Thursday, April 23, 2015

Twilight (#atozchallenge)

No, not this one…


A week ago we talked about noon, and mentioned that solar noon is the time when the sun is at its highest point in the sky at a given location. From our perspective, everything around us is fixed, and the sun rises in the east, makes an arc across the sky, and sets in the west. Sunrise is when it looks like the sun is rising over the horizon in the east, noon is when it looks like the sun is directly overhead, and sunset is when it looks like the sun is setting behind the horizon in the west. I say looks like because the horizon is an illusion, created by the curvature in the earth and the rotation of the earth on its axis.

A fun thing to do, a sort of rite of passage, really, is getting up early and watching the sun rise. This was great in Chicago, because Lake Michigan is east of the city, and it looked like the sun was coming up out of the lake. About a half hour before the sun comes up, it starts getting light outside. So, even though the sun comes up at 6:30 AM, it starts getting light out at around 6:00 AM. Likewise, if you were standing on the beach in Santa Monica, California, at sunset, you would see the sun drop behind the horizon over the Pacific Ocean at 8:00 PM, but it would stay light until 8:30 PM or so. We call these times dawn and dusk, respectively, but the true name for them is twilight.

There are three kinds of twilight: civil twilight, nautical twilight, and astronomical twilight. Dawn and dusk are civil twilight; they’re the times that it’s getting light or getting dark. The technical definition of civil twilight is when the sun is 6° or less below the horizon. You can think of civil twilight as when it’s light out enough to drive without your lights on, or when they turn the streetlights on or off. Nautical twilight is when the sun is from 6° to 12° below the horizon, and astronomical twilight is when the sun is from 12° to 18° below the horizon. They’re an average of 30 minutes apart over the course of the year. Here are the values for Atlanta on March 31, 2015:


from The Sound of One Hand Typing

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