Answered from cited sources · retrieved September 20, 2026
why do we have leap years
Leap years keep our calendar synchronized with the Earth's orbit around the Sun, which takes approximately 365.2422 days rather than exactly 365.
Leap years exist to keep our calendar in sync with the Earth's orbit around the Sun. A solar year—the time it takes for Earth to complete one orbit—is approximately 365.2422 days, not exactly 365 days. Without leap years, the calendar would drift, causing seasonal events like equinoxes and solstices to shift over time. To correct this, we add an extra day (February 29) to the calendar. Because adding a day every four years is slightly too much correction, the Gregorian calendar includes exceptions: century years are not leap years unless they are also divisible by 400.
- Correcting for orbital drift. A standard calendar year is 365 days, but the Earth takes about 365.2422 days to orbit the Sun. This difference of nearly six hours per year would cause the seasons to drift significantly over time if left uncorrected.
- The leap year algorithm. To account for the extra time, a leap day is added to the calendar. However, adding a day every four years is slightly too much, so the Gregorian calendar skips leap years in century years (divisible by 100) unless they are also divisible by 400.
- Maintaining seasonal alignment. Without these adjustments, the calendar would fall out of sync with the seasons. For example, the summer solstice would eventually shift to a different time of year, which would disrupt agricultural and seasonal planning.
- Why February?. February was chosen for the extra day because it was already the shortest month, making it a convenient place to add the additional 24 hours.
Sources
- Doing the Math on Why We Have Leap Dayjpl.nasa.gov · published January 16, 2024
- Leap yearen.wikipedia.org · published March 17, 2026
- Why Do We Have a Leap Year Anyway?scientificamerican.com · published February 23, 2024
- Why do we have leap years? And how did they come about? | Live Sciencelivescience.com · published January 5, 2024
- The Science of Leap Yearairandspace.si.edu · published March 1, 2024
- Why do leap years have 366 days?pbs.org · published February 25, 2024
- The mathematical muddle created by leap yearsbbc.com · published February 28, 2024
- This year is a leap year. Here's why we have leap years and why the extra day falls in February - ABC Newsabc.net.au · published February 28, 2024
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