Happy Fall Equinox

Today is the official start of Fall (or Autumn), because it’s the Equinox.   Google says the Autumn Equinox comes at exactly 4:02 pm EDT on September 22nd.  Equinox, since it’s derived from the word “equal”, means daylight and nighttime are exactly the same.  But like everything in this world, it’s complicated.

According to EarthSky, there are actually about eight minutes more of daylight than nighttime today at mid-temperate latitudes.  There are two reasons for this —

— The sun is a disk, not a point, so it takes some time for our parent star to disappear below the horizon.  This adds two and a half to three minutes of daylight.

— The Earth’s atmosphere acts like a lens or prism, uplifting the sun about 0.5o from its true geometrical position whenever the sun nears the horizon. This is known as atmospheric refraction (lower illustration), and it both advances the sunrise and delays the sunset, thus adding almost another six minutes of daylight at mid-temperate latitudes.  Also, the sun’s angular diameter spans about half a degree as well (http://earthsky.org/astronomy-essentials/why-arent-day-and-night-equal-on-the-day-of-the-equinox? ).

It gets better.  Again according to EarthSky, the fastest sunsets (and sunrises) occur at or near the equinoxes, and the slowest sunsets (and sunrises) occur at or near the solstices.

Why?  At every equinox, the sun rises due east and sets due west.  So on the day of an equinox, the setting sun hits the horizon at its steepest possible angle.  Conversely at a solstice, the sun is setting farthest north or farthest south of due west, so the angle is shallower, and the sunset is longer.

So if you live in Denver or Philadelphia, (latitude 40 degrees north),  on the day of equinox, the sun sets in about two and three-quarter minutes.  But the solstice sun sets in about three and a quarter minutes (http://earthsky.org/tonight/fastest-sunsets-around-equinox-time? ).

Who knew the setting sun could be so complicated?

The photos are from the websites.





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