2000 Divided by 400 Equals Leap Year

The year 2000 exposes a basic flaw of the calendar—each year is about 11 minutes too long to keep the solstices and equinoxes where they should be. The problem began more than 2,000 years ago, when Julius Caesar created “leap year,” slapping an extra day onto the end of February every four years.

Leap year stuck, but it wasn’t a smooth ride. In Caesar’s day, about 45 BC, most people followed the lunar calendar. People figured out the day of the month by checking the phase of the moon, which orbits Earth every 29 days. The ancient Babylonians began each new month on the day of the new moon. The full moon marked the middle of the month, and as the moon waned back to new moon, the month was over. As the years progressed, the lunar calendar became out of step with spring, summer, winter and fall. Caesar noticed this error and enlisted Sosigenes, an astronomer from Alexandria, to invent a new calendar.

Sosigenes knew that it takes Earth 365 days, 5 hours, 48 minutes and 46 seconds to orbit the sun. He decided each calendar year should be 365 1/4 days, with one “leap day” added every four years. To honor his boss, Sosigenes named his creation the “Julian calendar.”

The Julian calendar remained unchanged for more than seven centuries, until a monk known as the Venerable Bede calculated that the 365 1/4-day Julian year was 11 minutes, 14 seconds too long to keep perfect time with Earth’s journey around the sun. At the time, people ignored Bede’s finding because the difference between the Julian calendar and the sun’s trip around Earth added up to an error of only one day every 128 years.

But those years added up, and the calendar fell out of sync with the sun. In 1582, Pope Gregory XII noticed that the spring equinox occurred on March 11 instead of the date upon which it should have fallen, March 21. He repaired the problem by getting rid of 10 days—October 4, 1582, was followed by October 15, 1582.

Pope Gregory XII decided he needed to fine tune Caesar’s calendar. He made new leap year rules that apply to only the first years of each century. He declared that if the year beginning a new century divided by 400 left no remainder, it would be a leap year.

The Gregorian calendar knocked out leap years in 1800 and 1900, but we’ll have one this year. Those folks living in the year 2100 will be denied a leap year thanks to the pope.

Though the pope’s calendar is used almost universally today, Russia didn’t accept the Gregorian calendar until 1918. When the United States purchased Alaska in 1867, Russians were still using the Julian calendar, and those in the territory of Alaska at the time lost 11 days.

Leap years also have special significance to Alaskans. Summer solstice happens 18 hours earlier than in other years. This year’s summer solstice will occur on June 20 at 5:49 p.m., instead of the traditional June 21.