Blinding Lights, Losing Light

If you had the misfortune of driving west at sunset this last Saturday, September 22, then you experienced one of the notable characteristics of equinox days, namely that the Sun sets due west (cardinal west) for everyone on Earth.

Another interesting characteristic of equinox days is that the length of day is roughly 12 hours for everyone on Earth.  This is because the Sun is shining directly over (is zenially shining over) the equator.  This means that the terminator of the Sun’s light on Earth goes directly through the north and south poles of our beloved planet.  So everyone, rotating on their latitude circles, experiences 12 hours of sunshine, and 12 hours of twilight and night.

Because the Sun is shining directly over the equator, we see it descending due west, regardless of where we are on Earth.  Here in Chicago, pointing to the Sun at sunset means pointing to a point on the equator 180 degrees west longitude (aka the international date line).  Looking on the globe, this is around Howland island, where Amelia Earhart’s plane went down.

Put another way, when we Chicagoans point west, we are not really pointing along the latitude circle we live on: instead we are pointing along a great circle arc from Chicago to Howland island.


Because of Mr. Kepler’s second law of planetary motion, we know the Earth is moving faster through its orbit as we approach Winter Solstice (when Earth is roughly closest to the Sun), so concomitantly, we expect to lose quite a bit of time per day.

And losing time we are.  Right now we are losing 2 minutes and 47 seconds per day.  Since September 1st (the first day of meteorological Fall, a mere 3 weeks ago), we have lost an astounding 57 minutes of direct sunshine.  By the time Winter Solstice arrives (December 21), we will be down to the depressing amount of 9 hours and 7 minutes of direct sunshine.

One consolation: the amount of indirect lighting (twilight) stays the same year round, as it is a function of latitude only.  So we always have about 30 minutes of civil twilight in the morning(aka dawn), and 30 minutes of civil twilight in the evening (aka dusk).

Oddly, every day is an equinox day for people who live on the equator.  Their dawn and dusk twilight times are only 17 minutes each.

One last equinox observation: on an equinox day, you can “see” your latitude with the setting Sun.  The angle the Sun’s celestial path makes with due west and a line vertically upward from it is your latitude.

Click on the image to learn more.

To view sunrise and sunset times for Chicago year round, click here.

Author: Bob Mahoney

Physics teacher

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