If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

### Course: Class 11 Physics (India)>Unit 2

Lesson 1: Physical quantities and their measurement

# Intro to parallax

## Jumpy fingers

Hold up a finger and focus on something in the distant background. While looking at the background alternate opening your left and right eye one at a time. You will find that your finger jumps back and forth (or splits into two). This is the parallax effect in action:
Parallax is this apparent shift caused by viewing an object from two different vantage points. Next, gradually pull your hand away from your face while still alternately opening and closing your right and left eye. Does the gap between the two images change?
Yes: the greater the distance, the smaller the gap.
Below is an interactive illustration with the red dot representing your finger and a tree representing a distant background. You can click and drag the red dot to bring it towards and away from your eyes to see what happens. Press c to cross/uncross your eyes:
This simple observation is the key to how we can measure things at a distance. Notice what happens when you direct your eyes to your finger instead of the background (this will cause you to cross your eyes slightly).  Now the background splits into two images which are separated according to how far away your finger is.
The same effect is observed when we look at objects in our solar system from different positions on Earth. In this case the distant stars act as our fixed background and our finger is replaced by a distant object, such as Mars. If you took two pictures of Mars on different sides of the Earth you will find that it seems to jump positions, while the background stars do not. Observing objects from these different positions has the same effect as opening and closing our eyes, except now it’s over a much greater distance.
This means that the Moon would exhibit a greater parallax (or jump) as compared to Mars. Convince yourself of this using theinteractive illustration below.Click and drag the planets and move the time slider to view the parallax effect from different positions on earth:
Challenge question: Could we use the parallax effect to measure the distance to the Moon?

## Want to join the conversation?

• In the article, it says `If you took two pictures of Mars on different sides of the Earth you will find that it seems to jump positions, while the background stars do not.` Why is that? I don't quite get it...
• In the way as when the object in the background when you hold up a finger remains in the same place while your finger seems to move. when we look at mars from different positions the moon seems to move as it is clser than mars(in the same way your finger is closer than the tree).
• Am i not wrong if we focus on background then we experience parallax for the finger and if we focus on finger we experience the same for background?
why is this so?
• Actually, parallax happens for everything, even the background. But, when we take the background, we take it in such a big scale and so far away, that the difference in parallax is not that noticeable. As you must know, parallax becomes lesser as we place the object farther from the viewer. So the aim of choosing a far away background, is simply that we are not able to notice its parallax as well as the objects parallax. But it occurs in both.
• Is it possible to measure the distance from the Moon to Mars, or even the surface area of Earth by using parallax?
(1 vote)
• yes. for example, we can assume ourselves to be on moon. since we know what the diameter of the moon is, we can easily calculate the difference in distance between the two viewing points. And when we view the planet mars from moon (apparently), we can calculate the distance between them. To make this easier to understand, you can just say that parallax helps you calculate the distance between any two objects if only you can figure out what the diameter of the viewing planet is. or viewing platform, that is.
• The moon is closer than the Mars. Then why'll moon a greater parallax?
• closer the distance larger the gap. So you will notice parallax more in viewing of moon instead of mars
(1 vote)
• Does this mean that eclipses happen with objects in space, for example, two stars?