Parallax |
You read in chapter 1 about the concept of parallax and how the ancients tried to use it to determine the distances to stars. (Because the stars were too far away for the ancients to note parallax, they decided that the stars were infinitely far or at least unimaginably far way.) If you view the same object from two different angles, the perspective will change. Hold a pencil out in front of you at arm's length and view it first with your right eye (left eye closed), and then with your left eye (right eye closed). Click on the image to change. When you switch eyes, you are changing the angle of your view, such that you view the scene from a slightly different perspective. This is an "angular displacement." You could actually calculate the distance from your eye to the pencil (that is, the effective length of your arm) if you knew the exact angle of displacement and the distance between your eyes. There is a very precise relationship between the angular displacement and the distance to the object observed. The greater the angle observed, the closer the object. The smaller the angle, the farther the object.
This is how the ancients hoped to determine the distance to the stars. Unfortunately, the displacement caused by parallax was simply too small for the ancients to notice or measure, and it is too small for us to measure, too. However, parallax is somewhat more tractable if we consider planets or the Moon, and especially if we use our computer planetarium! That's what the activity is all about.
Even astronomers with powerful telescopes are severely limited in parallactic determinations. The limitations of optics are compounded by the obscuring and blurring effects of the Earth's atmosphere.
The distances to even the nearest stars is so great, and the angles involved so small, that astronomers invented a new distance measurement to be used in such calculations. It is called the parsec , and is equal to about 3.26 light years. A parsec is just the distance at which a star's parallax is equal to one second of arc (1/3600th of a degree or about 1/1800th the width of the Full Moon. ). Put another way, an observer in space one parsec away would perceive the distance between the Earth and the Sun as one second of arc.
While the distances to perhaps 10,000 stars can be determined through telescopic observations from Earth, the difficulties in so doing keep the actual number down to just a few thousand. No more than 1500-2000 of those distance are known well.
However, a recent spacecraft known as Hipparchos obtained much data from above the atmosphere, from which the distances to maybe 100,000 or more nearby stars can be calculated.
Copyright ©1999 The McGraw-Hill Companies. All rights reserved. Any use is subject to the
McGraw-Hill Higher Education is one of the many fine businesses of
The McGraw-Hill Companies
.
For further information about this site contact
mhhe_webmaster@mcgraw-hill.com
.
or let us know what you think by filling out our
site survey
.
Table of Contents