Published: February 08, 2008 08:34 pm         

The sky offers an unrivaled tapestry

Remarkable sky sights

Dawn and dusk, the hour of twilight that begin and end the day are the great times to see sky beauty. It’s then that the sunlight is striking the air and clouds high over our heads, resulting in some delicate tints that you are not likely to see any other time.

On days when there are high clouds at sunset, they will often glow pink, being lit by the sun’s rays that have been heavily filtered by the Earth’s atmosphere. If you were up there in the clouds, you would see the sun highly reddened, shining below the horizon. (The lower the sun is, the more air it shines through and then only red or orange sunlight gets through.)

On a very clear day, about a half hour after sun sets, one may see the deep purple shadow of the Earth creeping upward in the east. The brighter region of sky above the Earth’s shadow is known as the Belt of Venus.

Have you ever seen beams of sunlight streaming from behind a cloud that covers up the sun? These streams are called crepuscular rays. These rays can be traced backwards to the sun. Their spread is a matter of perspective as the sun’s rays are nearly parallel. (Just think how the tracks of a train seem to merge together in the distance, another product of perspective.)

It is helpful to know angle widths of sky sights. From the top of the sky to the horizon is 90 degrees. In the middle of the day at this time of the year, our sun crests at 36 degrees. The width of your finger held at arm’s length is about one degree. If you make a fist and stretch out your hand, your fist is about 10 degrees wide.

On cold days when the clouds are made of ice crystals, one may sight little patches of color on either side of the sun. These glows are called sun dogs or parhelia, caused by the transmission of light through tiny hexagonal ice crystals; the inside of the sun dogs are reddish.

These sun dogs appear at a 22 degree angle from the sun. Sometimes only one sun dog can be seen. Even more unusual is a sun pillar, a beam of light pointing upward when the sun is very low in the sky. These sun pillars are caused by the reflection of the sun’s light off the flat base of ice crystals.

The most eye catching sights are the rings around the moon when nearly full. These haloes are caused by the moonlight being bent or refracted by ice crystals. The halo formed has a radius of 22 degrees, owing to the geometry of the ice crystals. So the entire halo is about 45 degrees across, half the width from the horizon to the top of the sky.

Another great sight is the gentle glow about the moon called the coronae. The coronae are caused by the moon shining through tiny droplets of water in the clouds. The innermost part of the corona is bluish while the outer part has a red tint. If the water droplets vary greatly in size, then we have a glow called an aureole, lacking any separation of color. The reason why most clouds appear white is because they are made up of droplets of varying sizes, adept at scattering sunlight of all colors.

Rainbow hunters have their best chance to see a rainbow either early in the morning or late in the afternoon when the sun is low in the sky. For the center of a rainbow is the anti-sun, the point in the sky directly opposite the sun.

When the sun is high in the sky, the anti-sun is nearly straight down and its bow lies on the ground and is impossible to see. The color in the rainbows is due to dispersion, the spreading out of colors as the sunlight emerges from the raindrop after being reflected from its backside.

Violet light is bent the most so it appears farthest from the sun while red light is bent the least so it appears on the outside of the rainbow. The outer red edge is about 51 degrees from the anti-sun while the inner violet edge is 48 degrees from the anti-sun.

Moon and planets

Our evening moon grows to half full this Wednesday, offering the best views of its craters and mountain ranges through binoculars or telescope. Along the left (straight) edge of the moon, the sun is rising, lighting up the crater rims and mountains peaks. This Friday, the moon appears near the bright planet Mars.

Late Feb. 20, the moon will be full and appearing near the planet Saturn in the star group Leo. The moon will also pass through the Earth’s shadow late that evening. My next column on Feb. 17 will treat that eclipse in detail.

Now featured at the Frostburg State Planetarium is “Calendars Around the World” with free public programs each Sunday at 4 p.m. and 7 p.m. in Tawes 302. Tawes Hall is near the Performing Arts Center, the Compton Science Center, the Lane Center and the FSU Clock Tower.

Following our planetarium presentation, anyone interested is invited to see the magnificent Cavallaro Collection of Specimens from Five Continents in the Compton Center Exploratorium. For a free bookmark for the Planetarium/Exploratorium, please call (301) 687-7799 and leave your name and mailing address.

Any readers with comments or questions are invited to email Bob Doyle at rdoyle@frostburg.edu.