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Saturday, April 2, 2011

Polar Twilights

We often hear that at the pole there are six months of day and six months of night, but I can now attest that this is far from the truth.  We have loooong twilights.
Before we can really call it night we get to experience and enjoy civil twilight, then nautical twilight, and finally astronomical twilight. 
Civil twilight starts at sunset and ends when the sun is 6 degrees below the horizon.  At medium latitudes it lasts about a half hour.  Here at the pole it lasts 13 days, until April 5.  During civil twilight there is still plenty of light to move around outside.  The brightest planets and stars will appear by the end of civil twilight.  It has now been 10 days since sunset, and I have still been able to enjoy running and skiing outside without any impairment, except for the wind. 
Nautical twilight is the period when the sun is 6 to 12 degrees below the horizon.  At the end of nautical twilight the horizon is still visible, and most of the stars are visible, too.  It is during this period that we turn on our cameras to observe and record the auroras.  This period at the pole lasts 17 days, until April 22.
The last one, astronomical twilight, is defined by the sun being between 12 and 18 degrees below the horizon.  The horizon is no longer visible.  It does not get any darker after the end of astronomical twilight, so that is when the night really starts.  Astronomical twilight will last 20 days, until May 12.  At that point there will only be 6 weeks left to the winter solstice, so the polar night should really only last about 3 months.
I look forward to observing the slow evolution of lights, from twilight to the night sky, with the appearance of the stars and of the auroras.  I look forward to the next moon rise, and to the lunar eclipse, which we will experience around mid-June.  Meanwhile, I am enjoying civil twilight.  On March 30 I took my camera with me on a 6.7-mile ski tour to document what the South Pole looks like at this very special time.

View towards the sun on March 30: 7 days after sunset.  The sun is about 3 degrees below the horizon.

View of the station on March 30.  There is still plenty of light to see the buildings and the features on the snow.  On this day the wind was very light, less than 5 knots, and the smoke out of our power plant created a slender and compact plume slowly drifting away.

This is a view of our beautiful 10-mt telescope against the sky opposite the sun.  While the sky was pinkish in the direction of the sun, it was of an intense blue in this direction.  Note the crust of ice on the sides of the building, produced by ice fog and wind in the preceding days.

I skied all the way out to one of the three wind turbines that were installed last summer and are being tested to support remote science experiments.  This one is located 1.5 miles away from the station, in the direction opposite to the skiway.  The path to the wind turbine is well marked with flags.

On the way back from the wind turbine I met my friend Robert, who was walking back to the station from the telescope.  He took this photo of me.  With a wind of less than 5 knots (6 mph) the -63 C (-81 F) temperature felt very comfortable.

Loving it here at the South Pole!

Robert is an astronomer, or, I should rather say, a polar astronomer.  He has already spent 6 winters here at the South Pole operating a variety of different microwave telescopes.  When not on the ice, he lives in the lovely town of Garmisch, in the Bavarian Alps (the site of one of the most famous downhill courses in the world cup ski circuit), or teaches nature classes on board cruise ships around the world.  He is also teaching an 11-hour college-level astronomy class here at the South Pole, every Monday after dinner.

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