Final Sunset… What the world’s ending?

by wonderfullyrich on April 23, 2013

Nope, but Antarctican’s won’t see it again for several months, so like a friend who gets lost in the wandering, we tend to forget about them.  Although the sun is a big friend and it’s hard to miss our friend’s effects… so maybe that’s a fib.

Unlike many who’ve been writing and showing sunset here in Antarctica, I thought I’d talk and show the mechanism involved.  Future posts may also have photos showing the beautiful and quick sunset.  It rises tomorrow April 23 2013 at 12:16 PM and sets at 1:27 PM. 

Why though do we get months without sun south of the Antarctic Circle?  It is a question many of you probably know the answer to, but only abstractly.  We all know the earth is tilted and I think most people know that it relates to seasons, which also is the reason we get long periods of darkness in the polar regions.  How exactly they are related is what I believe many get confused on, so in an effort to make it more concrete and show why we only get an hour fifteen minutes of sunlight tomorrow I’ll describe it.

Being tilted at 23.5 degrees changes how directly overhead the Sun a little bit every day, this is called the angle of incidence.  The equinox days are the points where the sun is directly overhead above the Equator, meaning it is at 90 degree angle of incidence across the middle of the earth.  Similarly solstice days are the points where the sun is at it’s most indirect overhead above the Equator or 66.5 degrees. (90 – 23.5 = 66.5) It’s also the point where the sun will be the most direct and indirect point overhead along the Lines of the Tropics.  For example on June 21th the sun will be at 90 degrees over the Northern Hemisphere Tropic of Cancer, 66.5 degrees over the Equator, and 44.5 degrees over the Southern Hemisphere Tropic of Capricorn. I didn’t make a visual of this, but a youtube video exists here if this requires more explanation.

The Earth has two more circles of note, and also relate to the sun.  You probably can see where this is going by now.  The Arctic and Antarctic circle are the lines where the the sun will–at least for one 24 hour period per year–remain overhead and similarly be missing overhead.  The following is a good chart representation of this as stitched together by Jared Knox last summer. The further south you go the longer the period of darkness, above the Antarctic circle (say in Palmer and Christchurch) it will never actually go away.

This chart lacks geography, so the animated graphic to the right is a visual from space I built with the help of Google Earth and ImageMagick.  It shows the progression at noon Auckland time (same as McMurdo and South Pole) twice a month for the entire year.  In this way you can visualize the changing angle of incidence better.  Note that this is not a sunset and sunrise although it looks sort of like it, this is actually driven by the tilt of the earth and so its progression is slow and easiest to visualize when sped up.  Click on it for a larger slower version.

Indeed the following visual show you even more clearly what’s going on. As you watch this video I’ve spliced together from Celestia and Google Earth, you can see the angle increasing slowly on the left.  This view is as if you are in a parallel orbit with the earth directly below it looking up, as compared to the right which is a stationary view of Antarctica orbiting with earth on the right.  Look again at the chart above before you watch this video, and take a look at how the darkness on the right slowly increases and then looks as if it’s a spinning top about to fall over. 

A few notes about this video, the sync is just a little off I believe, but Celestia doesn’t have a proper satellite image of Antarctica so it’s hard to match up visually.  If it’s not possible to read the dates, they start on January 1st and end on June 24th, i.e. the middle of austral summer to the middle of austral winter.

I hope you’ve learned something about the beautiful final sunsets all my Antarctican friends will be posting in the near future. 

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