2008 is a leap year, so add an extra day

ST. LOUIS — Another year, another trip around the sun. Almost.

Earth hasn't quite completed the lap it started Jan. 1, 2007. There are still a little more than 400,000 miles to go before the planet completely orbits the sun.

And the 2007 mark is 400,000 miles shy of where it began in 2006, which is 400,000 miles short of the 2005 starting line.

But never fear, leap year is here.

This year February will get an extra day — a so-called "leap day" — which will allow Earth to trek about 1.6 million miles, keeping the calendar in sync with the seasons.

"It's the seasonal year that we set our calendar to, not the solar orbit," said Robert Buchwaldt, a geologist at Washington University.

But another twist is that the speed at which Earth spins isn't coordinated with how long it takes to get around the sun, said Erika Gibb, an astronomer at the University of Missouri-St. Louis.

The speed varies depending on the pull of the sun and Earth's "delicate, vibrating dance" with the moon and other planets, said Michael Wysession, a geophysicist at Washington University.

Earth is slowing down. Four hundred million years ago, a year was 420 days long. That is, the Earth was whipping around at a rate of 420 revolutions per year. Now it makes about 365 and a quarter turns per year. Storms and ocean tides speed or slow Earth's spin by fractions of fractions of seconds each year.

"Even huge earthquakes minutely change the length of day by redistributing mass. It's like sticking a piece of clay on a top; it causes a little wobble," Wysession said.

No matter how fast you spin, "it still takes the same amount of time to get back to where you started," Gibb said.

Earth takes slightly more than 365 rotations, or days, as we call them, to get around the sun. To be precise, it takes 365 days, 5 hours, 48 minutes, and 46 seconds to complete an orbit. That means the calendar year of 365 days is a smidge shorter than the solar year by about a quarter of a day. Over years, those extra quarters add up to days, then weeks and months until the seasons are shifted relative to the calendar.

Calendars were invented to help keep track of the seasons so people would know when to plant, when to harvest, when to pay taxes and so on.

In 46 B.C., Julius Caesar consulted an astronomer, borrowed concepts from the Egyptians and decreed a calendar with 365 days divided into 12 months be used. The calendar introduced a leap day every four years to account for those extra quarter-days it takes to circle the sun.

But a year is actually 365.24 days long, so the Julian calendar added too many leap days.

By the 14th century, the spring equinox had slipped 10 days to March 11 instead of March 21.

Finally, in 1582, Pope Gregory XIII convened a panel of scientists to fix the calendar. Because the Julian calendar was so far ahead of the season, 10 days were chopped out of October that year, so that Oct. 4, 1582, was followed by Oct. 15, 1582. To prevent the calendar from getting ahead of itself again, the scientists proposed to hobble some of the leaps.

Now, leap years occur in years divisible by 4, except for years ending in 00. If a century is divisible by 400, it is a leap year. So 2000 was a leap year, but 2100 will not be.

England, Ireland and the American colonies (now the United States) didn't start using the Gregorian calendar until 1752. By that time, the Julian calendar had slipped another day, so 11 days had to be cut out of September 1752.

The good news is that even though Earth is slowing down and the Gregorian calendar eventually will incorporate too many leap days, the system works pretty well.

"We're not going to run into any problems for thousands of years," said Frank Podosek, a planetary scientist at Washington University.