Every four years, we add an extra day to the calendar in the form of February 29th. This is known as a leap day. So why do we need leap days?
What is a leap year?
A leap year is a calendar year that contains an additional day added to keep the calendar synchronized with the astronomical year or seasonal year. This additional day is added to the end of February, giving it 29 days instead of the usual 28.
Leap years occur every four years. The last leap year was 2020, and the next leap year will be 2024. Leap years help keep our calendars aligned with the seasons by adding this extra day.
Why are leap years needed?
Leap years are needed to keep our calendars in sync with the seasons because the length of the astronomical year does not perfectly match the length of our calendar year. Here are the key reasons why leap years are necessary:
- The astronomical year (the time it takes Earth to orbit the sun) is approximately 365.2422 days long.
- Our calendar year is 365 days long in a common year.
- This difference of around a quarter day each year adds up over time.
- Without leap years to add in extra days, the seasons would drift over the calendar.
- Leap years resynchronize the calendar with the seasons.
Essentially, leap years bring the calendar back into alignment with astronomical events and the seasons. The quarter day difference each year means the calendar would be off by about 1 day every 4 years without leap years.
Astronomical influences
There are two major astronomical influences that make leap years necessary:
Earth’s orbit around the sun
Earth takes approximately 365.2422 days to orbit the sun. This is known as a tropical year. Because our calendars use a whole number of days, an extra day needs to be added periodically to synchronize the calendar with Earth’s orbit.
Earth’s rotation on its axis
Earth’s rotation on its axis is gradually slowing down over time. This makes the mean solar day slightly longer than 24 hours now. In the past, days were shorter. This slowing rotation also contributes to the difference between the calendar year and astronomical year.
Together, Earth’s changing rotational speed and its consistent orbital period mean that a quarter day is unaccounted for in our calendar every year. The leap day every four years realigns things.
History of leap years
Leap years have been used for over 2000 years. Here is a brief history:
- 45 BC – The Julian calendar introduced leap years with an extra day added every 4 years.
- 1582 – The Gregorian calendar modified the leap year rules.
- Now – The Gregorian calendar is the internationally accepted civil calendar with leap years.
Julian calendar
Julius Caesar introduced the Julian calendar in 45 BC. This calendar had a leap day added every 4 years, giving those years 366 days. The leap day was added after February 23rd.
However, too many leap years were added, making the Julian calendar drift noticeably over centuries. By the 1500s, it was 10 days out of alignment with the seasons.
Gregorian calendar
The Gregorian calendar was introduced in 1582 to fix the issue. It modified the leap year rules:
- Leap years occur every 4 years, except…
- Century years not divisible by 400 are NOT leap years.
This removed 3 leap days every 400 years, better aligning the calendar. Some days were also removed in the transition to align it with the seasons.
Gregorian calendar leap year rules
The Gregorian calendar has clear rules dictating which years are leap years. These are the leap year rules still in use today:
- Leap years occur every 4 years.
- Years divisible by 4 are leap years, except…
- Years divisible by 100 are NOT leap years, except…
- Years divisible by 400 ARE leap years.
So:
- 2020, 2024, 2028 are leap years (divisible by 4)
- 2100, 2200, 2300 are NOT leap years (divisible by 100 but not 400)
- 2000, 2400 ARE leap years (divisible by 400)
This keeps the Gregorian calendar aligned with the seasons to within 1 day over a period of around 3000 years.
When does the leap day occur?
The extra leap day is added to the end of February, between February 28 and March 1.
So in a leap year, February has 29 days rather than the usual 28. The rest of the year has the normal number of days:
| Month | Days in a leap year | Days in a common year |
|---|---|---|
| January | 31 | 31 |
| February | 29 | 28 |
| March | 31 | 31 |
| April | 30 | 30 |
| May | 31 | 31 |
| June | 30 | 30 |
| July | 31 | 31 |
| August | 31 | 31 |
| September | 30 | 30 |
| October | 31 | 31 |
| November | 30 | 30 |
| December | 31 | 31 |
| Total | 366 | 365 |
Why is February affected?
February is chosen to have the extra leap day because it has the fewest days of any month in the Gregorian calendar. Adding the day to the end of February causes the least disruption compared to other months.
February normally has either 28 or 29 days. Keeping the leap day at the end means that the date of March 1st is not affected in leap years.
Examples of recent leap years
Here are some recent years that were leap years according to the Gregorian calendar rules:
- 2004
- 2008
- 2012
- 2016
- 2020
As you can see, leap years tend to occur every 4 years. But century years not divisible by 400 break the pattern.
Upcoming leap years
Here are some upcoming leap years we can look forward to:
- 2024
- 2028
- 2032
- 2036
- 2040
- 2044
- 2048
The next century year leap year will be 2400. Years like 2100, 2200, and 2300 are not leap years, even though they are divisible by 4.
Leap day traditions and superstitions
There are some interesting traditions and superstitions surrounding leap day:
- Women propose to men – In many countries, there is a tradition that women can propose marriage on leap day. If the man declines, he is supposed to give the woman money or buy her a gown.
- Leap day babies – People born on February 29 are “leap day babies”. They often celebrate their birthday on February 28 or March 1 in common years.
- Unlucky to marry – In Britain, there was a belief that marrying on leap day was unlucky. Greeks also felt it was unlucky to marry during a leap year.
- Frog proposals – In the late 5th century, an Irish legend linked leap years to an unfortunate proposal. If a man refuses a woman’s proposal on leap day, he must buy her 12 pairs of gloves to hide the embarrassment of her unadorned fingers.
Calendar variations
While the Gregorian calendar is widely used, some other calendar systems handle leap years differently:
Julian calendar
The Julian calendar adds a leap day to February every 4 years without exception. This creates too many leap years, causing excess drift over time.
Lunisolar calendars
Many lunisolar calendars like the Chinese and Hebrew calendars add an extra month rather than a day. This is done to keep in sync with the lunar cycle.
ISO 8601 calendar
The ISO calendar used in computing adds a leap week rather than a leap day. This keeps each month at 28 days but may disrupt weekly cycles.
Why not add more than 1 day?
You may wonder why our calendar doesn’t add 2 or more days to sync with the astronomical year. Here’s why:
- Adding just 1 day is the smallest adjustment needed.
- Keeping months at 28, 30 or 31 days is convenient.
- Adding multiple days would disrupt weekly cycles more.
One day is the least disruptive adjustment. The 400 year leap year cycle also neatly keeps the calendar aligned without ongoing tweaks.
Summary
In summary, leap years add an extra day every 4 years to the calendar to realign it with the astronomical year. This keeps seasons occuring at the same calendar dates.
The Gregorian calendar has clear leap year rules – years divisible by 4 are leap years, unless it’s a century year not divisble by 400. The extra day falls at the end of February.
Leap years have been used for over 2000 years. They provide a clever method to synchronize the man-made calendar with Earth’s orbit around the sun and the gradual slowing of Earth’s rotation.