StarWatch: Moravian College Astronomy

StarWatch for the greater Lehigh Valley

JULY 2008

JULY STAR MAP | STARWATCH INDEX | MOON PHASE CALENDAR

Print Large Sky Charts For 10 p.m. EDT: NORTH | EAST | SOUTH | WEST | ZENITH

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620 JULY 6, 2008: One More Book: “No more books,” I emphatically stated to my wife as she inquired what I might be interested in receiving for my birthday. “I’ve just got too many of them.” “Fine,” she said, and I honestly thought that was going to be the end of it. The next day, I found myself rummaging around the dinning room surveying the piles of papers that had to be cleaned up over the summer break. My wife, Susan, is a senior high English teacher with a staggering amount of paperwork to correct from September through June. My eye caught a book with an orange star cluster on its front cover and titled Universe: The Definitive Visual Guide, ISBN 978-0-7566-3670-8, DK Publishing, 2008, $24.95. “Another pretty book,” I mused, “filled with NASA images but little substantive text.” I picked it up. It was heavy with the slight bouquet of printer’s ink which led me to believe that it was a quality product. When I opened Universe to a random page, my eyes were met with a cornucopia of exquisite images on black paper, diagrams, and charts, each with its own caption, telling a snippet of information, which in this case was about comets. I turned the page, and now the paper was white with black text. Most of the pictures were of comets that I had actually seen, well not the Great Comet of 1680, and again, each of the 50 to 150 word captions highlighted a different aspect of cometary astronomy. I snuck the book downstairs into my study for further examination and found the information to be well-written, concise, and completely up-to-date. It should be because its general editor, Martin Rees, is Astronomer Royal of the UK. When I showed the book to my wife, she smiled and said, “Happy Birthday.” I said, “I’ll definitely take it.” I have since ordered 60 additional copies for my astronomy students this fall. Thanks, Sue. I am once again humbled.

621 JULY 13, 2008: The Moon Illusion: This is a great week to watch the moon blossom in brightness as well as approach and pass the bright planet Jupiter low in the SE. It is also a great week to partake in an experiment which is called the Moon Illusion. Everyone has seen a full moon on the horizon and made the comment about its size. When the moon is near to the horizon, it appears much larger than when it is higher in the sky. Actually, you can prove that the moon really does not change in size by holding a dime at arm’s length and using it to cover the moon when it is in a rising position. The fit should be tight. Wait a couple of hours until the moon has gained altitude and appears smaller. Repeat the experiment, occulting the moon with the dime, and you will notice that the fit will be just as tight as when the moon was close to the horizon. What is happening to convince the mind that the moon appears larger when it is nearer to the horizon is a conflict between the rational and intuitive parts of the brain. The intuitive brain sees the sky as a flattened dome, believing that objects closer to the horizon are considerably farther away than objects which are located overhead. Puffy clouds in a blue sky offer a good example of this effect. Clouds nearer to the horizon are farther away and appear smaller and more foreshortened than clouds which are above the observer. This mindset of our intuitive brain is transferred to our perception of the sky at night. Because the moon is far away in comparison to the clouds, birds, and planes of the sky, the moon’s size basically does not change. Our rational brain perceives no alteration in the size of the moon whether it is near the horizon or high in the sky. This cerebral conflict causes the brain to interpret the horizon moon as closer, and therefore, larger in size. See the Moon Illusion in the diagram below.

622 JULY 20, 2008: Bright Light in the Southeast: Is it Christmas in July? What is that bright star low in the SE after sundown? If you live where I live in Coopersburg, PA, then it even points in the general direction of Bethlehem—that’s Bethlehem, PA. Currently, with Venus too close to the sun for easy viewing, it is the third brightest object in the sky. By Jove, if you’re thinking Jupiter, then you are right on target. Jupiter is not only the largest and most massive of the eight planets in our solar system, but it has influenced our neighborhood in space second only to the sun. In fact, because of its large gravitational attraction, Jupiter, and to a lesser extent Saturn, have helped to cleanse and protect the inner solar system from the myriad of comets that are sent sunward by other stars passing close to Sol. We witnessed this “Jupiter Effect” in July of 1994 when Comet Shoemaker-Levy 9, first broken into 21 fragments by Jupiter’s strong gravitational field, slammed into Jove to produce the best solar system fireworks ever witnessed by humankind. This same gravitational field holds 63 moons in its grasp. Most are small, captured asteroids that wandered into Jupiter’s influence, but its four largest satellites, Io, Europa, Ganymede, and Callisto, create a solar system in miniature, depicting changes in composition and size which reflect what we see happening in our own planetary landscape. The inner solar system is composed of smaller, denser worlds which are composed of materials that have higher melting and boiling temperatures. The same can be said about Io and Europa. Outer moons, Ganymede and Callisto, are larger, less dense, and composed of ice and rock. The outer four planets of our solar system are larger, more massive, and made up of matter with a lower melt-boil temperature. I’ll talk more about Jupiter next week.

623 JULY 27, 2008: Fun with Jupiter on Hazy Summer Nights: Where I live in eastern PA, the white and hazy skies of summer have taken over. If you’re into astronomy like I am, but with only 20 stars visible from your backyard, what possibly can you do? As a kid, I would have just sulked, but older and hopefully a bit wiser, there’s still adventure in even a milky sky. Jupiter, that bright star like object shining with an even light in the SE after dark, presents a wealth of astronomical changes that can be fun to witness. Because of its brightness, Jupiter can act as a frame of reference for the motion of other objects. Take, for instance, the Earth. If you place Jupiter within the upper part of a tree so that it can appear and disappear among its smallest branches, you can watch the Earth rotating on a windless night. All you have to do is hold your head very still, and you’ll perceive the slow but irreversible westward march of Jove. Earth’s yearly revolution around the sun changes the sun’s position against the background of stars, causing celestial objects to cross the same point in the sky about four minutes earlier each evening. Place Jupiter over a telephone pole or a streetlight, let’s say, at 10 p.m. Come back to the same location at the same time several days later. You will notice Jupiter to the right of its original position, mainly because of the Earth’s orbital motion. With binoculars, a spotting scope, or ever better still, a small telescope, Jupiter’s nightly motion against more distant background stars can be witnessed. Presently, Jupiter is moving towards the west or retrograding in the sky. Even more interesting are the orbital motions of Jupiter’s four largest satellites: Io, Europa, Ganymede, and Callisto. They are endlessly changing positions as they circle Jove. Online photos of Jupiter and its moons are available at the URL posted with this article.

[July Star Map]

[July Moon Phase Calendar]