A leap year is a calendar year containing an additional day, February 29th, making it 366 days long instead of the usual 365. This adjustment keeps our calendar synchronized with Earth’s orbital period around the Sun. Without it, the calendar would gradually fall out of alignment with natural seasons, causing events like equinoxes and solstices to shift over time.
The Astronomical Need for Leap Years
Earth’s orbit around the Sun does not take exactly 365 days. The precise duration for Earth to complete one full cycle of seasons, known as a tropical year, is approximately 365 days, 5 hours, 48 minutes, and 45 seconds, or about 365.24219 days. If our calendar consistently used only 365 days, this accumulated fraction of nearly a quarter of a day each year would cause a drift. For example, after just 100 years, the calendar would be off by approximately 24 days, and in about 700 years, summer in the Northern Hemisphere could begin in December. Adding an extra day every four years accounts for these accumulated hours, keeping the calendar aligned with the astronomical year and maintaining seasonal predictability.
The Rules for Calculating a Leap Year
Specific rules determine when a leap year occurs. The primary rule states that a year is a leap year if it is evenly divisible by four. This accounts for the approximate quarter-day surplus accumulated over four years, adding a full day to February.
However, this simple rule would still lead to a slight overcorrection because the tropical year is not precisely 365.25 days. Years that are divisible by 100 are generally not considered leap years. For instance, the year 1900 was not a leap year, despite being divisible by four. This exception prevents the calendar from gaining too many extra days over centuries.
Years that are divisible by 400 are also leap years. This means that while 1700, 1800, and 1900 were not leap years, the year 2000 was a leap year because it was divisible by 400. These rules create a calendar system that very closely approximates the actual length of the tropical year, maintaining long-term accuracy.
Practical Effects of the Extra Day
The extra day in a leap year has minor impacts on daily life. Individuals born on February 29th, often referred to as “Leaplings,” have a unique birthday that only appears on the calendar every four years. In non-leap years, they typically celebrate their birthday on either February 28th or March 1st.
Most annual events and holidays are unaffected, as schedules are easily adjusted. For employment, salaried workers work one more day for the same annual salary, resulting in a slightly lower effective daily wage for that year. Conversely, hourly workers receive additional pay for the extra day they work. Some contractual agreements, such as monthly rent or public transportation passes, might offer a benefit as they cover an additional day without extra cost.