Sunrise, Sunset

ŠPat Holleran, Shannon Image Technologies, 1994-1997

"Even if you care little for the scientific explanations for what you're seeing, observing the sky just after the Sun sets can be most rewarding. The heat of the day is abating. It's a bit quieter than during the hustle and bustle of the day. Just sitting in a comfortable reclining lawn chair and watching day give itself up to night can be an important source of energy for recharging your personal batteries"

Your Guide to the Sky by Rich Shaffer page 62.

I must say that I agree with Mr. Shaffer. "Watching the day give itself up to night" has always been an one of my favorite things. It is also one my favorite prayer times. It is even more enjoyable now that I know what is really going on. You see, the more I learn about natural phenomena, the more I appreciate the omnipotence of God. Let's see if it does the same for you.

Color Me Red

Why is the sky increasingly red after sunset? And why is the Sun itself very red when we see it on the horizon?

When the sun is first pokes its head above the horizon, its light rays must penetrate a long way through the earth's atmosphere before reaching us. Many of its rays are scattered by the air molecules or other things in the air. Since red rays do not scatter as easily as the rays of the other colors of the visible spectrum, we see a red sky and a red sun.

As the sun travels higher in the sky, its rays has less atmosphere to travel through which causes less scattering of the light rays. More of the Sun's rays are visible when the sun reaches the zenith then at any other time during the day, so the sun looks almost white.

Refraction and the squashed Sun

There is another fun phenomenon that you and your family will observe at sunset. Have you noticed that as you watch the Sun set it appears to be flattened? That is caused by refraction. Think back to your science classes. Refraction is the bending of a light ray when it passes from one wave medium to another wave medium like from atmosphere to water or from less dense atmosphere to more dense atmosphere. What does that mean to the average Joe watching a sun set?

"At these times it appears close to the horizon where the density of the air differs greatly. The air near the ground is denser than the layer of air just above it, and the layer of air above that is less dense still, and so on upwards until the Earth's atmosphere peters out at some 400 km. Now consider what happens when the Sun is setting. When the Sun is at the horizon, light from the top of the disc is going through the air at a different angle than that from the lower part. So the rays are bent by different amounts before they reach the observer's eye. The result is that the bottom part of the Sun's disc appears to be lifted up. In consequence the Sun's disc appears slightly compressed."

he Practical Astronomer by Colin A. Ronan page 16.

As the sun sinks lower and lower, the air gets denser and denser making the Sun look flat. That isn't all that is happening. As Mr. Shaffer puts it in his book Your Guide to the Sky page 62, "Since rays from the Sun are bent down toward us by the atmosphere, we can actually see the Sun after it is physically below our horizon. That means a bit more than half of the Celestial Sphere is being squeezed into the hemisphere above our horizon than we would see if refraction wasn't happening. "

Fun, huh? I think about this every time I watch a sunset.

The Green Flash

"Refraction also plays tricks on the last image of the Sun as it sets. Sometimes the turbulence through which the light from the setting Sun must pass is so great the image breaks into several distinct layers. At other times, when there is little turbulence, the last image of the Sun will be green. This 'green flash' happens very quickly, because green light is bent a bit more than red. When the 'red image' of the Sun has set, the 'green image' is still up. Images corresponding to even shorter wavelengths might exist, but many of their photons are scattered by the atmosphere, so they are too dim for us to see. This phenomenon is seen very rarely, mostly because people don't look for it. But if you look at a lot of sunsets, eventually you'll see the green flash."

Your Guide to the Sky by Rich Shaffer page 62

I am still looking for it. I will let you know when I see it.

ŠPat Holleran, Shannon Image Technologies, 1994-1997

North, South, East, West

Serious sunset observers will notice that in the summer the Sun sets in the northwest and in the winter the Sun sets in the southwest. You will remember that the ecliptic is the path of the sun, moon, and planets across the Celestial Sphere. You will also remember from your science book that the earth's axis is tilted 23 1/2 degrees . That means the planes of the equator and the ecliptic are inclined to each other by about 23 1/2 degrees. This is why we have seasons. The tilt of the earth's axis causes the Sun's path through the Celestial Sphere to be tilted 23.5 degrees to the celestial equator.

December 22, the sun is at its southernmost point in its travels along the ecliptic. The results of this? The south pole is enjoying the Midnight Sun. The North pole is 'enjoying' almost twenty-four hours of darkness. This is the winter solstice, the shortest day of the year and the first day of winter. On this day the sun is as far south of the ecliptic as it will ever get. If you can tear yourself away from Christmas cookies long enough to go to see the sun set, you will see that it is setting in the southwest. This is as far south in the sky as the sun gets.

If you continue to watch from December 22 on, you will see that the sun is setting a little farther north every evening. This continues until June 22. On June 22, the north pole enjoys the Midnight Sun and south pole has almost twenty-four hours of darkness. The sun is as far north of the ecliptic as it will ever get. On this evening you will see the sun setting north west, as far north as it will go in the sky. This is the summer solstice and the longest day of the year. From June 22 on the sun will set a little further south every night.

About halfway between the winter and summer solstice, the Sun crosses the celestial equator and enters the northern hemisphere. That happens on March 21 and is called the vernal equinox, the first day of spring. The result? The day is exactly twelve hours and the night is exactly twelve hours.

The autumnal equinox happens halfway between the summer solstice and the winter solstice on September 23. The sun crosses the celestial equator on its way south. Again the day and night are equal in length. This is the first day of autumn.

 

The more I know about how these things work, the more I am in awe of Jesus Christ, the Creator of these things. This increases my pleasure in them. These facts make sunsets more fun for me. I hope it does for you. too. Most of all, I hope it turns your thoughts to the Creator.

 

All photographs ŠPat Holleran, Shannon Image Technologies, 1994-1997

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