PARALLAX

What exactly is parallax?

Parallax is a displacement or difference in the apparent position of an object when viewed along two distinct lines of sight,

and it is measured by the angle created between those two lines.

Parallax can be used to determine distances of various object by using a simple concept that near objects have more parallax as compared to distant objects.

As you can observe, far buildings appears to move slowly than the near ones.

Applications of parallax.

All the images of galaxies that we see contain more information than we think. See this representation which is made by manipulating this information by using parallax and thus detecting distance between the nearer and farther stars.

It is also used by humans for perceiving depth. The two eyes receive different images and they are overlapped in our brain. The difference in these two images is used to sense depth. Try it by closing one of your eyes and you will have difficulty catching a ball thrown at you. This process is known as stereopsis.

Parallax also affects optical instruments such as rife scopes, binoculars, microscopes, and twin-lens reflex cameras that view objects from slightly different angles.
There is a device called a parallax range finder that uses it to and range, and in some variations also altitude to a target.

Measuring distance using parallax.

Parallax effect can be used to measure the distances to nearby stars. As the Earth orbits the sun, a nearby star will appear to move against the more distant background stars, in the same way the tree appears to move against the more distant mountains in the diagram. Astronomers can measure a star's position once, and then again 6 months later and calculate the apparent change in position. The star's apparent motion is called stellar parallax.

There is a simple relationship between a star's distance and its parallax angle:

d = 1/p

The distance d is measured in parsecs and the parallax angle p is measured in arcseconds.

This simple relationship is why many astronomers prefer to measure distances in parsecs.

Limitations of Distance Measurement Using Stellar Parallax

Parallax angles of less than 0.01 arcsec are very difficult to measure from Earth because of the effects of the Earth's atmosphere. This limits Earth based telescopes to measuring the distances to stars about 1/0.01 or 100 parsecs away. Space based telescopes can get accuracy to 0.001, which has increased the number of stars whose distance could be measured with this method. However, most stars even in our own galaxy are much further away than 1000 parsecs, since the Milky Way is about 30,000 parsecs across.

Here is a short video to help you understand better

For more detailed information about astronomical parallax refer Attachments

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