The Parallax view

Some of you may recall a 1974 movie, the political-thriller The Parallax View, starring actor Warren Beatty. This movies cryptic title illustrated the visual effect of looking at objects, both near and far, against an unchanging backdropin the movies case, mysterious moving objects against the backdrop of international politics. With a nod to the movies title, lets look at the parallax view of how todays astronomers figure out the vast distances between the Earth and other celestial objects such as stars, galaxies and ancient quasars. A law of mathematics that astronomers use when measuring distances in space is the so-called "inverse-square law." At its heart, the inverse-square law involves the concept of this parallax view. Parallax is easier to understand when you try it for yourself. You probably already discovered the inverse-square law as a child, just as I did, when left alone to amuse yourself on a rainy Saturday afternoon. Remember, parallax results when nearby objects appear to shift their positions relative to objects farther away. Remember that rainy day childhood discovery? As a child you may have held a finger up at arm's length. Then, you looked at your fingertipfirst with one eyelid closed. Next, you opened your eyelid and then closed the other. Magic appeared to be the resultyour finger jumped in space! Well, this phenomenon turns out not to be very magical; the apparent movement of your fingertip was the result of a change in your perspective or parallax view; in this case, a mere two or three inches as the fingertip jumped from one eye to the other. This happens when astronomers look at an object through an Earthbound telescope or an orbiting space telescope. According to a fact sheet about parallax appearing on the McDonald Observatorys web site: As Earth revolves around the Sun, astronomers invoke this same (finger jumping) principle (of parallax) to determine the distance to nearby stars. Just like your fingertip, stars that are closer to you and me shift positions relative to more distant stars that appear to be fixed in space. By carefully measuring the angle through which the stars appear to move over the course of the year, and knowing how far Earth has moved, astronomers are able to use basic high-school geometry to estimate the star's distance. So the inverse-square law is the best mathematical method to use to figure out the fascinating parallax effect. This will explain why a star that is closer to us is brighter than a star farther away. Lets imagine two identical starsjust like our yellow Sun. Example: Star A is twice as far away as Star B. Thus, Star B will appear four times dimmer than Star A. The inverse-square law tells us that the amount of dimming is the relative distance squared. Astronomers like to use the inverse-square law when they locate stars that are clearly similar in age and composition. They then compare the brightness of each staragain, well use the example of Star A and B. If Star B is nine times less bright than Star A, then from the inverse-square law, astronomers deduce that Star B must be three times more distant than Star A. Whats in the Sky: Nov. 3-4: The Moon is the big performer in a stellar lineup of stars and planets at dawnweather permitting. Check out Venus, the "morning star"; it appears below the Moon. Then look for the ringed-planet Saturn located above the Moon. Youll see the star Regulus, too, to the upper right. Special thanks to the McDonald Observatory/Stardate staff for technical assistance with this weeks column.

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