Question

Describe what parallax is.

Answer 1

Parallax is the way an object’s position or direction seems to change depending on viewing angle. To experience this phenomena, simply cover one eye and focus on an object. Then move the cover to the other eye. Because each eye provides a different viewing angle, the object will appear to move.

To measure large distances, such as the distance of a planet or a star from Earth, astronomers use the principle of parallax.

For example, if you look at some nearby object and move your head a little from side to side, the object looks like it is moving back and forth. Another way to observe parallax is to look at some object while you sit still but alternately cover your right and left eye. You will see the object "jump back and forth" as you do that.

Parallax is what allows us to estimate the distance to nearby objects. We use it all the time in such simple actions as picking up a pencil. Our right eye sees the pencil in a slightly different direction than our left eye sees it and our brain combines the two images to tell us instinctively how far to stretch. You can test the importance of this by trying to pick up a pencil while you keep one eye covered. It is much harder to do without the clues given by parallax.

Because stars are extremely far away, the angle by which they shift in position is very tiny - only fractions of a degree. The shift is so small, that astronomers use a smaller unit of angle equal to a sixtieth of a sixtieth of a degree called an arc-second. That is, an arc-second = 1/60x1/60 = 1/3600 degree. Such a tiny angle is imperceptible to the naked eye, but can be measured with special instruments.

The amount of the shift caused by our motion around the Sun depends on the star's distance from us. Just as a nearby object in the room shifts more than a more distant one if you move side to side, so a nearby star shifts more than more distant star. Thus, the amount of shift gives a clue to the star's distance.