Across much of the United States, this has been an unusually mild winter, especially for those living east of the Mississippi. Not a few people have noted that spring seems to have come early this year. Of course, in a meteorological sense that could be true, but in 2012 it will also be true in an astronomical sense as well, because this year spring will make its earliest arrival since the late 19th century: 1896, to be exact.
The vernal equinox — the first day of spring — will arrive tomorrow (March 20) at 05:14 Universal Time, or 1:14 a.m. EDT. Even more intriguing is that for those in the Mountain and Pacific Time zones, the equinox will actually arrive tonight (March 19).
Astronomers define an equinox as that moment when the sun arrives at one of two intersection points of the ecliptic (the sun's path across the sky) and the celestial equator (Earth's equator projected onto the sky). One such intersection point is located in western Virgo; the sun arrives there on Sept. 22 or 23, and appears to cross the equator from north to south, marking the beginning of autumn in the Northern Hemisphere.
The other intersection point, in eastern Pisces, is where the sun will be tomorrow. The sun is now migrating north of the equator, hence this is the "vernal" or spring equinox. At 5:14 UT next Tuesday, the sun will be shining directly over the equator from the point of view of a spot in the Indian Ocean, 757 miles (1,218 km) southeast of Colombo, Sri Lanka. [Earth's Equinoxes & Solstices (Infographic)]
Why so early?
Now maybe this rings false. After all, when some of us of a certain age were growing up, the first day of spring was always on March 21, not March 20, right? Now all of a sudden spring is coming on March 20, and as we just noted, even earlier — March 19 — for some.
Is this unheard of? Not if you look at the statistics. In fact, did you know that during the 20th century, March 21 was actually the exception rather than the rule? The vernal equinox landed on March 21 only 36 out of 100 years — most of these occasions coming during the early and middle part of the 20th century. Yet from 1981 to 2102, Americans celebrate the beginning of spring no later than March 20. Still, for many March 21 is ingrained in our culture as the traditional first day of spring, even though it's been more than 30 years since that last happened.
There are a few reasons why seasonal dates can vary from year to year.
- A year is not an even number of days, and neither are the seasons. To try to achieve a value as close as possible to the exact length of the year, our Gregorian Calendar was constructed to give a close approximation to the tropical year, which is the actual length of time it takes for Earth to complete one orbit around the sun. It eliminates leap days in century years not evenly divisible by 400, such 1700, 1800 and 2100, and millennium years that are divisible by 4000, such as 8000 and 12000.
- Another reason is that Earth's elliptical orbit is changing its orientation relative to the sun (it skews), which causes Earth's axis to constantly point in a different direction, called precession. Since the seasons are defined as beginning at strict 90-degree intervals, these positional changes affect the time Earth reaches each 90-degree location in its orbit around the sun.
- The pull of gravity from the other planets also affects the location of Earth in its orbit.