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The parallax view

Some of you may recall a 1974 movie, the political-thriller "The Parallax View", starring actor Warren Beatty. This movie's cryptic title illustrated the visual effect of looking at objects, both near and far, against an unchanging backdrop-in the movie's case, mysterious moving objects against the backdrop of international politics.

With a nod to the movie's title, let's look at the "parallax view" of how today's 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 fingertip-first with one eyelid closed. Next, you opened your eyelid and then closed the other. Magic appeared to be the result-your 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 Observatory's 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."

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