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## Class 11 Physics (India)

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

Lesson 1: Physical quantities and their measurement

# Stellar distance using parallax

Stellar Distance Using Parallax. Created by Sal Khan.

## Want to join the conversation?

• Aren't star charts usually drawn with West on the right and East on the left? I always think of it as you are lying down on the ground. North is still up, but east is now to your left, and west is to your right.
• Yes, star charts are usually drawn that way, for exactly the reason you suggest. But it is not intuitively obvious for most people who aren't used to looking at the stars, so I think Sal was just assuming the 'normal' way round for East and West because that's what most people deal with most of the time.
• Since the universe is expanding, wouldn't it be true that the star we are trying to measure would have moved away from the earth after six months making it so that the triangle is not an isosceles triangle making our calculations incorrect or is this distance negligible within the time of six months.
• Within a galaxy, and to some extent between some nearby galaxies as well, gravity plays more of a part than the expansion of the universe. So yes, the universe is expanding, but gravity holds our galaxy together more strongly, so there is no net expansion.
• We measure the angle by a sextant for example right? How we get a fix center point to measure theta then if we got theta why we need to wait a half year to measure the other theta? Is that just to get a better estimation (precision?). You don't need the second theta to apply the trigonometry on that issue. How to be sure that your centered point is absolutly the same between those half years?

Thanks for helping me understanding that issue. Sorry for english misstakes, I am still learning it (from Germany). One simple other question, can i do that double progressive form one sentence before? :)
• You would want to phrase it as "Thanks for helping me to understand this issue" hope this helps! good luck on learning english (i'd like to learn german!)
• How do we measure those angles?
• What about the idea that during the 6 mths for earth to rotate halfway around the sun, the sun and the star have already moved relative to one another...be it due to their own orbits or due to space expansion. What about curvature of light? The light from the star to us would have probably curved a little due to any large bodies it passes nearby. Even if we had precision devises that could measure angles up to 0.00001 degrees, wouldn't variations due to orbit/expansion/curvature be significant?
• The sun and the star will indeed move relative to each other. But the amount they are moving is so tiny compared to the distance between them, even over six months, that it does not significantly affect the measurement. Remember that any star you are measuring with parallax will be in our own galaxy, so its motion through space will be quite similar to the Sun's anyway.

As for curvature of light, any body large enough to significantly bend light around itself will be large enough for us to notice it (either because it shines under its own light, or we notice its effects on other light). So we can measure the effect and take it into account.
• Does "straight-up" mean the same thing as "zenith"?
• Well is there any way to measure these angles at home lets say a star i might want to find the distance for?
• The parallax of the nearest stars is just under 1 arcsecond, which is about the limit of resolution for most amateur telescopes. It might be possible, if you had a relatively large amateur telescope with angle measuring equipment or imaging hardware, and persisted in taking measurements and/or images under near-perfect conditions for a year or more, you might be able to detect motion against the background.
• Did astronomers use this same method to first calculate the distance between the earth and the sun?