No Darkness for Us

Posted by lwpetersen | April 10, 2011 | Fairbanks

A few days ago I mentioned that it is staying twilight until really late. My friend Jason pointed out on facebook today that as of yesterday the Sun no longer goes lower than 18° below the horizon. By the definitions given by the U.S. Naval observatory, this means that at the darkest part of the night we are officially in ‘astronomical twilight’.

Astronomical twilight is defined to begin in the morning, and to end in the evening when the center of the Sun is geometrically 18 degrees below the horizon. Before the beginning of astronomical twilight in the morning and after the end of astronomical twilight in the evening the Sun does not contribute to sky illumination; for a considerable interval after the beginning of morning twilight and before the end of evening twilight, sky illumination is so faint that it is practically imperceptible.
USNO Rise, Set, and Twilight Definitions

I took a few screenshots from Stellarium to illustrate the geometry:

At solar midnight, the Sun will be located somewhere behind the horizon in the north direction.

If you take away the ground, the ‘elevation’ of the Sun behind the horizon can be seen:

As we move toward summer solstice the Sun will keep getting closer and closer to the horizon until it is only about 2&#0176 below the horizon. If you drive a little further north to the arctic circle the Sun won’t even set, it will just do a little bounce off the horizon on the solstice.

Leave a Reply

Related Posts

Watching the first sunrise of 2018 from Ester Dome. Fairbanks, Alaska.
Posted by lwpetersen | January 3, 2018
I’m not big on New Year’s resolutions, but I’m still resolving to write more and take more photos in 2018. I’ve been occupied with work the last couple years and...
-40 degrees in front of the UAF sign
Posted by lwpetersen | February 7, 2015
One of my best friends, Jenn from New Hampshire just made her second visit to Fairbanks, this time braving the bitter winter temps in hopes of seeing the aurora. After...
Posted by lwpetersen | December 22, 2014
Fairbanks is dark in the winter. The official length of the day on the winter solstice is 3 hours, 41 minutes, and some change. Luckily, we benefit from having a...