Pick a finger, any finger, and hold it out at arm's length. Close one eye. Now open that eye and close the other. You may have noticed that your finger appeared to shift in position, even though you kept it still. That shift is called parallax. In this case, it happens because each of your eyes is looking at your finger from a different position in space.

Parallax is formally defined as the apparent change in position of a nearby object relative to a distant object when the observer moves to a new position. In the example above, your finger is the nearby object, the background is the distant object, and looking first with one eye and then the other is your change in position. Parallax is an important concept in the study of astronomy because it makes it possible to calculate the distance between different celestial objects and the Earth. The parallax method of calculating distance entails the use of different lines of sight to an object to determine that object's distance. If not for the parallax method of finding distance, astronomers would be at a loss as to how to measure the distance to an object that may be many trillions of miles away.

Although parallax makes it possible to gauge distance, astronomers can't use the finger trick when working with distant celestial objects. The shift in position looking from one eye and then from the other wouldn't make much difference when observing something as far away as another star. Instead, astronomers use opposite points of the earth's orbit around the sun to measure stellar parallax. For example, an astronomer would look at the position of a fixed star today, and then look at it again six months from now. The star will appear to have shifted its position in the sky, but actually, the star is merely being viewed from the opposite side of the Earth's orbit around the sun. It's like looking first out of one eye, and then out of the other, on a much larger scale. Using what they know about the distance of the Earth's orbit and the angle of observation from each end of the orbit, astronomers can use trigonometric ratios of similar triangles to calculate the approximate distance of a star from the Earth or from our own star, the sun.

To determine distance on a slightly smaller scale, we can also measure parallax from opposite points on the Earth's surface. This method is useful in studying closer objects, such as the moon, or a comet that is passing close to Earth.