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Weather Whys: Refraction and how it tricks our eyes

A setting sun can still be seen on the Slope, thanks to refraction
Undated photo.
Undated photo.(KTUUU)
Published: Nov. 23, 2020 at 9:18 AM AKST
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ANCHORAGE, Alaska (KTUU) - The sun officially set for the last time this year in some areas of the Arctic Circle. However, each day like clockwork the sky still lights up and the sun can be seen. But wait, I thought we just said the sunset for the final time in 2020. So why can you still see the sun and just what exactly is polar night?

Let’s take you on a journey into the science of refraction and how it tricks our eyes into seeing our sun long after it dips below the horizon.

You may recall studying in science class the term refraction and even did an experiment with a pencil, water and the readily available light that surrounds us daily. Refraction itself is simply light that bends as it travels from one substance to another or traveling through the same substance but at varying densities.

We see refraction at work daily and it’s one of Mother Nature’s favorite tricks to play on our eye. From the bending of a pencil in the water to rainbows in the sky and even to being able to see the sun long after it set. Refraction is at work all around us and comes in many forms.

Pretend you are an observer, the day is just beginning and the sun is set to rise at nine in the morning. But wait, it’s 8:45 and you already see the sun. What’s happening? It’s Atmospheric Refraction at work. While the actual position of the sun is below the horizon, the light still travels through the Earth’s atmosphere, which is constantly refracted depending on the density of the atmosphere. This density can change and is greater at the bottom depending on the pressure, temperature, moisture and wavelength of the light that enters the atmosphere. It’s the combination of these conditions that bend the light, much like you see when you place a pencil in a cup of water. Because there is more atmosphere for the Sun’s light to travel through it becomes refracted numerous times displacing the sun.

As the sun continues to rise there is less refraction, meaning an object that is directly above you will be in the correct position, while those that are on the horizon will appear higher up than they actually are.

This is what leads to twilight and still being able to see even long after the sun has set. It’s also why Polar Night is not complete darkness for those north of the Arctic Circle. While the sun will gradually get lower and lower below the horizon, it’s not until the sun is 18-degrees below the horizon that it’s officially dark.

For Utqiagvik, the highest point below the horizon the sun gets from now through January 22 is -5 degrees. This not only means the sun can still be seen due to refraction but that it doesn’t truly stay dark through the winter months for 24 hours despite what you might think.

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