Up in the Air

Sun dogs and shooting stars

October 14, 2012 

Daniel B. Caton is a physics and astronomy professor and director of observatories at Appalachian State University. Email: catondb@appstate.edu.

This October is a witch’s brew of things to see up in the air. Closest to us are rings around the sun or moon. Although they can be seen any time of the year, as we enter fall and winter the jet stream will push down more cold air to create the cirrus clouds with ice crystals that cause the rings. Each crystal acts as a prism, splitting the light into a rainbow, and the geometry of the crystals means the radius is about 22 degrees – two fist-widths at arm’s length.

The rings around the sun are brighter and show the colors. Often near sunset or sunrise you just see pieces of the ring, south and north of the sun. These little chunks of rainbow are called sun dogs. The rings around the moon are fainter, and the eye’s color sensitive cones can see only a touch of red and blue extremes at best.

About 10 times higher in our atmosphere than where we find these ice crystals is the mesosphere, where incoming chunks of rock enter our atmosphere at high speed and burn up as meteors – “shooting stars.” Every October we pass through the orbit of a broken-up comet that provides some of those rocks, and this year the peak of the meteor shower is this Saturday night. The shower seems to radiate from the direction of the constellation Orion, so it is named the Orionids.

Before dawn, you will notice Venus high in the eastern sky before sunrise. It is the brightest thing, outshining Jupiter, which is high in the western sky at that time. A small telescope will show the phase of Venus and the four Galilean moons of Jupiter – two of Galileo’s observations that disagreed with an Earth-centered model of the solar system.

If you’re lucky to have a dark sky, free of light pollution, you may be able to see the zodiacal light, a wide, faint pyramid of light that extends up from the horizon in the direction of the yet-to-rise sun. At our latitude it is best seen in the pre-dawn sky in autumn, when the ecliptic (the path of the sun in the sky) and its zodiacal constellations stand highest in the sky. The light we see is sunlight scattered off dust particles in space, between the planets.

The final ingredient to add to our cauldron is the full moon, which occurs Oct. 29 and will still be full enough to light the path for trick-or-treaters on Halloween. The full moon rises about sunset to provide a spooky evening.

Daniel B. Caton is a physics and astronomy professor and director of observatories at Appalachian State University. Email: catondb@appstate.edu. More on this month’s column: www.upintheair.info.

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