Apple #420: Sunsets

While I was in Florida, I took some pictures of sunsets.


(Photo of Sarasota Bay by the Apple Lady)


So here are a few of my photos along with some sunset facts.

• Sunset happens when the earth has rotated far enough so that the place on the planet where we're standing no longer has light from the sun reaching it.

Diagram showing how the earth's rotation spins us from daytime to night. Sunset happens when our patch of earth spins out of the lighted area into the darker side away from the sun.
(Diagram from a NASA StarChild page)

• But since we perceive sunsets not from a rocket's eye view above the planet but from where we're standing on the earth, sunset is defined as the moment when the top edge of the sun disappears below the horizon.

• But there are all sorts of factors which confuse the moment when this happens.

• First, the earth's atmosphere causes the light from the sun to "bend" so that even after the sun has disappeared below the western horizon, light is still coming to us. In fact, we're still able to see light from the sun for about 3 minutes after the sun has set.

• (This span of time is actually twilight, which is just after sunset but before dusk. Dusk is the first onset of darkness, when the sun is about 18 degrees below the horizon. Both twilight and dusk are very brief.)

Twilight. The sun is below the horizon, but light is still sneaking up from the sun and giving everything just a tinge of light. A few minutes after this, and it was definitely dusk.
(Photo of Sarasota Bay by the Apple Lady)

• Another thing that makes the moment of sunset confusing is, in a word, meteorologists. The times for sunrise and sunset that get posted in the weather page of your newspaper or by your local TV weather forecaster or by weather.com -- all those times are only approximations.
  

• The time of sunset will differ depending on your exact latitude and longitude (where you're standing on the planet), and from one day of the year to the next. But it also varies depending on the terrain around you.

• If you're standing on a mountain, the sun will set sooner than if you're standing on an open plain. But those meteorologist-published times are all based on the assumption that that particular location, no matter where, is at sea level.

• So most likely, the time at which you see the sunset will be slightly different from the time posted by your local news station or newspaper or online.

• Now, about those colors. Sunsets are reddish-orange because of the interaction of light and the atmosphere through which it travels.

• Light is made of up of lots of different colors, which each have different wavelengths. Darker colors, like blue and purple, have shorter wavelengths. Reds and oranges have longer wavelengths.

Wavelengths of the different colors in visible light.
(Diagram from NASA)

• Think of wavelengths as kangaroos. Shorter wavelengths mean the kangaroo bounces and hits the ground many more times than a longer wavelength will.

• If there's something on the ground, like a boulder, and the kangaroo hits that boulder, it'll bounce off of it in a crazy direction.

• If the kangaroo has a longer wavelength, it'll sail over the boulder and when it touches down on the other side the boulder, there might not be anything on the ground in its way, so it can keep going on its same path.

• At sunset, there are a lot more metaphorical boulders in the way. The atmosphere the sun has to travel through at sunset is about 10 times thicker than at noon.

Diagram explaining why sunlight has to travel through a lot more atmosphere at sunset than at noon.
(Diagram from Weathersavvy.com)

• Also there may be additional particulates in the air, like from pollution or smog or smoke from fires.

• (What has the greatest effect, though, is volcanic dust. Volcanic eruptions result in spectacular sunsets for hundreds miles away from the volcano for months afterward.)

• With more bumps in the atmosphere at the end of the day, the light coming from the sun has more metaphorical boulders in its way. The shorter, blue wavelengths (blue kangaroos) are more likely to encounter those boulders and get scattered (bounced) off in a different direction.

• The longer, red wavelengths (red kangaroos) don't hit as many of those obstacles so they keep going all the way through the atmosphere to us. So this is why we see sunsets as primarily red and orange.

Most of the blues in the light have been scattered away, and the yellows and oranges remain.
(Photo of Sarasota Bay by the Apple Lady)

• Each day, there will be different amounts and kinds of particulates in the air. Each day, we are standing on a slightly different spot on the earth. Each day, the earth is going through its rotations in a different place in its annual path around the sun. Therefore, each sunset is unique. We'll never see exactly the same sunset twice. Ever.
  

These sunset photos were taken at Lake Michigan. You can see how the quality of light is very different.


Sunset at Lake Michigan in the summer. This one has even more oranges and reds in it. Perhaps because the latitude is higher north, the light had more atmosphere to travel through?
(Photo taken by the Apple Lady's mom)



Same sunset at Lake Michigan, a little bit later. More reds yet.
(Photo taken by the Apple Lady's mom)



Sources
Inspiration Line, What's the technical definition of a Sunset or Sunrise?
Absolute Astronomy.com, Sunset
US Dept of Energy, Ask a Scientist, Orange Sunset, May 24, 2004
Cottage Life, Fascinating facts about sunsets