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## Class 11 Physics (India)

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

Lesson 1: Physical quantities and their measurement

# Parallax in observing stars

Parallax in Observing Stars. Created by Sal Khan.

## Want to join the conversation?

• Just a curiosity! Millions of stars in the universe. Why does universe appear dark? It would appear bright!
• Actually they are more than millions...

Anyway, you have just restated a famous problem in cosmology, the so-called Olber's paradox, that we can summerize stating: if there are stars in every direction we look, why isn't the night sky glowing with light.

The Big Bang model of the Universe answers to this question because it shiws how the light from the older stars has been redshifted to non-visible radiation due to the expansion of the Universe itself. For instance, there was an early age of Universe when it was dominated by radiation (really bright), but as the billion of years passed, it redshifted in the very weak comsic microwave backrgound radiation.
• In this case, the assumption seems to have been made that the lengths of both the purple lines to the 'purple' star are equal, but is this the case in all situations? What if the 'purple' star which we are looking for, is above the horizontal plane of the sun in this video? Surely, the purple line at summer (in this case) would be longer than the purple line at winter? (and therefore different angles?) Is that correct? If I've misunderstood, then how are they always the same length/angle (at both summer and winter)?
• If this were the case, they would have chosen two other points in time from where the "purple lines" would be equal.
This is because we NEED those angles to be equal, watch the other parallax videos to understand more.
• at ,sal says when sun is just setting,why not observe it as sun is just rising; like in the previous case?
• If you looked at the star as the sun was rising exactly 6 months later, while on the other side of the sun, you would be facing the wrong direction. At , Sal says that at sunset 6 months later "straight up is the same direction".
• if you were to shine a torch in the night sky for about a minute how far would it go into space
• 1 light minute or about 18 million km. However, it would be so diffuse by that distance that it would most likely be undetectable.
• This concept of parallax is amazing.
Well, let's suppose we are looking at a mountain range at a distance. Could we there make a measurement, then walk 50m (for example) by our side and make another measurement. And then, knowing the angle formed, and knowing that we walked 50m, discover the distance between us and the mountains?
If so, please explain it to us! Do we need great instruments for this?
• The ancient Greeks have used this already. And before GPS kicked in it was a common technique in navigation at sea.
• what if you wait 3 month instead of 6 month?
• The baseline of the triangle used for calculate the distance will 1/2 of what it would be at 6 months so it is not able to work for distances as far if you waited for 6 months.
• What are some other examples of Parallax? Could someone please list some?
• Well parallaxes don't have to always be related to space. When, driving through a village or the countryside, you can see that the trees near you appear to be moving faster than those away from you. That's an example of parallax. Stellar parallaxes are related to the parallaxes of the stars.
• But Won't our angle measures be wrong due to the atmospheric refraction and we are seeing the sun rise 2 minutes earlier than the ACTUAL sunrise?
• You don't actually do it at sunrise. I don't know why Sal chose that way to illustrate it. You would do it at night, because that's when you can see the stars.