“It does no harm to the romance of the sunset to know a little bit about it.” –Carl Sagan
I would argue the exact opposite, in fact: the beauty of a sunset, in all of its varieties and variations, is only enhanced the more you know about it.
The next time you watch the Sun descend through the sky, towards the horizon, you might marvel at how the Sun remains the same size all the way down. At just slightly over half-a-degree, the Sun appears to drop at a constant rate throughout the afternoon and into early evening.
But there are some small changes that are extremely important if you want to understand the beauty behind the sunset.
The first and most obvious is the change in coloration of the Sun, as well as a severe drop in the Sun’s brightness. On an airless world like the Moon, the Sun at sunset would look no different than at any other time. But it’s the Earth’s atmosphere that makes sunsets so special.
When the Sun appears progressively lower and lower on the horizon, its light needs to pass through more and more of the atmosphere to reach our eyes. You might not think of the atmosphere as being a very good prism, but when you pass through around 1000 miles of it just before the Sun dips below the horizon, it starts to add up.
The bluer wavelengths of light get scattered away, leaving only the reddest wavelengths that reach your eye. As the sun drops towards the horizon, it progressively loses violets and blues, then greens and yellows, and finally even the oranges, leaving only the reds behind.
You may not even realize it, but by time you’d see a sunset like the picture above, the Sun has already technically set, it’s only due to the fact that the atmosphere bends light that we’re still seeing it like this.
This is why, if you time a sunset, it will take longer than the expected 120 seconds to go from the moment it touches the horizon to the moment it dips below, even during the equinox at the equator, where it rises and sets as close to completely vertical to the horizon as possible. The Sun appears to linger due to the refraction of our atmosphere.
Also, despite its red appearance, there really still is blue and green light coming from the Sun, of course, while this is going on. But these shorter (i.e., bluer) wavelengths refract slightly more than the lower frequency ones, meaning that the reds come in at a different, shallower angle than the greens and blues, that come in at a slightly steeper angle.
Given a clear path to the horizon — such as over the ocean — this means that there’s a slight region of space just above the reddened Sun where only the shorter wavelength light is visible!
And when that happens, in addition to the normal color gradient that comes with a sunset, you can also get a small, separate region above the disk of the Sun that appears yellow, green, or even blue!
This optical phenomena is always most clearly visible over a flat area in pollution-free skies, and is known as the green flash. It occurs in many different stages, sometimes appearing at the limb of the Sun or just above it, but most it commonly appears just after the disk of the Sun has set, in a literal “flash” lasting just a few seconds, just barely above the horizon.
Although there’s a lot of green light in the Sun, the bluest wavelengths refract even more than the green ones do. In principle, you could get a “flash” of any wavelength — yellow, green, blue, or even violet — if the atmosphere cooperated. Although green and yellow flashes are the most common, under just the right atmospheric conditions, you can see even blue colors flashing at a high angle above the top of the Sun!
This applies to any very bright, white-light object that encounters our atmosphere as seen just barely above the horizon. So that means the Moon, which reflects sunlight back at us, should also exhibit a green flash under the right atmospheric conditions. And although I’ve never seen it with my own eyes, some diligent astrophotographers have captured the sight to share with us all.
You may be wondering, if greens and blues appear slightly above the disk of the Sun (or Moon), could we ever see a red flash slightly below the disk?
Under just the right, favorable atmospheric conditions, that’s exactly what happens!
Way back on the old blog (some four years ago), I posted a short explanation of the green flash, and little did I know that years later, I would receive the following message from Don Arnold of Chattanooga, TN:
I thought this was a hoax every time I visit Costa Mesa..so last week we were on the costa mesa pier and had my good Nikon set the motor drive to max and took 30 frames right at sunset. So I think I have a good one. You will have to zoom in but it looks good…thanks for the great explanation on this!
Here was the image he enclosed.
And here is the zoomed-in-version (my apologies for my lousy image processing skills):
The sunset is beautiful to anyone’s eyes, and the clarity or dustiness of the horizon, the quality and turbulence of the atmosphere, and position of the Sun give us a great diversity of beautiful sights.
But when you see a color gradient on the Sun, a red lip at the bottom, or a yellow, green, or blue rim at or above the top, will you see less beauty or more for having read and understood this? To me, at least, everything is more beautiful the more you know. Thanks for sharing the beautiful physics of sunsets with me!