Bad Astronomy

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, daytime starts getting longer. But the difference day-to-day is small; one day may only be a few seconds longer than the day before. At the equinox (today!) that difference can be several minutes from one day to the next. The amount each day is longer is itself getting bigger every day from the winter solstice to the vernal equinox. After today it slows down, and near the summer solstice in June each day will only be a few seconds longer than the one before it… and then the whole thing reverses. Speaking of which, reverse all that for the southern hemisphere. And again, the Earth’s elliptical orbit complicates things, but hey, close enough.

  • And finally, it means you can stand an egg on end… as you can do any day of the year. Here’s proof!

    [embed width=”500”][/embed]

    So, what are you going to do with your equinox?