Without leap years, today would be

Average time between leap days:

Leap years, like the one this year, keep the calendar on track. Try for yourself: can you keep Jan. 1 close to the start of the solar year?

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What happens without leap years?

Not much, at least at first.

But little by little, the calendar year would get out of sync with the solar year. That’s because our calendar year usually lasts 365 days. But not exactly. It actually takes 365 days, 5 hours, 48 minutes and 45 seconds for Earth to complete its journey around the sun.

Every year, the calendar falls about one-quarter of a day behind the solar year. Over time, Jan. 1 would come earlier in winter, then in the fall. After about 780 years, New Year’s Day would coincide with the summer solstice.

If we never had leap years, today would be .*

This date is calculated by adding previous leap days to today’s date.

That assumes you start counting from the time of Julius Caesar and Cleopatra in 46 B.C. Through Cleopatra, Caesar met Egyptian astronomers on his trips to Alexandria. The Egyptians had figured out something the Romans missed: Days don’t fit neatly into a year.

There are about 365.2422 days each year. And that number is shrinking as tidal forces slow Earth’s orbit ever so slightly.

"The timepiece that we use, the Earth, is not as accurate as we need it to be in modern society," said David Ewing Duncan, author of the book “Calendar: Humanity's Epic Struggle to Determine a True and Accurate Year.”

"Natural objects have always been our timepiece, but we've always had to adjust," he said.

Hence the leap year and leap day on February 29th.

With Caesar’s new calendar, the Roman Empire would have three 365-day years followed by one with 366 days. This would “leap” the calendar forward, aligning it with the solar year before the two got too far out of whack.

Each year would be 365.25 days.

To get ready for the switch Caesar had to get the Roman calendar caught up. Until then, the Romans had been using a lunar calendar. So in 46 B.C., Caesar added 80 days to the calendar, in what became known as the “year of confusion.”

Unfortunately, Caesar’s plan to add a leap year every four years overshot the mark.

The difference between the extra .2422 of a day in the solar year and the extra .25 of a day in the calendar year amounted to 11 minutes and 14 seconds, according to Duncan.

It may not sound like much, but after four years the calendar was off by about 45 minutes. After about 125 years, the calendar was off by a day.

The Julian calendar remained unfixed for 1,600 years. Finally, in the mid-1570s, Pope Gregory XIII was convinced that something had to be done. The dates were off so much it made it hard to track holidays, particularly Easter.

So he organized a calendar reformation with the goal of figuring out when Easter – and every other day – really and truly was.

The reform commission confirmed that the Julian calendar was adding leap days too frequently. In fact, they calculated that they were off by 10 days.

The panel recommended removing those 10 days from the calendar and adding a rule to the leap year system: Skip three out of four leap years that ended in “00.”

If the year was divisible by 400, the leap day remained. That’s why 2000 was a leap year. But 1900, 1800 and 1700 were not. Leap day will be skipped in 2100 if the Gregorian calendar remains unchanged 84 years from now.

Roman Catholic countries quickly adopted the pope’s new calendar. But Protestant countries were suspicious of edicts from Rome. Many of them didn’t accept the calendar until 1775.

Britain and its American colonies made the switch in 1752, Duncan said. By then they had to skip 11 days to catch up with the Gregorian calendar.

The Gregorian calendar also has its faults. Each year, it’s off by about 26 seconds.

Duncan said this will difference will add up to one day by the year 4909.

“We could decide to shift everything around again,” he said.

That probably won’t be necessary. Our timekeeping system is now pretty much divorced from the sun. It is based on the atomic clock.

Instead of defining seconds as a fraction of a day, we now use computers to count the oscillations of atomic cesium: 9,192,631,770 of them add up to one second. That’s much more precise than tracking Earth’s movements around the sun.

But it doesn’t entirely streamline our calendar, with months that seesaw between 30 and 31 days (or 28 and 29).

Other calendars have attempted to fix these problems.

The French introduced a calendar in 1792 that had 12 30-day months. Each week had 10 days. And they added five days (or six in a leap year) to the end of each year. These were holidays.

Another calendar introduced in 1902 contained 13 months with 28 days each. Each month would begin on a Sunday and end on a Saturday.

Each year would end with “year day.” In leap years, a leap day would be added in the middle of the year.

With all this precision, scientists can tell just how far off Earth is from atomic time. And every once in a while, they introduce a leap second to get our clocks back in sync with our slowing planet.

“The agreed definition of time is atomic time,” said Bob Tjoelker, a supervisor working with NASA’s Deep Space Network at the Jet Propulsion Laboratory in La Cañada Flintridge. “The Earth-based day has instability to it. It wiggles and jiggles.”

Tjoelker should know. His team manages an array of antennas that use time to track distant spacecraft.

“Leap seconds are a big, big deal for deep-space navigation,” he said. Being a nanosecond – a billionth of a second – off is the equivalent of a foot in space, he said.

NOTE: The “year” described above is the solar, or tropical year, which is a measure of time between the vernal equinoxes. The sidereal year begins and ends at the same fixed point. The two differ because of Earth’s wobble. The math used in the graphic at the top of this page simulates the tropical year.

Additional programming by Armand Emamdjomeh