"Twilight Edition"

Katy McNitt - August 6, 1995
Expert on Ozone and Climate Change

    Greetings from the South Pole, where the "long dark" has just ended. It's still plenty dark outside, but the moon has been up since the first of the month, getting fuller every day, and once the moon sets we'll see a faint glow of twilight rolling 'round the horizon. In fact, one person (who will remain nameless because they have been teased enough already) has claimed to have seen twilight since the 25th of July... long before the sun was eighteen degrees below the horizon... which leads me to the topic of this note: What the heck is twilight, anyway?

Well, as far as I know... "dawn" and "dusk" are mostly poetic terms, but twilight occurs both before sunrise and after sunset. And there are different kinds of twilight, too. Here at the South Pole, we have just leaped from the realm of "astronomical night" to ASTRONOMICAL TWILIGHT, because the center of the sun's disk is fewer than eighteen degrees below our horizon. (The equator is our celestial horizon, so a positive declination of eighteen degrees is the same as "eighteen degrees below our horizon.") Technically, you're not supposed to be able to see any trace of twilight glow during astronomical twilight, but even small changes in atmospheric refraction can cause some pretty bizarre images. I'll tell you more about those when we get closer to sunrise.

By 8 September or so, the center of the sun will be fewer than six degrees below the horizon, and we'll enter CIVIL TWILIGHT. At that point, the glow will be obvious, and first-magnitude stars will become hard to find as the sky gets brighter. The sky-brightness will double every two days, increasing by a factor of 1000 between 2-21 September!

Finally, by the end of September, the sun will cross the equator again, setting at the North Pole and rising at the South Pole, rolling 'round and 'round the horizon, higher and higher in our sky, reaching its highest point during the December solstice... not to set again until next March.

Of course, the sun isn't really moving up and down: these are just apparent movements caused by the tilt of the earth's axis. Make a model of your own if it's confusing, using a bare lamp as the sun and some sort of ball or globe as the earth. Once you decide where the North and South Poles are on your ball, tilt the ball about 23 degrees from vertical. If you hold your fingers at the Poles and spin the ball, you'll witness one earth day with each rotation. (If you're holding the North Pole up, spin the ball counterclockwise as you look down on it. See why the sun rises in the east and sets in the west?) Now, without changing the tilt of your earth, walk in a slow circle around the lamp, and you'll see what causes the seasons, and you'll understand why the Poles are dark for six months and light for six months. One complete circle around your lamp will equal one earth year (don't trip over the cord!) :)

Quote of the week: "Can you imagine living in a place where the sun rises Every Day?"

cheers, for now: LT. Katy McNitt, NOAA
Amundsen-Scott South Pole Station

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