Mysteries of the Sun
If you've ever watched the lazy summer Sun redden as it settles with a stalling sigh into the welcoming bosom of Earth's horizon, you might have thought it grew a little fat around the mid-section. Time seems to suspend itself as the fading star spreads out, gripping the evening stage like a performer reluctant to allow the next act, twilight, to begin.
Strange things happen in the gathering dusk.
A few people claim they've seen a brilliant and instantaneous green flash as the top of the fiery blob dipped out of sight. More common are reports of light pillars shooting straight into heaven just before the day's goodbye.
Wrapped up in these strange phenomena are a curious combination of fact and myth -- quite possibly not in the combination you imagine. The commonly held belief that the green flash is a myth, for example, is itself a myth. We've got a picture to prove it.
And the idea that the Sun gets fatter and larger as it sets is flat wrong. Oh, it seems to. But what really happens involves some relatively simple atmospheric science and, to keep things interesting, a strong dose of illusion and a dash of mystery.
The flat Sun (and Moon)
Let's imagine you're not at your computer today. Instead, you're wiggling your toes in the sand at Malibu.
Everyone around you is half-naked, of course, and when the reason for that soared directly overhead at Noon, its waves of radiation fried a lot of the exposed flesh. But now it's evening, the Sun is preparing to depart, and the harshness has gone out of the rays. You know intuitively that some of the energy isn't getting through.
The electromagnetic waves -- light -- are muddling through more atmosphere en route to your spot on the beach.
To envision why, you can draw two circles, one to represent the atmosphere and another one inside the first and slightly smaller. This is Earth. Now draw a vertical line that connects the 12 o'clock positions of both circles. From the same point atop the Earth circle, draw a horizontal line to the left until it intersects the outer circle of atmosphere; this line will be longer, and it points toward your imaginary setting Sun.
Back in the real atmosphere, molecules of water, oxygen and other things get in light's way, refracting some of it, sending it off in new directions. With more atmosphere comes more refraction.
Here's the important part: When the Sun is at the horizon, light from its bottom travels through just a bit more air than light from the top. This lower batch of light gets refracted upward, and so the Sun appears squashed, as though it's being drawn up like a set of mini-blinds. Yet light from the left and right sides of the Sun travel through the same amount of atmosphere, so the Sun does not spread out -- it's horizontal dimension remains the same.
The squashed Sun effect happens to the Moon, too, when it is near the horizon.
The extent of the squashing changes with altitude and temperature, both of which alter air density. In a study this spring, Romanian scientist Zoltan Neda and colleague Sandor Volkan worked out the math on all this. They found that on a normal day in a place like Malibu, the Sun can appear 1.2 times wider than it is tall. But in the Arctic, the Sun can appear twice as wide as its height.
And from an airplane or a space shuttle, the width can appear 2.5 times greater than the height, Neda said in an e-mail interview.
This refraction causes another effect you might have noticed. Because the bottom of the Sun is visually drawing up, the Sun really does appear to slow down just before it sets, explained Neda, who is a professor of theoretical physics at Babes-Bolyai University.
Now as you ponder all of this from the beach in Malibu, you would probably not be alone if you argued, "Hey, you said the Sun does not get wider, but I can see with my own eyes that it is much larger overall at sunset than at midday."
I wouldn't argue with you, but I would ask Philip Plait about your perplexing observation.
Debunking a myth
In his new book, "Bad Astronomy" (Wiley & Sons, 2002), Plait argues across 10 pages against a long-held myth that the Moon is larger when near the horizon. You've no doubt noticed this when a full Moon rises and looms ridiculously huge, only to shrink an hour later. Scientists don't know exactly what's going on, but they do know that the whole thing is an illusion. If you don't believe them, Plait suggests you conduct your own test.
Hold a pencil eraser at arm's length, he writes, and you'll see that the Moon measures the same size when it rises as when it's high in the sky.
The illusion of a big Moon is all in our heads, Plait and other astronomers say. It has to do with the fact that most people (whether they know it or not) perceive the sky to have bounds, and they imagine it's shaped more like an upside-down, shallow cereal bowl than half of a true sphere. Thus, our brains tell us the sky is farther away at the horizon than it is directly overhead. Our brains are further warped: When confronted with two same-sized objects, the more distant one is interpreted as being larger (this is called the Ponzo Illusion).
Combined, the two imaginings lead us to think the Moon is bigger when it rises, Plait says. In his book, however, he does not address whether the widely discussed "Moon Illusion" applies to the Sun (he's busy dispelling other myths). So I asked him.
"Yes, the setting Sun suffers the same illusion as the rising Moon," he said. "Anything on the horizon in the sky does that."
Pillars and the green flash
Other weird things happen when the Sun sets, but you'll need some patience and luck to spot these.
A solar pillar is a striking sight. Again, the Sun itself has little to do with the effect, other than sending earthward the necessary light.
A pillar is typically rendered in cold air by ice crystals falling from high clouds, explains Robert Nemiroff, an astrophysicist at Michigan Technological University and a researcher at NASA's Goddard Space Flight Center. The crystals are sometimes flat, and air resistance will cause them to float flatly, rather than knifing downward on edge. Sunlight reflects off the crystals to generate what appears to be a column of light soaring into space.
The pillar looks like it comes from the Sun, but in reality it's just a few miles away.
Finally, the rarest of all these tricks of light is the green flash. You'll recall that we decided you're at the beach, gazing out over the Pacific. You are there because it is a good place to see this rare and fleeting phenomenon. A low horizon with a long view is needed. A prairie can suffice, and an airplane will do nicely, too.
The key to the green flash mystery again lies in refraction, along with the fact that the Sun's white light is actually made of many colors.
"The Earth's atmosphere acts like a prism," Nemiroff says. "Blue is refracted the most." Red light is refracted the least. Green is somewhere in the middle.
At sunset, when refraction is most pronounced, there can actually be three images of the Sun -- a blue one on top, a green one in the middle, and a red one at the bottom. Each overlaps the others.
So why don't we see a blue flash? Because the blue light is scattered so severely that it doesn't even reach our eye, Nemiroff says. Once in a while though, and typically for less than a full second, a bit of the green light gets through.
And all the while, as you feel the sand growing cooler and prepare to bid the Sun farewell, the red light has the most success travelling unimpeded through the extra atmosphere in its evening path, which explains the general color of your final moments in Malibu.
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