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

Earth hasn’t quite completed the lap it started on Jan. 1, 2007. There are still a little more than 400,000 miles to travel before the planet makes a complete orbit around 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.

There’s really no need to worry if Earth gets “behind” in its orbit, said Robert Buchwaldt, a geologist at Washington University.

“It’s the seasonal year that we set our calendar to, not the solar orbit,” Buchwaldt said.

But setting the calendar turns out to be a task with astronomical problems. 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.

On Jan. 3, when Earth is at its closest point to the sun, it will be hurtling through space at 67,779 miles per hour, Gibb said. When Earth is farthest from the sun – in July – it will be traveling at a mere 65,542 mph.

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.

Meanwhile, Earth is whirling on its axis at a rate of 1,029 miles per hour at the equator, Gibb said. (St. Louis and other locations at this latitude are spinning at about 803 miles per hour, she said.)

But 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.

And for Earth, it 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 to be used in Rome. The calendar introduced a leap day every four years to account for those extra quarter days it takes to circle the sun.

And it would have been a fine calendar, too, if a year really were 365.25 days long, said Frank Podosek, a planetary scientist at Washington University. 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 21st. That threw off the date of Easter, which falls on the Sunday after the first full moon following the spring equinox, said Daniel Bornstein, a professor of history and religious studies at Washington University.

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. In order 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.

The Gregorian calendar also established Jan. 1 as the start of the year.

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 to compensate.

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

“We’re not going to run into any problems for thousands of years,” Podosek said.

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(c) 2007, St. Louis Post-Dispatch.

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AP-NY-12-31-07 1917EST