Why Leap Day is every four years

Marcia Moore, The Daily Item, Sunbury, Pa.
·2 min read

Feb. 28—SELINSGROVE — Most people around the world will celebrate Leap Day Thursday and Jennifer Carter, assistant professor of physics at Susquehanna University, is breaking down what it means.

"The leap day is the extra day added to the calendar during a leap year (every four years)," she said. "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."

Banking six hours per year for four years is a full day and on Leap Days, the bank time is spent, Carter said. The process is intercalation. It has been used on many different calendars, including the one used today, the Gregorian calendar.

"Leap years do not occur every four years like you might expect," she said. "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.

"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."

So, why is Leap Year celebrated in February?

"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," said Carter.

She said most calendars 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," Carter said. "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."