By David Ward
Tired of trying to stay awake until dark to see something up there? There are all kinds of cool things to see in the daytime besides blue sky and clouds.
The most obvious object above us is our own personal star, the sun, but do not dare to look at it, not even a glance. It is far too bright for our eyes. Our sun is mostly a ball of plain old hydrogen gas, the most plentiful stuff in the universe. But deep down in the core of the sun, 320,000 miles beneath the surface, lurks a fiery inferno that Dante himself could not have imagined.
There, temperatures and pressures are high enough to produce non-stop nuclear fusion, the energy equivalent of 91 billion nuclear bombs exploding every second, 24 hours a day, 365 days a year, and this has been going on for 5 billion years! Photons, tiny bits of light, are created during this complex process and they bounce around inside the sun for hundreds of thousands of years before escaping the grasp of that big ball of fire. Once free, they fly through space at the speed of light and reach the earth in 8 minutes and 19 seconds.
Here on our little planet, photons interact with the atmosphere and oceans to produce weather. Hitting your skin, they can give you a nice tan, a nasty burn or, even worse, skin cancer. These same little bits of light striking water droplets and ice crystals in the air can create some of the most amazing colors to be seen in the daytime.
Rainbows are the most familiar display of the sun’s colors, and they are mostly seen in the summer because the sun has to be shining through a rain shower. You will never see one if the sun is more than halfway up in the sky, so do not look for one at noon.
Each little droplet of water in a sun shower acts like a prism, bending and breaking the sun’s light into separate colors in a process called refraction. If you are gazing at a rainbow with a friend, the two of you are looking at separate rainbows created by different droplets of water. You always see your own personal rainbow that no one else can see because the geometry of the sun, the water droplets, and you always puts the shadow of your own head at the center of the arc of the rainbow you are looking at.
It is not too unusual to see a second rainbow outside the primary one. It is always fainter and its colors are reversed. Notice that the sky between the two bows is unusually dark. If you want to impress that friend with you, tell them that it is called Alexander’s dark band, named after a Greek guy who saw it 1,800 years ago.
Halos and sundogs
Then there are halos and sundogs. You can see these almost anytime there are thin wispy clouds in front of the sun. They are as plentiful as pack rats in the summer and are caused by ice crystals in those high clouds. Just hold up your hand to block the sun and look for a ring circling it. Often there are one or two bright colorful spots on that ring on either side of the sun, never above or below. Often called sundogs or mock suns, the scientific name is parhelion, if you are still trying to impress that friend.
Another easily seen colorful display is called cloud iridescence. Look for it on the fringes of white clouds while wearing sunglasses. It is not caused by refraction, the separation of light colors, but by diffraction, the interference of light waves, canceling some colors and accentuating others. These are crazy psychedelic hot pinks and strange aquas that are not emitted by the sun or any other star in the universe.
One last display of color is rarely seen because it does not happen too often and no one has ever heard of it. Even the name is weird enough to trip you up — the circumzenithal arc. Look for it when the sky is blue or decorated with slightly wispy clouds. When the sun is sort of low, about a third of the way up the sky, use your hand to block the sun’s intense glare and look straight up overhead. It will either obviously be there or not, an arc, about one-third of a circle, perfectly surrounding the zenith, the top of the sky. You will not see it very often, but if you do, you will witness perhaps the most spectacular display of color in the daytime sky.
On July 3, Earth will reach that point in its orbit known as aphelion, the farthest we get from the sun in 2014, about 3.5 million miles more distant than in January. Those little photons flying from the sun will need another 19 seconds to reach us.