#### February 28, 2024

2024 is a leap year and more specifically, Feb. 29 is leap day, the day on which our planet’s yearly journey around the sun seemingly expands from 365 to 366 days.

But what is leap year and how long has it existed? Why is it in February? Jennifer Carter, assistant professor of physics, is here to answer all your questions about the intercalary holiday.

SU: What is a leap year? What is leap day?

JC: The leap day is the extra day added to the calendar during a leap year. A leap year generally occurs every four years, on each calendar year divisible by four. In the calendar used by the world today, leap day is Feb. 29.

SU: Why does leap year occur every four years?

JC: The reason we need a leap year is because Earth does not travel around the sun in a whole number of days. One year — the time it takes for Earth to go around the sun once — takes about 365 days and six hours. If we did not use a leap year, the seasons would slowly shift earlier in the year.

By banking the extra six hours, a whole day will be built up every four years. We use a leap day to spend the banked time. This process is called intercalation, and it has been used in many different calendars including ours, the Gregorian calendar.

SU: Does leap year always occur every four years?

JC: Leap years do not occur every four years like you might expect. A leap year is skipped at the beginning of every century when the year is not divisible by 400. For example, the year 2000 was a leap year, but 1700, 1800 and 1900 were not. But even this system is not perfect — it is only correct to about one day in 3,300 years.

SU: Do we know who established leap day? Or how long it has existed?

JC: The use of a leap day is likely as old as written calendars, dating back to the Bronze Age. Our modern leap day traces its roots to the calendar instituted in 46 BCE by Julius Caesar.

SU: Why is leap day observed in February?

JC: February was the last month of the year in the Roman calendar and already their shortest month. Using Feb. 29 as leap day seems to have occurred over time; it appears increasingly in legal documents from England in the 15th century.

SU: Do all areas of the world observe a leap year in their calendar?

JC: Most calendars must include a method of banking and spending days. Some calendars prioritize the motion of the Earth around the sun and try to ensure that the seasonal dates remain consistent. These are called solar calendars and include the Julian, Gregorian, Indian national, Ge’ez and Solar Hijri calendars.

Lunisolar calendars are based on the motion of the moon and sun and use leap months to line up the solar and lunar years. In these calendars, every second or third year will have 13 months. Finally, a lunar calendar is based only on the motion of the moon, ignoring the seasons, and does not require the use of leap days or leap months.

SU: How will you celebrate leap day?

JC: I tend to track the cycles of the year in terms of the equinoxes and solstices rather than the motion of Earth. Currently, we’re in early spring and it’s a time to look for signs of new plant growth as we approach the vernal equinox in March.