The Arctic Circle and the Suspended Sun

Apologies in advance. This blog will get a little dense, but I think it’s interesting. Here’s to hoping you do too.


 

One of my favorite things about the trip to Alaska that Adric and I recently took was watching the sun and enjoying our impossibly long days. Alaska in summer is of course one of the few places on earth that experiences the so-called “midnight sun”.

Adric and I were unfortunately a few weeks too late to see the midnight sun in action, but I was still stunned by the length of sunlight and how shallow the sunsets were.

But more than anything, I was surprised at how oblique the sun’s path was across the sky. It was something I’d never really stopped to think about, and was totally dumbfounded upon witnessing, as I’d never before been in an arctic or polar latitude.

It took me a couple days just watching the sun to try to understand what was going on. By the end of the trip my biggest realization was wrong I’d been in my understanding. It was a fun thought experiment to figure out how the sun moved and why.

The sun’s path, day length, and really all seasons themselves have to do with the sun’s changing position (relative to the Earth) over the course of the year and where it is above the earth. The horizontal lines we draw across the earth- latitudes- help us measure the sun’s position.

To explain it, I think it would first be valuable to explain the five most important latitudes on the planet. These are the Equator, Tropic of Cancer, Tropic of Capricorn, Arctic Circle, and Antarctic Circle.

With the exception of the Equator (which I’d say is widely known as the horizontal “middle” of the earth, represented as zero degrees) the importance of these lines is due to the sun’s presence above the earth on the equinoxes and the solstices. Each of the lines represents a critical apex or position of sunlight over the course of the year.

“Solstice” and “Equinox” themselves are the words used for the days when the angle of sun is at its most northern or southern extreme (solstices), or when it is directly above the middle of the earth (equinox).

Now all of this is possible for one reason, and one reason only. Earth is crooked. It does spin uniformly from left-to-right, but the vertical axis of its spin is off by 23.5 degrees.

Axial Tilt
We often assume that the earth spins like the left image; in a perfectly straight, left-to-right fashion. In reality, it spins like the image on the right, where the vertical axis is off by 23.5 degrees.

Now those 23.5 degrees are very important. They are the single biggest factor that dictates seasonality worldwide. If the earth’s rotation was not crooked, the Equator would always be the only place on the planet receiving direct sunlight, which would also mean that sunlight levels would not differ from day to day or month to month.

Every place on the planet would receive 12 hours of daylight and 12 hours of night, and the only thing that would be different is the angle of the sun above the horizon at your given latitude. In other words, every day would be an equinox and there would never be a solstice.

But we obviously know this is not the case. Earth is crooked. So what does that ultimately mean? This means that as the Earth does 365 spins in one trip around the sun, the position of its ‘crookedness’ changes.

Earth's Rotation1
As shown here. When Earth travels around the sun, the angle and spot of direct sunlight (marked by the yellow X) changes.

Now twice a year, the sun’s direct sunlight is hitting in the middle of the Earth, the Equator. Here it is represented by the red line. When the sun hits the red line, with the way the earth spins, it means that everywhere on the planet will have an equal 12 hours of day, and 12 hours of night…. essentially negating the 23.5 degree crooked tilt. This is of course the aforementioned Equinox.

But twice a year as well, the Earth’s crooked tilt means that it will be in a position where the sunlight is direct either 23.5 degrees above the Equator or 23.5 degrees below the Equator; the sun’s most northern or southern extreme. This is the solstice.

These two lines, 23.5 above, and 23.5 below, are the Tropics of Cancer and Capricorn. On June 21, when the sun is as high as it gets above the northern equator, the Tropic of Cancer is acting as the solar equator. This is to say that it is receiving direct sunlight. The same is true for the Tropic of Capricorn on December 21st.

earth daigram2.png
The ‘X’ on the right represents direct sunlight on the Tropic of Cancer on June 21st. The ‘X’ on the left represents direct sunlight Tropic of Capricorn on December 21st.

