AUGUST  2002

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310    AUGUST 4, 2002:   Perseids Take Center Stage

There's a party in the sky, and you're invited to participate. The most famous of all meteor showers, the Perseids, is taking center stage during the next 10 days to bring you its annual glitz of shooting stars. As the Earth revolves around the sun, it passes near enough to the orbit of periodic comet Swift-Tuttle to plow into some of the bits and pieces of debris shed by this cosmic nomad during scores of passages around the sun. Presto, shooting stars seem to radiate from a region of the heavens between Cassiopeia and Perseus. See the map at web StarWatch. This year, the moonless morning of Tuesday, August 13 should provide the best chances of catching the fireworks, but do not expect a display like last November's Leonids. Rates in rural locations north of the Valley could top as many as 60 meteors per hour right before dawn. In the city, I would expect no more than 5-10 meteors per hour. Observing after midnight will also enhance your chances of viewing Perseids. That is because during the morning hours the Earth spins us forward allowing us to intersect more meteors in a similar fashion to how the front windshield of a moving car always catches the most raindrops. Before midnight our location places us in a position analogous to the back windshield of a car-there is little action. To test this fact, start your observations about midnight, looking towards the NE. A lounge chair or an air mattress with a pillow will do nicely. Don't forget the sleeping bag, plastic dew tarp, flashlight, clipboard, paper, pencil, a caffeinated drink, and a snack. If you observe until 3 a.m., you will notice the increase in rates after 1 a.m., but watching until dawn will really make the biggest difference. Rates of between 30-60 meteors per hour can normally be expected between 4-5 a.m.

Perseid Meteors will be plentiful on the morning of August 13. Look NE starting around midnight on August 12 to catch the year's most popular shooting star display. Graphics by Gary A. Becker...

311    AUGUST 11, 2002:   Watch Those Facts!

My wife, Susan, handed me a filler from World Features Syndicate which appeared on E1 of The Morning Call for Tuesday, August 6th. "Here," she said, "This may give you some ideas for a future StarWatch article." This column does have its supporters and to boot my wife's an English teacher. It was titled "Faster than a speeding bullet," and featured six different speeds. They were "SPACE DEBRIS travels 44,000 miles per/second; OPEN OCEAN TSUNAMIS (tidal waves) move more than 500 miles per/hour; EARTH ORBITS THE SUN at 18.5 miles per/second; OCEAN CURRENTS travel at 3-5 miles/hour; METEORITES enter Earth's atmosphere at 40 miles/second; and finally SOLAR WIND zooms at a million miles/second." Houston, Einstein, Newton, we've got some problems here, but not really with tsunamis, Earth's orbital velocity, ocean currents, or the speed of "meteoroids" as they enter Earth's atmosphere. However, SPACE DEBRIS, and I am assuming that these are bits and pieces of human-made satellites, do not travel nearly two times around the Earth in a mere second. Even 44,000 feet/second translates into 8.3 miles/second, which is greater than Earth's 7 mile/second escape velocity. A much more realistic speed for space debris would be 25,000 feet per second or about 17,000 miles per hour. Einstein would have a fit if the solar wind, basically protons and electrons ejected from the sun, traveled over 5 times the velocity of light. No material object, including light itself, can travel faster than 186,000 miles/second in a vacuum. Change the one million miles/second to miles/hour and that is a very realistic speed for an average solar wind particle. The more energetic ones travel at twice that speed.

312    AUGUST 18, 2002:   Double Rainbows

This may be the year of the double rainbow. I have seen at least a dozen of them so far. A rainbow is certainly one of nature's finest jewels and never fails to elicit gasps of wonder, as well as lots of picture taking. The bow's geometry works like this. Rays of sunlight that pierce falling raindrops are refracted (bent) upon entering and exiting the drop, as well as reflected one time against the backside of the drop. Although sunlight can enter the raindrop at any location, the light which emerges is concentrated around an angle of 42 degrees from the optical axis of the drop. The optical axis is simply a line drawn from the sun through the center of the spherical water drop. Since sunlight is a combination of all the colors we see, the refraction that is occurring inside the drop bends each color or wavelength of light uniquely. Red light is bent the least, so it appears closest to the critical angle of 42 degrees or along the top of the bow. The bow is always antisolar, circling the position exactly opposite to the sun. When the sun is on the horizon, the bow can be at a maximum height of 42 degrees or about half way up in the sky. While the primary bow is being created, a smaller amount of sunlight is also entering each raindrop in such a manner that it undergoes two internal reflections before emerging. The light coming from each drop is now concentrated at an angle of 51 degrees to the antisolar position and can form a secondary bow if there is enough rain and sunlight. The double reflection inverts the order of the colors. Red light is still bent the least and appears closest to the 51 degree angle. Blue light which is refracted more, now appears above the red. View my double rainbow taken this summer in New Mexico at web StarWatch.

As Good as it Gets: Rainbows are spectacular by themselves, but couple a double bow with sunset and the landscape becomes magical. This double rainbow was snapped at Chaco Culture National Historical Park in New Mexico on July 7, 2002 about 8:30 p.m. Photography by Gary A. Becker...

313    AUGUST 25, 2002:   Moon's Path Similar to Sun's Path

Frank Kasper of Quakertown posed an interesting question about the moon's orbit. In his July 24th e-mail he said that he assumed that the moon and sun followed similar paths in the sky. Yet he had noted from recent observations that the evening moonrises were occurring in a direction much farther south to the sun's daily rising position. How could this be true if their sky paths were similar? The moon's path does indeed follow the sun's path very closely. The moon's orbit is inclined (tilted) about five degrees to Earth's yearly orbital path around the sun. This close proximity of the sun and moon guarantees at least two lunar and two solar eclipses somewhere during the year. When the moon is new, that is between the Earth and the sun, the moon basically rises and sets in a similar position to the sun. The problem is that we cannot see the moon when it is in its new phase. As the moon's angular distance from the sun rapidly increases from night to night, the moon moves into locations of the sun during different seasons. The moon therefore rises and sets in similar positions to where the sun would rise and set during that time of the year. The full moon is always opposite to the sun. The full moon closest to the summer solstice would be near the sun's position around the time of winter solstice. At winter solstice the sun rises in the SE, stays low in the sky during the day, and sets in the SW. The full moon nearest to the summer solstice rises similarly to the winter solstice sun-in the SE, remains low in the sky, and sets about sunrise in the SW. Over the next four months the sun's daily path in the sky will become lower, and the days shorter. Watch how the full moons of each succeeding month become higher in the sky, and the full moon's sky path increases.