# Equinox sunset at South Pole? Promptly at ?? o'clock

You'd think the answer would be quite simple: What time does the sun set at the South Pole on the equinox?

In a perfect world, the sun would be exactly half-way down the horizon at both poles at the moment of equinox -- which is 10:32 a.m. our time on Saturday. For the South Pole, it's the first time the sun is attempting to set since late September. For the North Pole, it's the first hello to the sun since then.

But the answer is not that simple. Actually, I'd say it's not even somewhat complex -- it's *really* complex.

Dr. Dale Ireland -- he of the great time lapse camera over in Silverdale that we feature frequently here -- has a friend who is at the South Pole working on the CUBE neutrino detector down there. They received an e-mail from a NOAA worker detailing just how difficult it is to figure out just when the sun is actually going to disappear and not return.

The short version: we make a lot of assumptions based on ideal conditions that are rarely ideal. For one, the Earth is not a perfect sphere. Two, official sunset is calculated when the sun drops below the horizon, as in all the way, not just half way. Third, and most importantly, is how changes in the atmosphere can affect how the sun appears.

These atmospheric effects make figuring the actual time the sun appears to set below the horizon to someone standing at the pole quite variable from year to year, and makes it a guessing game for those down there when the sun will appear to disappear.

Some specifics of the math involved: Based on the sun's size, it takes about an extra 16 hours at the South Pole for the sun to go from "half gone" to "all gone" below the horizon. (Remember, the sun is still travelling horizontally around the horizon as the day progresses. We're talking vertical movement there). Also, the atmosphere bends lights and the degree of refraction is related to the thickness of the atmosphere. But this effect is seen worldwide -- the sun actually sets a few minutes before you actually see it set. At the South Pole though, the air is so cold and dense that it nearly cancels out the thinness from their elevation.

Anyway, they figure this refractive effect is good for another 36 hours of keeping the sun appearing above the horizon (even though it's not really there).

Also a part in the madness -- how tall you are, and the elevation of where you are standing, and where the sun will actually be on the horizon when it finally sets. Since there's all these other factors in play for what time the sun will set, they don't know where on the horizon it will finally set, so variations in topography could affect the sun's disappearance time -- if it sets behind a "hill", that'll make it disappear earlier than if it sets behind a minor valley. (Yes, there are some minor topography variations at the South Pole.) The e-mailer says this can add anywhere from 0-10 hours to the sunset time.

All in all, they figure the sun will finally set there -- sometime between 2:30 p.m. and 10:30 p.m. our time on *Monday*.

OK, that was the "short version". If you want the long version, I have copied the entire e-mail onto this web page. He also has some great photos of the sunset and "Green Flash" which is actually the "Blue Flash" down there. I've pasted some of the images below here as well.