This is all of course a roundabout way of me explaining the significance of the polar regions, and specifically why the Arctic sun is so oblique.

That 23.5 degrees will keep popping up.

Now at the very beginning I went ahead and assumed that everyone knew that the Equator was not only the middle circle drawn around the Earth so to speak, but also that it was “line zero”. Zero degrees north or south. I think assuming this is common knowledge, it’s probably also fair to assume that the North Pole being at 90 degrees north is common knowledge as well.

Now, because the earth is titled 23.5 degrees as said above, it means that you have an extra 23.5 degrees of sunlight that is traveling over the top of the earth when the sun is pointed at the northern half of the Earth (when the sunlight is direct at 23.5North). If we express it as a simple math problem, sunlight traveling 23.5 degrees beyond the north pole would be 90-23.5=66.5. Not 90+23.5 because 90 is the north pole and the biggest number.

What this means is that anything between 66.5 and 90 degrees north during the summer solstice is getting sunlight all the time, even if it is 2am, or more famously, midnight.

The diagram below helps illustrate this point. The sun is sitting above 23.5N (where the X is) and this means that it is a northern hemisphere summer, June 21st. Everywhere with yellow is in daylight, and black is night time. Notice the green “X” on the upper part of the Earth. Though it is in the dash-line half of earth (and therefore should be night) it is receiving low angle sunlight. This is midnight sun.

earth daigram3

The other interesting part of this phenomena is that because the angle of sunlight is so low and oblique, it makes for an odd pattern of the traveling sun as viewed from Earth.

The closer to the equator, the more simple and linear the sun’s pattern is. It rises perpendicular to the horizon, at midday is straight over head, and sets perpendicular to the horizon.

Equator
The sun’s path near the equator. The sun moves perpendicular to the horizon

The further we move away from the equator, the more acute the angle becomes.

sacramento June 21st
For example, Sacramento, at 38N degrees latitude. The sun will be 75 degrees overhead on June 21st, because the sun is a perfect 90 degrees above 23N, Sacramento is 15 degrees above 23N, so the difference of 15 between Sacramento (38N) and the Tropic of Cancer (23N) is recouped by the sun being 15 degrees lower

Adric and I were above the Arctic Circle at the small town of Prudhoe Bay for a day. The two most important variables for the sun’s path while we were there was Prudhoe Bay’s latitude (70N, above the Artic Circle) and time of year (August 4-6). Because Prudhoe Bay is so far north and it was still near peak summer, the angle of the sun there would have looked something like this;

Prudhoe Bay August 6thIf you’re still with me I want to make one last point. Notice on that last diagram how acute the angle of the sun’s travel is when it approaches the horizon. When the sun is overhead it’s hard to track as there is nothing to relate its path to. But as the sun approaches the horizon we start to notice how oddly it moves, as if it is flirting with the horizon, but not wanting to go down.

That is what fascinated me so much. Because the sun’s angle of rising and setting was so oblique, sunsets and sunrises lasted hours. The sun’s presence was perpetually felt as even when it went below the horizon it seemed to create a perpetual sunset. Not in view, but still keeping a glow on the horizon for hours until it came back up.

If Adric and I had been above the Artic Circle only 4-5 weeks earlier, the sun would have never set, and half of the day would’ve been a sunset that slowly morphed into a sunrise.

North Pole June 21st
The midnight sun. A sunset that slowly morphs into a sunrise, and the only thing breaking one day from the next is the sun dipping as low as it does in the day sky before rising again.

It was an amazing thing to see. And though Adric and I were a few weeks out of season to see the midnight sun, it was still unlike anything I’d seen before.

img_0332
Rainbow at 10:45 PM

img_0326

pic1
The title image from above?… Had Adric and I been on the Equator, the sun would set like the yellow lone, completely perpendicular to the horizon. We were at about 70N when I snapped this photo, so the sun moves like the red line, and sunsets last 3-5 hours. Amazing.
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