Vernal Equinox – how we know it’s the first day of Spring

Lots of folks in the U.S. are celebrating the fact that today is officially the first day of spring.  After a very rough winter, a good portion of the country is hoping for no more ice, snow, nasty storms, or sub-zero temps.  Another chunk of the country is desperate for rain and anxiously anticipating a rough fire season.  Either way, we’re all ready for a change of pace, and so with spring comes great hopes for some improved weather.  So how do we know it’s now spring?

Our seasons are marked by our orbit around the sun.  Winter and summer are marked by the solstices, and spring and fall are marked by the vernal and autumnal equinoxes, respectively.  The winter solstice is the shortest day of the year, the summer solstice is the longest day of the year, and the equinoxes are those in-between points where the day and night are the same length (the root words help you remember this one – equi/equal, nox/night).  Equinoxes happen when the plane of the earth passes through the plane of the sun, which happens twice per orbit (year).

Remember that our planet isn’t perfectly upright in our orbital plane – we’re tilted.  As the planet travels around the sun, for half of the orbit our pole will be tilted away from the sun, and for the other half it will be tilted towards the sun.  This is also what makes the seasons opposite in the two hemispheres.  When we are tilted towards the sun (summer), Australia is tilted away from the sun (winter), and vice versa.  Spring begins at the vernal equinox, when the sun shines right on our equator.  From here until the summer solstice in June, our days will just keep getting longer!

And now I’m going to shut up and just let this National Geographic video explain it better than I can with just typing words and trying to do hand gestures you can’t even see…


Also, look at today’s Google doodle if you haven’t already.  It’s pretty!


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Filed under Geek parenting resources, Geeking out

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