Thursday, August 18, 2016

Midnight Sun

     Sunset in my opinion is the most peaceful time of the day. Sunsets are amazing, and we're lucking to have them. Sunset represents a transition from day to night, and the resulting gradual disappearance of the Sun drowns the sky in awing colors that we don't get to see during the day or at night. What exactly makes the sunset so pretty? Why doesn't the sky go from blue to dark blue? It's all based on how light interacts with our atmosphere. As the Sun heads towards the horizon, the Sun's light hits more molecules and scatters farther over the atmosphere, pushing out the shorter blue wavelengths and allowing us to see the longer wavelength reds, oranges, and yellows. You are closer to the Sun at midday, and blue reaches down to Earth better because blue wavelengths have a shorter distance to travel, but when the Sun is at the horizon, the sunlight has to travel a farther distance - as much as 30 times. The blue light can't handle the distance but the longer wavelength red light can; red travels farther and reaches our eyes. This phenomenon is called Rayleigh scattering. The technical definition of a sunset is when the trailing edge of the Sun dips below the horizon. Once the Sun dips completely below the observable horizon, sunset ends and twilight begins. Twilight is that time period where the Sun can't be seen, yet there is still light in the sky. There are three recognized stages of twilight: civil, nautical, and astronomical.
     Civil twilight occurs when the Sun is less than 6 degrees below the horizon. Many outdoor activities can typically still be carried on during civil twilight without the need for lights, including reading. Also during civil twilight, only the brightest objects in the sky can be seen. Once the Sun dips past 6 degrees below the horizon, nautical twilight begins. Nautical twilight occurs when the Sun is between 6 and 12 degrees below the horizon. The term "nautical" is used because sailors would use the stars to help navigate. The brighter stars can be seen during this phase, and the horizon is still barely visible. While you can typically still see objects outside during nautical twilight, lights are required for outdoor activities. When the Sun lands between 12 to 18 degrees below the horizon, astronomical twilight is in play. If you live in a place that's abused by light pollution, you may not be able to tell the difference between astronomical twilight and night. Once the Sun dips past 18 degrees below the horizon, its light is no longer visible and true nighttime begins.
     What time you experience sunset and when you get to enjoy twilight depends on where you live on Earth. The closer you are to the equator, typically the shorter the twilight. On June 21st, the summer solstice, twilight in Washington, DC (38.9 degrees N) lasts 3 hours and 59 minutes. Manaus, Brazil (3.1 degrees S) meanwhile only gets to experience 2 hours and 29 minutes of twilight. Reykjavic, Iceland, one of the highest latitude cities in the world at 64.1 degrees N, doesn't even experience nautical and astronomical twilight on June 21st; just 2 hours and 51 minutes of civil twilight. This is a consequence of being so close to the Arctic Circle, which is explained below.
     Places closer to the western end of their time zone will experience later sunsets as well. Boston, Massachusetts, one of the farthest east cities in the Eastern Time Zone will experience sunset tomorrow, August 19th, at 7:38 PM. Louisville, Kentucky on the other hand, one of the farthest west cities in the EST will experience sunset at 8:31 PM. While this may not be that much of an issue in the United States, in China it is problematic. Mainland China and the continental US are basically the same size. Even though they're size twins, the continental US has 4 times zones, but China only has one, because they feel like it. Sunset in Shanghai, on China's east coast, will occur at 6:33 PM, but in the city of Kashgar, China, the Sun won't set until 9:46 PM...while in the same time zone. Kashgar should really be 2 hours behind Shanghai, if not 3, but I guess if Kashgar still gets to experience sunsets, then it's alright. Kind of.
     If you happen to live near the poles of the Earth (which you probably don't), then you're in luck! You get to experience the phenomenon known as "midnight Sun." At the poles, the Sun only sets (and rises) one time a year. Six months of the year, the Sun is continuously in the sky, while for the other six months, the Sun is below the horizon, creating 24 hours of day and 24 hours of night. How? Well just do the math. 90 - 23.5 (the tilt of the Earth) = 66.5. This is the Arctic Circle.While there are periods of total nighttime darkness, most of the time the night is really just an extended twilight. All three twilights can be seen depending on location.
      Ever go outside at twilight and notice a pinkish band above the horizon? Say hello to the Belt of Venus. The Belt can be seen around sunset and sunrise, and hangs 10 to 20 degrees above the horizon. The belt of Venus is caused by red light from the sunset (or sunrise) being backscattered by dust particles.
     To me, sunset and twilight are the most peaceful times of day and the most exciting. Don't let them slip by, because as you see, they don't last all that long. And be sure to put seeing a midnight Sun and polar night on your bucket list. Svalbard, Norway and Bodo, Noway are waiting for you.

*Sources: Time and Date, Wikipedia, Web Exhibits, The Atlantic, Gizmodo, Space Weather Gallery, How It Works, photo from Wikipedia Vberger

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