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Why do we use parallax?

Updated: 6/30/2023
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14y ago

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As Earth orbits the Sun individual stars seem to move their position against the celestial background. The nearer a star is to is, the greatest that apparent move is. That apparent change in the stars position is known as its parallax. A star close enough to show a change of 1 second of an arc is said to be at a distance of one parsec. No star is actually that close. Proxima Centauri, the nearest start to us after the Sun, is 0.75 of a second of an arc. One parsec is equivalent to 3.76 light years. The farther away a star is, the smaller its parallax. Stars over 50 light years away have a parallax that is too small to measure, even with the most powerful of telescopes. Only about 1000 stars have an accurately measured parallax. Beyond that, the absolute magnitude of a star is used to estimate its distance, which relates to its brightness.

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15y ago
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8y ago

Parallax is the apparent displacement of a relatively close object compared to a more distant background as the location of the observer changes. Astronomically, it is half the angle which a star appears to move as the earth moves in its orbit from one side of the sun to the other.

To see a quick example, close one eye and hold up a finger and align the finger with a tree, light pole, the edge of a door or the like. Then close that eye and open your other one. Your finger appeared to move, but it was not actually displaced. Your point of observation changed. This is parallax.

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Here's a practical example:

Imagine you are standing on a sidewalk on a straight, flat street with evenly spaced lightposts. All traffic is blocked for you so you can move safely. As you walk out to the street, look down the row of posts. The post closest to you "seems" to move across your field of vision much more quickly than the posts farther away. That difference is parallax, and it is one way that triggers our brains to understand depth. It can also be used in astronomy. When earth is at one end of its orbit, we can look out toward the stars and observe their positions. When earth is then at the other end of its orbit six months later, we can observe the positions of the stars, and if the stars are not unimaginably far away, we might make measurements that will relate to their distances based on the same idea of parallax as with the lightposts. The more a star seems to move (change positions over the 6 months) the closer it is.

Another example:

When traveling in a car (not driving) in the countryside, you might notice a mountain top in the distance. If you look at the mountain, you notice everything between you and the mountain moving across your field of vision very rapidly. The closer it is, the faster it "seems" to move. In fact, if you fix your gaze on something half way between the car and the distant mountain, it almost "seems" that everything in front of that point is moving in one direction, and everything behind is moving in the other direction! This is just another way to think about parallax. The apparent motion of objects different distances away, as YOU move from side to side.

The farther away something is, the less it appears to move relative to you as you move. This is why the moon near the horizon seems to travel along with you when observing it from a moving car. You notice this even if you are walking when the moon is visible in the sky. Of course the moon is moving, but relative to your motion on earth, it seems to be motionless.

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12y ago

Well it's pretty simple, you know what a parallax is, as you experience it all the time. Like your in the car, and your moving... you can see the stationary objects closer to you seem to be moving faster than the objects further away. The apparent change in position is the parallax, and it is measured in the terms of "The angle of parallax", the greater the angle, the closer the object is.

So in terms of nearby stars, it is used to measure the distance, by calculating and observing how they move relative to very distant stars, when the earth is in different parts of it's orbit. And this distance unit is called a parsec.

Umm... it's measured per say when earth is in January and in July during it's obit, and is observed to see how it has moved... so the distance from earth to that star, along with knowing that earth is exactly 1 astronomical unit away from the sun, creates a right angled triangle per say. And using that triangle and trigonometry, we can measure the angle of the paralax, to work out the stars distance.

Hope this helps!

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12y ago

The star's apparent position in the sky changes, due to Earth's movement around the Sun. Earth's orbit has a diameter of 300 million kilometers, so Earth has this much change in position each year.

The distance to a star (in parsecs) is equal to the reciprocal of its parallax (in arc-seconds). For example, a star that has a parallax of 0.5 arc-seconds is at a distance of 1 / 0.5 = 2 parsecs.

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11y ago

Parallax is a method of estimating an objects distance by observing it's position against a background from two different locations.

To illustrate the technique, hold your thumb out at arms length. Close one eye, and line your thumb up with an object across the room. Now, close the other eye, and see what the thumb lines up with now. Repeat this with your thumb at different distances from your eyes.

If you measure the change in the thumbs position, and the distance between your eyes, you can find the distance to your thumb without actually taking a rulerout.

Stars are so far away we can't travel the distance and measure how far it is. So we use parallax to find the distance. For our "eyes" we look at the star from the extremes of earths orbit, 6 months apart (so from each side of the sun as we orbit). We measure how far the star as shifted compared to other more distant stars and can find the distance.

This is a key technique, because we can only use it to tell distances out to ~100 light years, and is the most direct way of measuring distance we have. With accurate Parallax measurements, we can determine other methods to find distances, which can be used to determine greater distances.

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12y ago

The relationship is very simple. If the parallax is 1 arc-second (1/3600 of a degree), the distance to the star is 1 parsec. If the parallax is 1/2 arc-second, the distance is 2 parsecs. If the parallax is 1/10 of an arc-second, the distance is 10 parsecs. Etc.

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13y ago

Take a picture of the sky (at night of course). Six months later take another picture of the same piece of sky. Compare the two picture.

Now; most of the objects are so far away that they seem "fixed", but the nearer ones will appear to have moved. Knowing the base line (186 million miles) the distance to those objects can be calculated.

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13y ago

a paralax can be used to measure the relation of the distance between one star to another.such as, when you take a picture of a star, and another of that same star six months later, with the paralax you can see how far they moved.

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14y ago

The parallax of a star indicates how far the star is. The smaller the parallax, the farther the star.

The parallax of a star indicates how far the star is. The smaller the parallax, the farther the star.

The parallax of a star indicates how far the star is. The smaller the parallax, the farther the star.

The parallax of a star indicates how far the star is. The smaller the parallax, the farther the star.

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9y ago

Parallax is used by astronomers to measure the distance to nearby stars. As the astronomer moves between two viewing positions they see the same tree but it will appear to be moving against the background.

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