AUGUST  1998

AUGUST STAR MAP | INDEX

101    AUGUST 2, 1998:   Moon Illusion

During this week the waxing gibbous moon will travel eastward among the stars, illuminating the landscape with increasing brilliance. By Saturday it will be in the full phase, positioned opposite to the sun, and rising at sunset. Many of us will catch the yellowed moon as it first climbs into the dimming twilight and remark to ourselves about its huge appearance in the evening sky. Actually, the moon is no larger when rising than when it is high in the sky. You can demonstrate this to yourself with a dime, which is barely large enough to hide the full moon. By holding the dime at arm’s length, you can occult the full moon when it is rising or high in the sky. But the illusion does appear very real as we watch moonrises around the time of its full phase. What is really happening? Trees, homes, telephone poles, and other distant objects near the horizon appear smaller because they are farther away. Yet we know through common experience, that if we were nearer to these objects, their size in comparison to us would be much larger. Now, let’s put the moon into the setting. Our eyes see the moon in back of distant objects which our brain knows to be much larger than they actually appear. This results in a psychological effect which causes the brain to perceive the moon as a larger object than it actually is. The same effect works for the constellations which can appear absolutely huge when in a rising position.

102    AUGUST 9, 1998:   Perseid Meteors

The second full week of August is the time when Perseid meteors fly. Unlike last year when the heavens were devoid of lunar influences during the morning hours, this year’s show is marred for most of the night by the glare of the moonlight. Still, the Perseids represent the best chance of seeing meteors because of their abundance, warm weather, and summer vacations. Your best opportunity for catching Perseids are after midnight on the morning of August 11th and throughout the night of August 11/12. Before and after these dates however, Perseids can still be easily seen. The moon rises about 9:50 p.m. on the 10th and 10:30 p.m. on the 11th. By midnight the radiant, which is the location from which Perseids will appear to be diverging, will be in the NE about one-third of the way up in the sky. By 4:00 a.m. Earth’s rotation has carried the radiant to nearly 70 degrees above the NE horizon. Make sure you have a sleeping bag or blankets to keep you warm, a lawn chair or air mattress for comfort, flashlights, snacks, something warm to drink, and mosquito repellent. Position your equipment to avoid looking at the moon directly. StarWatch columns for July 27, August 3, and 10, 1997 dealt with meteor observing in greater detail. These are archived on the home page noted below.

103    AUGUST 16, 1998:   Cygnus X-1

Astronomers believe that roughly 90 percent of the mass of our Milky Way galaxy is in the form of nonluminous material. A certain amount of this dark matter is relegated to black holes, gravitational wells, where a once massive star had gone supernova. In the hellish end scene of the star’s existence, as its core rapidly collapsed, some of its material was squeezed so tightly that it disappeared from space and time forever. Remaining around the collapsed center of the exploded star was gravity, so strong that not even light could escape it. Today, almost all astronomers believe that black holes are scientific fact. One of them Cygnus X-1 lies very near to a star named Eta Cygni in the constellation of Cygnus, the Swan. To find it, face east about 11 p.m., and look directly overhead. You’ll see the Great Summer Triangle straddling the zenith. The faintest member of the triad is Deneb. It is the brightest star in Cygnus. You probably won’t see a swan, but you should notice a large cross with its staff extending well into the triangle. You’re looking at a part of the swan called the Northern Cross. Where the "staff" and the "cross" meet is a fairly bright star called Gamma. The next star along the staff is Eta, fainter than Gamma, but still visible from an urban location. Look just on the Gamma side of Eta Cygni, and that is the location of Cygnus X-1, the most famous black hole in the heavens.

104    AUGUST 23, 1998:   Double, Double

More than half of the stars that we see in the sky are multiple systems, meaning that there are two or more stars in orbit around a common center. Some of these systems can be seen with only binoculars or spotting scopes. One very easy binocular double is located in the Great Summer Triangle which is nearly overhead about 10 p.m. The brightest star of this threesome is blue-white Vega in the constellation of Lyra, the Harp. It is the sixth brightest luminary of the heavens. Facing east with binoculars, and centering on Vega, you’ll notice four stars in the shape of a parallelogram to the right of Vega. This represents the strings of the harp. Another star is situated below and slightly to Vega’s left. Now look again at this star called Epsilon Lyrae. It is really two stars--a double star system separated by about 1/15th of a degree. Both stars are virtually the same brightness. But what makes Epsilon Lyrae so special is that each of these stars is also a double star. You’d need a good telescope at 150-200 power to resolve them easily. Look again with binoculars at the left component of the double called Epsilon 1 Lyrae. Remember that a good telescope can split this star into two components separated by nearly three seconds of arc. The brighter star of this double is also a double star. Getting confused? Me too! The bottom line is that there are double stars everywhere you look.

105    AUGUST 30, 1998:   Rainbows

The rainbow is nature’s universal translator, for within its spectrum of colors hides the codes which allow us to decipher the secrets of the cosmos. The spectrum when accurately prepared and recorded can allow astronomers to discover such diverse properties as temperature, composition, mass, distance, and movement. Extend the spectrum into its invisible parts which can be recorded by sophisticated instrumentation, and you can now obtain a portrait of the heavens at different energy levels. If you’re vigilant, you will probably have at least a half dozen opportunities to see a natural rainbow during the course of a year. Three conditions must be met. It needs be raining, a low sun must be shining through the rain, and you must be located between the sun and the falling raindrops facing away from the sun. A late afternoon thunderstorm with rapid clearing offers the best hope of catching a natural bow. Rays of sunlight shining through the falling rain are internally reflected and dispersed into the colors that we see. In the primary bow, red will be on the outside, while blue will be on the inside. When a secondary bow forms, you’ll notice that the colors are reversed. Impatient for a natural rainbow? Then use a garden hose turned to a fine spray on the next, hot sunny day. While you’re having fun dousing the kids, make sure your back is to the sun, and you can enjoy the beauty of the rainbow too!