Spring has sprung a bit earlier than usual - in fact, it's the earliest vernal equinox in over a century.

We commonly associate the start of spring with March 21, but this year, the vernal equinox officially happened on March 20 at 5:14 UT (1:14 am EDT) when the center of the sun shone down directly over the equator, right over Singapore, according to the U.S. Naval Observatory.

After the equinox, the sun's path through the sky will appear to creep north of the equator as Earth's orbit, combined with the planet's tilted axis, points the Northern Hemisphere more toward Sol.

The earliest equinox before Tuesday came in 1896, which fell on March 20 at 2:23 UT - or late on March 19, for much of the Western Hemisphere.

We will continue to see the time of the equinox shift a bit earlier every four years until 2100, says Geoff Chester, a spokesman for the U.S. Naval Observatory.

The reason for the varying start of the season, Chester says, comes from the Gregorian calendar, which is a good approximation of the time it takes the Earth to travel one complete circuit around the sun, but isn't perfect.

Fortunately, the calendar contains some corrective measures. For instance, most years divisible by four, such as 2012, are designated as leap years. But the calendar eliminates leap years in century years not divisible by 400, such as 2100. In that year, February will have just 28 days, giving the equinox a chance to catch up with the calendar.

In 2102, the equinox will fall on the 21st for the first time since 2007, Chester says.

Conventionally, we might think the equinox comes when night and day are the same length - but in 2012, that already happened before Tuesday for most places north of the equator. For instance, on Tuesday in New York City, daytime will be slightly longer than nighttime by about 10 minutes, according to the U.S. Naval Observatory.

The reason for the slight discrepancy arises from several factors. When determining day length, the times of sunrises and sunsets are calculated when the upper edge of the solar disc crosses the horizon, not the center. Also, Earth's atmosphere bends the sun's light, meaning that the sun appears to be crossing the horizon sooner than it actually is.

Weather-wise, the National Oceanic and Atmospheric Administration is forecasting a dry spring thanks to a less snowy winter.

We're not forecasting a repeat of recent historic and prolonged flooding in the central and northern U.S., and that is a relief, Laura Furgione, deputy director of NOAA's National Weather Service, said in a statement last Thursday.

Note to our readers in the Southern Hemisphere: don't forget to replace all instances of vernal equinox with autumn equinox, reverse daytime and nighttime, and take the sweaters out of storage.