StarWatch for the greater Lehigh Valley
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JULY  1998

JULY STAR MAP | INDEX
 
097    JULY 5, 1998:   Summer Stars
The night sky this week is dominated by the moon, which is full just after noontime on Thursday. The moon will certainly "look" full to the casual observer for several days before and after this time which makes the week a bust for anything except the brightest sky objects. Tonight and tomorrow, watch as the moon passes above the bright star Antares in the constellation of Scorpius, the Scorpion. Their separation will be about 10 degrees or one clenched fist held at armís length. I will return next week to this gem of the summer sky. In the early evening, to the right of Antares by about four to five fists twinkles Spica, the brightest star of Virgo, the Virgin. By 10:30 p.m. they are both at the same elevation, so finding them should be easy. Viewing them with binoculars will show the distinct color difference between them. Antares is a red supergiant on its way to stellar demise, while Spica is a normal hydrogen-burning, blue-white star. Now turn towards the east. Youíll see the three principal stars of the Great Summer Triangle rising triumphantly. The brightest star highest up will be Vega of Lyra, the Harp. The faintest, Deneb of Cygnus, the Swan, is to the left, while Altair is to the right. The size of the triangle will look absolutely huge, so donít be surprised if it takes up nearly your full field of vision. Arching across the sky, through the Great Summer Triangle to the left of Antares flows the "river" of the Milky Way invisible this week because of moonlight.
 
098    JULY 12, 1998:   Scorpius, the Scorpion
One of the most beautiful constellations of the season is Scorpius, the scorpion, one of the twelve zodiacal constellations which the sun moves through during the course of the year. From our latitude much of its splendor is masked by its low altitude as it hugs the horizon in the south right after darkness. Its brightest star, Antares, should be an easy target, even from urban locations. It twinkles with a reddish hue. Indeed its location, color, and brightness gave this star its name. Antares means the "rival of Ares." Ares, the Greek god of war, the planet Mars to most of us, can pass very close to this star. When it does, the similarity in appearance can be uncanny. The rest of the constellation is far more inconspicuous, more for a dark sky site. This is not a result of the stars being particularly faint, but because the Earthís atmosphere, coupled with dust and moisture, causes the diminution of a starís brightness when near the horizon. For suburban and country observers away from any direct illumination, youíll notice a delicate line of stars moving down and to the left of Antares, then hooking back to the right near the horizon. Youíre looking at the body and tail of the Scorpion. Three fairly bright stars to the right and slightly above Antares completes the head of the Scorpion, noted in classical mythologies for having killed the greatest hunter of them all, Orion.
 
099    JULY 19, 1998:   Observing the Milky Way
Without moonlight to bother us this week, we are presented with an opportunity to view our own Milky Way galaxy now gaining prominence in the evening sky. Indeed all the stars that we see scattered across the heavens are part of its giant pinwheel-shaped structure. The Milky Way may contain as many as 2-6 hundred billion stars, and most astronomers agree our visible galaxy is only the tip of the iceberg. More than 90% of the Milky Way is composed of matter that is not shining. Some of this dark matter, in the form of giant clouds of interstellar dust shroud most of the galaxy from our view. When we gaze from the countryside along the plane of our galaxy, the sky glows with a soft iridescent light, along a wide band of the firmament. Currently about 11 pm, the Milky Way spatters the sky from due south, then up across the Great Summer Triangle, high in the east, then arcs downward through the rising "W" of Cassiopeia in the northeast. By using binoculars, observers unable to escape the lights of the city can also glimpse the Milky Way. Look high in the east before midnight. As you move your binoculars back and forth, youíll notice an increase in the star count which reveals the location of where the Milky Way is secretly hiding. View from the country and the beautiful starscapes of our galaxy will begin to tantalize you with the insignificance of our place in the vast universe which surrounds us.
 
100    JULY 26, 1998:   Aristarchus and the Sun's Distance
The moon debuts this week as a waxing crescent, reaching first quarter phase by mid-afternoon Friday. Look for the moonís terminator, the region between light and dark, to appear as a straight line on Friday evening. The Greek astronomer Aristarchus of Samos (about 310-230 BC) used the straight terminator to determine a relative distance of the Sun from the Earth. Aristarchus knew that when the moonís terminator was straight, the Earth-Moon-Sun angle was exactly 90 degrees. He argued that if the Sun was relatively close to Earth, the period of time between the first and third quarter moon would be a longer interval than from the third to first quarter. Aristarchus also assumed that the Moonís orbital path was a circle, and that it revolved at a constant speed. By measuring the exact times of the quarter moon and knowing the moonís orbital period, Aristarchus could proportion the number of degrees which the moon had traversed during each interval and thus make his geometric sketch. Aristarchus felt that the period between the third and first quarter moon was about one day shorter than between the first and third quarter phases. This resulted in an answer which put the sun about 20 times more distant than the Moon. In reality the true solar distance is approximately 20 times greater than what Aristarchus calculated, but it was still a brilliant quantitative beginning.
 
July Star Map

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