Today?s the vernal equinox!

Today, at 23:21 UT (19:21 p.m. Eastern US time), the Sun’s odometer resets, and it once again finds itself at the celestial coordinates of 0h0m0s Right Ascension, 0°0m0s declination.

Or, in other words, it’s the vernal equinox!

A lot of folks will say this is the first day of spring. I think it makes more sense to call this the mid-point of spring — as do many countries — but I’m less inclined to argue about it as much as I used to. What the heck; it’s getting warmer in the northern hemisphere after quite a long and adventurous winter, and I went biking in the sunshine yesterday. It’s sure starting to feel like spring. Good enough for me!

In real terms, the equinox means a few things, too:

Day and night are about the same length (12 hours each)… although the Earth’s non-circular orbit and atmospheric distortion mess that up a bit.

The Sun rises pretty much due east and sets due west.

In the northern hemisphere the length of daylight is increasing the fastest. That sounds funny, but it’s not too hard to understand. In the northern hemisphere, just after the winter solstice, ...

Full story at http://blogs.discovermagazine.com/badastronomy/2011/03/20/todays-the-vernal-equinox/