Leap Year | Purpose, History & Origin 

–Devon Denomme

What’s a Leap Year?

To understand leap years, other units of time measurement must first be introduced. In a normal year, there are 365 days, which is the time it takes for the Earth to complete one full rotation around the sun. A day is characterized by the time it takes Earth to spin one full rotation on its axis, or the time it takes for a specific point on Earth to be at the exact place relative to the sun once again. These measurements are important to recognize when conceptualizing the meaning and reasoning behind leap years.

Leap years are years that have 366 calendar days. They are are a special occurrence because they only happen once every four years. The extra day is placed at the end of the month of February, with the additional date being February 29. In actuality, it takes the Earth approximately 365.25 days to complete a full solar rotation and create a full year. This means that the Earth is technically six hours into its daily axis rotation when the orbit around the sun is finished.

For the sake of simplicity, astrologists have adapted the calendar in this manner to better track Earth’s rotation around the sun. The ordinary citizen would likely find it confusing to track an extra quarter of a day. Therefore, the four quarter-days are totalled and make up an additional day once every four years.

Origin of Leap Years

The origin of leap years actually dates back to ancient times and the implementation of leap months, an additional month added every few years to keep in line with the seasons. The tropical year, also known as the solar year, is the time it takes for the cycle of seasons to repeat and the sun to travel from and return to the vernal equinox. The cycle of seasons takes approximately 365.242199 days to repeat.

Historians are unclear as to how exactly ancient Romans, ancient Chinese, and other cultures kept track of the months; however, because calendars were often controlled by individuals in power who would alter the time for political gains, Julius Caesar is credited with reorganizing the Roman calendar and introducing the leap day.

Caesar’s calendar was based off of the Egyptian calendar, which included 365 days and occasional months that were added every time astronomers observed a specific alignment of the stars. Julius Caesar simplified this setup by standardizing the addition of a day to the 365 day calendar every fourth year. The extra day was added onto the end of February because Romans had long adjusted the length of that month to cope with the unpredictable winter season. The Julian calendar was first used in 45 B.C.E.

As revolutionary as the Julian calendar was, it did have one significant flaw. The Earth was discovered to rotate around the sun for more than simply 365 days per year–rather 365 days, 5 hours, 48 minutes, and 46 seconds (one tropical year). This discrepancy was noticed by the Catholic church, which was angered that Easter was becoming more distant from its original placement on the calendar. Pope Gregory XIII ordered the modification of the Julian calendar and the following changes were made:

• Leap day was kept in place on every fourth year
• On centurial years (1700, 1800, 1900, etc), leap day would not occur
• Leap day would only occur on every fourth centurial year (1600, 2000, 2400)

The Gregorian calendar, as it became known, first went into effect in 1582 and is still used today. The Julian and Gregorian calendars have been the most impactful developments in leap year history, but neither are completely perfect. One day would go unaccounted for approximately every 3,030 years due to the specific timing of solar years.

What is the Purpose of Leap Years?

The reason for leap years is to keep the timing between the solar or tropical year as exact to the length of the seasons as they are experienced on Earth. The leap day is an important addition to the calendar every fourth year. If this time was not accounted for on the calendar, the timing of seasons and other solar events would become confusing, inaccurate, irregular, and unpredictable. If a calendar that matched the solar year precisely was used, then the time each day starts would change every year.

Why do leap years occur every fourth year, though, as opposed to another amount of time? There are 24 hours in a day, but at the end of the year, an additional six hours (rounded up) remain unaccounted for. If those hours are multiplied by four, a full 24 hour period is created. Instead of adding six hours onto every year, which could become confusing, the multiplier of four is used to place an additional day on every fourth year. This keeps the calendar simplified and regular so that it is more understandable and manageable.

There is one irregularity with the Gregorian calendar, however. On every centurial year (the beginning of a century), leap year does not take place, except on the fourth centurial year. Again, the exact timing of the Earth’s rotation is the cause for this discrepancy. Since it takes the Earth an additional 5 hours, 48 minutes, and 46 seconds to make a full rotation beyond the standard calendar each year, and 6 hours are accounted for on every fourth year, a displacement of 11 minutes and 14 seconds are in excess. As these minutes begin to total, a day technically needs to be omitted from the calendar. Every 100 years, leap year is not observed to account for this imbalance.

Evidently, leap years can be confusing due to the precise nature of the mathematics involved. However, they are an important event that takes place every four years with few exceptions. Interestingly, Earth is not the only planet with leap years; Mars in fact has more leap years than regular years because of the length of its rotation around the sun.