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What is an Equinox?

Today is the long-awaited spring, or vernal, equinox for folks in the northern hemisphere.  If you’re one of the 99.9% of people who are sick of winter then this is fantastic news.  In the Mid-Atlantic we’ve had one hell of a winter!  For three months it was just snowstorm after snowstorm and lots, I mean lots, of cloudy skies.  We also had the polar vortex swing by twice dropping temperatures into the single digits with sub-zero wind chills.  I think it’s fair to say that I am not alone in saying “Sayonara winter!”

The beginning of spring is officially today at 16:57 UTC or 12:57 EDT.  Why the exact time?  Well that’s because the changing of the seasons on Earth is indicated by the tilt of our planet.  As you might know the Earth orbits the Sun in a slightly elongated oval-shaped orbit.  Somewhat counter-intuitively it is not our position in our orbit that changes the seasons.  Did you know that Earth’s closest point to the sun actually occurs in early January?  Likewise Earth is furthest from the sun in the summer time.  It is actually how the Earth is angled with respect to the orbital plane that determines what season it is.  As the Earth orbits around the Sun it also spins causing day and night to come and go.  This rotation occurs around the Earth’s axis which is tilted about 23 degrees.  Throughout each year the Earth “wobbles” as it goes around the sun thanks to the inclination of the Earth’s axis.  When the Earth is tilted a certain way in respect to the Sun we call it an equinox.

The tilting of the Earth's axis throughout the year causing the seasons to change.

The tilting of the Earth’s axis throughout the year causing the seasons to change.

Equinox is a Latin term that means “equal night”.  During an equinox day and night are approximately the same length.  When an equinox occurs the plane of the Earth’s equator crosses the center of the Sun so that the Earth is neither tilted away or towards the Sun, causing an equal amount of sunlight to shine on both the southern and northern hemispheres.  As you can see from the incredible video below the day/night line (called a terminator) moves throughout the year if seen from space.  During an equinox the terminator is completely vertical.  In the case of the spring equinox the northern hemisphere has been angled away from the Sun for several months and thus has been receiving less direct sunlight and shorter days.  After the winter solstice during which the northern hemisphere experiences its shortest day the Earth begins to angle itself to point the northern hemisphere back towards the Sun resulting in longer days and more direct sunlight…summer!

This video is really hypnotizing.  What you’re seeing here is quite fantastic.  The video was taken using images from NASA’s Meteosat satellite.  Meteosat is a geosynchronous satellite which means it orbits around the Earth at the same speed as the Earth’s rotation so it always sees the same part of the planet.  As a result the planet never seems to move throughout the year.  In this video courtesy of Astronomy Picture of the Day and NASA, you see an entire Earth year in just twelve seconds!  You notice that the Earth’s terminator swings back and forth.  That is the seasons progressing as the Earth tilts on its axis.  The video begins on an autumnal equinox in September 2010 then the terminator swings over to the right giving the southern hemisphere lots of direct sunlight.  It reaches the solstice then swings back the other way towards the spring equinox when the terminator runs from north pole to south pole before swinging to the left and giving the northern hemisphere more light.  The video ends on the September equinox 2011 completing a full orbit around the Sun and an entire year for the people of Earth.

Because of our busy lives we tend to forget how dependent we are on the celestial mechanics of the solar system for life.  We owe our very existence to the fact that everything in the solar system, indeed the galaxy, moves in predictable orbits.  Thankfully for Earth our solar system is stable but that is not the case elsewhere in the galaxy.  In alien solar systems that we’ve only recently discovered there are planets that are drifting away from their stars and getting colder.  There are also planets that are being drawn inexorably towards their stars to eventually be consumed by them.  Other planets cross orbits too close to one another and are knocked off their orbits never to return to their home solar system.

We are lucky that our solar system has time to work out its kinks before life began on Earth.  If the solar system did not have orbital stability there’s a good chance life might not have arose here.  So be thankful we live on such a marvelous planet in a marvelous solar system!  On this spring equinox 2014 take a moment to think about the science that is behind the changing of the seasons and if the weather is clear tonight (fingers crossed!) go outside and take a look at the night sky and think that everything you see up there obeys the same laws that govern the seasons here on Earth.

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