would start to occur in December. This seasonal drift would be more than just inconvenient; it could be devastating for agriculture and many other aspects of life that are tied to the seasons.
Adding an extra day, the leap day, approximately every four years is our way of correcting this drift and keeping our calendar aligned with the sidereal year. However, the system isn’t quite as simple as adding a day every four years without exception.
The Nuances of Leap Years: Not Always Every Four Years
If we do some simple calculations, we find that the extra time we accumulate over four years is not exactly 24 hours (which is one full day). Instead, it’s closer to 23.262222 hours. This is because that extra fraction of a day each year is not precisely a quarter of a day.
By adding a leap day every four years, we slightly overcompensate. We actually make the calendar year a little longer than the sidereal year by about 44 minutes and some seconds over those four years. While 44 minutes might not seem like much, these extra minutes also add up over centuries, and would eventually cause the seasons to drift again if left unchecked.
To address this overcorrection, there’s an additional rule in place. Leap years are skipped in years that are divisible by 100 but are not divisible by 400. For example, the year 2000 was a leap year because it is divisible by 400. However, the years 1700, 1800, and 1900 were not leap years, even though they are divisible by 4, because they are also divisible by 100 but not by 400. The next time we will skip a leap year due to this rule will be the year 2100.
Why the Term “Leap Year”?
The term “leap year” comes from how the extra day affects the days of the week. A common year has 365 days, which is 52 weeks and 1 day. This means that if your birthday falls on a Monday in a common year, it will fall on a Tuesday the following year.
However, with the addition of an extra day in a leap year, your birthday “leaps” over a day. Instead of your birthday being on a Tuesday the year after a common year, in the year following a leap year, your birthday will “leap” over Tuesday and land on a Wednesday. This “leaping” of days of the week is where the name “leap year” originates.
And for those born on February 29th, the leap day, it doesn’t mean you only get to celebrate your birthday every four years! In non-leap years, people born on February 29th typically celebrate their birthdays on March 1st. You still age every year just like everyone else.
In conclusion, leap years are a crucial correction to our calendar system. They ensure that our calendar year remains synchronized with the Earth’s orbit around the Sun, keeping our seasons in their expected places and maintaining a consistent and reliable way to track time throughout the year. Without leap years, our calendar and the natural world would slowly drift apart, leading to significant disruptions over time.
