Tuesday, March 19, 2013


Tomorrow is the Spring Equinox, so that means that it is the end of day and beginning of night at the South Pole right?

Sun appearing above the horizon 30 hours before equinox

Well, not really. It is complicated but here are the basics:

One Sunrise - One Sunset

There is only one day and one night per year at the South Pole. The sun sets in March and the rises again in September. After sunset in March the sun stays below the horizon, and after sunrise in September it stays above the horizon traveling around the horizon instead of across the sky.

At the Equinox

If the world was perfectly round (it isn't) and the South Pole was at sea level (again it isn't) and the horizon was completely flat (and once more it isn't) then at the Spring Equinox the sun would be half way above the horizon and half below the horizon. But sunset is when the sun is completely below the horizon. Here in Utah the sun goes from half visible to gone in a matter of minutes, however at the South Pole it takes another 16 hours for the last half of the sun to move bellow the horizon. 

Seeing Something that Isn't There

Now it gets crazy. The earth's atmosphere bends the rays of light coming from the sun. This makes it look like the sun is still up after it has set. At the South Pole this effect lasts for about 36 hours. So the sun will appear above the horizon for a day and a half after it has already set.

And it Doesn't Stop There 

The taller you are the more distant the horizon is. So, the taller you are the longer until sunset. Since the Sun is traveling around the horizon instead of across the sky and with all of the factors above it is next to impossible to figure out where the sunset will occur. This would be no big deal if the ice around the pole was flat, but it isn't. So where the sun is when it sets can change when it sets by as much as 10 hours.

So I was going to say good night as the South Pole "day" is about to end, but I guess there are still a couple more days of day left.

Here is a good article that explains about sunset time at the South Pole.

Oh well, good night anyway!

No comments:

Post a Comment