With excitement and anticipation, you make your way down to the Christmas tree, turn on the lights, and announce to the rest of the family that it’s Christmas Day. With a chill in the indoor air, you nestle under a blanket and sip your hot cocoa (or coffee if you prefer). Christmas morning, even as an adult, still ranks as one of your favorite mornings of the year. 

After your family opens some presents and enjoys a nice brunch, you head into the rest of your day. Stepping outside, the blistering 100°F temps remind you of what people used to describe as summer, but Christmas in the year 2523 is a different time. The lawn is dry, the sun intense, and the air conditioning bills high. No, global warming is not the culprit. This is life without leap year! Don’t like this situation? Give it a few more centuries and Christmas will once again occur during the part of the year we in the northern hemisphere call winter. 

What Is a Year?
To understand why we need leap years, a little background on years and seasons is required. The cyclical nature of years (summer, fall, winter, spring) and the existence of seasons are not arbitrary or random. Rather, the motion of Earth around the Sun determines the length of the cycle—defined as a year—and the tilt of Earth’s axis determines the seasons. As shown in the figure below, Earth’s axis is tipped relative to the plane of Earth’s orbit. This means that as Earth moves around the Sun, the amount of sunlight received at a given location will change. When the North Pole points away from the Sun, the northern hemisphere experiences winter and the southern hemisphere summer. Conversely, when the North Pole points toward the Sun, the northern hemisphere experiences summer and the southern hemisphere winter. 

Figure 1: Earth’s positions as it revolves around the Sun, with seasonal dates and configurations.

We define a year as the time it takes for the Sun to return to the same spot in Earth’s sky, and we call this quantity a tropical year.1 Although the precise length of a tropical year can vary by about 30 minutes, it averages 365.2422 days, or 365 days, 5 hours, 48 minutes, 45 seconds. Since we only have 365 days in a calendar year, the calendar year will slowly drift out of phase with the tropical years at a rate of about 6 hours/year. Since the location of Earth in its orbit determines the season, when a calendar year only has 365 days, the seasons will slowly drift through the calendar. 

How Does Leap Year Work?
Throughout most of human history seasonal occurrences like the beginning of harvest and other natural agricultural-related events drove much of life, and different cultures adopted various approaches to keep the calendar years synced with the tropical year. It was a common occurrence that calendar years varied from year to year. For example, the Hebrew calendar had 12 months with a total of 354 days. To compensate for seasonal drift, a thirteenth lunar month was added every 2–3 years. Starting in 45 BC, Julius Caesar sought to remove any reason to modify the year since it was often done for political motivations. Caesar modified the Roman calendar so that each month had 30–31 days (except February) because a superstitious previous king thought months with even numbers were unlucky. Additionally, Caesar instituted a policy of adding one day to the calendar every four years, placing that day before March 1. Thus, the average year in a Julian calendar is 365.25 days, which is much closer to the average tropical year of 365.2422 days. 

This Julian calendar aligned with the tropical years quite well, but the average Julian year is slightly longer than the average tropical year. By the 1500s, the Julian calendar had drifted by ~10 days relative to the tropical year. Thus, in October 1582, Pope Gregory XIII introduced the calendar that now bears his name. The most significant difference between the two is that the Gregorian calendar has only 97 leap years every 400 years but the Julian calendar has 100. With this change, the average Gregorian year is 365.2425 days, and the drift between the Gregorian calendar and tropical years is about 1 day every 3,000 years!

The rule for determining which years are leap years is this: 

According to the Gregorian calendar, which is the civil calendar in use today, years evenly divisible by 4 are leap years, with the exception of centurial years that are not evenly divisible by 400. Therefore, the years 1700, 1800, 1900 and 2100 are not leap years, but 1600, 2000, and 2400 are leap years.

Leap Year Reflects Order
I’ve always thought leap years were cool. Specifically, the year I finally had more birthdays than my mom (born on February 29) was memorable. And that’s something that most of her grandkids remembered as well! More importantly, the need for leap year reflects the incredible order in our universe. Think about it for a minute. Earth travels nearly 600 million miles each year in its orbit around the Sun, and that journey takes 525,948  +/- 15 minutes! And scientists can calculate those variations for each year by accounting for the gravitational influences of the Moon and other planets in the solar system. This knowledge allows us to adjust calendars accordingly. 

Scripture makes it clear that God has the power to create the universe, but it also reveals another attribute: God’s immutability. God is the same yesterday, today, and tomorrow. If God were to withdraw his hand from upholding the universe, it would tumble into nonexistence. In other words, God’s immutability results in the universe behaving reliably because God sustains it at every place, all the time. In fact, God sustains the universe so reliably that we can describe its behavior using terms like the “laws of physics” and, for science to function properly, that reliability is an absolute necessity.

Endnotes

1. There are other ways to define a year, such as a sidereal year, which is the time it takes for Earth to complete one revolution of its orbit around the Sun. However, for calendar purposes, we’re most interested in aligning the seasons (primarily determined by equinoxes) so the tropical year is used to define the Julian and Gregorian calendars.

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