Twinkle Twinkle

Visitors to Frosty Drew Observatory often say "I guess its an old wives tale, but I've heard that stars twinkle but the planets don't." Well, this folksy wisdom is basically correct. While planets occasionally twinkle, they are far less susceptible to twinkling than stars. Twinkling occurs when air momentarily diverts light from a viewer's line of sight. Optically, a star is a single point of light. Optically, a planet is a disk (a myriad of light points). It's more difficult to divert all the light from a multiplicity of points than from a single point. Simply put, that is why stars twinkle and planets do not.

You might wonder why stars don't show disks like planets. Certainly, the Sun, the nearest star shows a disk. The answer is that stars are many hundreds, even millions of times farther away than anything in the Solar System. With the exception of a very small handful of nearby red super giants, no star has a disk large enough to be seen by any Earth based telescopes. All of the planets, even tiny remote Pluto, have resolvable disks through large telescopes.

Anyone who has ever looked along a hot street in the summer has seen shimmering mirages. In winter, the air above a warm chimney roils and bubbles. Wind currents and air layers above the Earth add to the turbulence. Even moderate amounts of turbulence can cause point like objects to twinkle, but light from an extended disk almost always manages to shine through. Only on exceptionally unstable nights is air turbulence strong enough to cause planets to twinkle.

If you look at something about 51/4 miles away on the horizon, you will peer through as much air as if you saw a star above your head. (The atmospheric pressure halves every 33/5 miles in altitude. This means that while the atmosphere extends hundred of miles, most of the air hugs the Earth.) Looking towards a celestial body near the horizon means peering through as much as a dozen times the air above your head. Worse yet, turbulence generally increases greatly near the horizon, particularly at sunrise or sunset. Probably you've noticed distortions in the Sun's color and shape at sunset or sunrise. The same distortions affect planetary, lunar and stellar objects. While the planets, the Moon and the Sun may be free of twinkling, this does not mean we can see them clearly. Far from it! Their disks can wobble and blur infuriatingly on turbulent nights or near the horizon. This is why almost every astronomer wants to wait until the objects are at least 15o above the horizon before viewing.

Oddly, good viewing can occur when there is a high thin haze. High thin haze seldom occurs in turbulent air. So if you look out some night and see some haze, don't wait until a clear night when the stars twinkle beautifully to do some sky gazing. If so, you may pass up a night of wonderful viewing for a night of mediocre viewing!

We should be in for fireworks this fall. Several of the more significant meteor showers give indications that this might be an unusually spectacular year. With meteors, you can never be sure, but especially the Leonid shower should be very good. The Leonid shower's parent comet [Temple - Tuttle] has be sighted again and its position is favorable. The radiant (the apparent source of the meteors) will be located in the constellation which names the shower, but meteors can streak anywhere across the sky. The following calendar indicates the starting - [optimum] and ending dates of the shower:

Oct. 2 [20-23] Nov. 7 Orionids
Oct. 6 [8] 10 Draconids
Oct. 14 [18] 27 Epsilon Geminids
Nov. 14 [17-18] 21 Leonids {the one to really watch for}
Dec. 13 14 Geminids

Fall nights with their longer and longer dark periods, but relatively mild temperatures make a great time for meteor watching. Remember you best meteor watching tool is you eyes. You have almost no hope of seeing a meteor with a telescope, and even binoculars are usually too slow to use before the streak fades away. This is a great event for kids. Most kids night acuity is better than adults. They stand a better than even chance to be the best meteor spotters in a family outing.

Leslie Coleman
Leslie Coleman
Entry Date:
Oct 1, 1998
Published Under:
Leslie Coleman's Columns
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