Spring Equinox

March 20, 2012 by staff 

Spring Equinox, 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.

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