Fall begins!

Fall begins at 4:02 pm in Eastern, 3:02 pm Central, 1:02 am Pacific, 8:02 pm GMT on September 22, 2017. Here in the northern hemisphere this is called “fall equinox.”

“Equinox” means equal night, because the Sun is exactly over the equator, the Earth’s terminator runs exactly from pole to pole, with all parts of earth receiving 12 hours of daylight and 12 of dark… but wait!  Look at your newspaper.  The time from sunrise to sunset on Sept 23 is actually LONGER than 12 hours, by about 6-7 minutes.  Why?

On an equinox, sunlight falls directly over the Earth’s equator.  The “terminator” — the line dividing daylight from nighttime — runs vertically from pole to pole.

Answer:  Actually the answer has two causes.  One is the finite size of the sun.  Sunrise is defined as the first part of the Sun peeking over the Eastern horizon, and sunset as the last part of the Sun slipping below the western horizon.  This is twelve hours PLUS the time that the Sun takes to move its width across the sky.  The Sun is a half-degree across and the Earth rotates 15 degrees per hour (360 degrees in 24 hours), so the Earth rotates a half-degree in two minutes.  So two minutes of the difference is just from the finite size of the Sun.   What is the rest?   REFRACTION.

Light refracts when passing through a medium.  If the medium is uniform, light just slows down a little. But if the medium is not uniform in density or thickness, the light bends.  Since the air is more dense near the Earth’s surface, the light moves more slowly there than in higher elevations, causing the wave front to tip forward, following the curvature of the Earth.  So we actually see the sun BEFORE it actually breaks our horizon, and we see it for a few minutes after sunset too..  And that’s the other 5 minutes!  The refraction depends on the air temperature, surface temperature, etc, but in general is just larger than the diameter of the Sun – we see all of the Sun before any of the Sun really is above the horizon!

More info: www.weather.gov/cle/Seasons and www.timeanddate.com/calendar/september-equinox.html

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