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Parallax in observing stars

Parallax in Observing Stars. Created by Sal Khan.

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  • blobby green style avatar for user omaniruth
    How could I visualize this?
    (20 votes)
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  • leaf blue style avatar for user Stuffe
    But the star would have moved in the 6 months no? And how would you be able to pick a time of year when the line between the sun and that star was exactly parallel to the line of "looking exactly up"?
    (32 votes)
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    • male robot donald style avatar for user Nick Maes
      The star WOULD have moved, yes. As every thing in space is spinning and revolving and hurtling all over the place. However, the star is incredibly, vastly far away that any distance it would have shifted would be so absolutely imperceptible to anyone on our planet as to make the difference negligible.
      (48 votes)
  • aqualine seed style avatar for user Luisa11
    Just a curiosity! Millions of stars in the universe. Why does universe appear dark? It would appear bright!
    (14 votes)
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    • leafers ultimate style avatar for user Massimo Boscherini
      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.
      (28 votes)
  • leaf green style avatar for user Jamie Saunders
    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)?
    (14 votes)
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  • male robot hal style avatar for user Aditya Chaubey
    at ,sal says when sun is just setting,why not observe it as sun is just rising; like in the previous case?
    (7 votes)
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    • orange juice squid orange style avatar for user maren23
      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".
      (7 votes)
  • leaf red style avatar for user Sam Daliya
    if you were to shine a torch in the night sky for about a minute how far would it go into space
    (4 votes)
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  • leaf red style avatar for user Paze
    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?
    (3 votes)
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  • male robot hal style avatar for user Mrinmay Dhar
    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?
    (3 votes)
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  • male robot hal style avatar for user Anderson Lu
    what if you wait 3 month instead of 6 month?
    (3 votes)
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  • piceratops ultimate style avatar for user Tavares Wilson
    At around if we rotate around the sun is it possible that the sun's light can get so intense that we would not be able to see the other star that we are measuring using parallax
    (3 votes)
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    • male robot johnny style avatar for user Robert van Spanje
      No, that's why we wait for nightfall so we can observe stars better, without the sunlight during daytime :) Mind you (and this is explained in the videos in detail) The parallax is measured in six-month intervals -the maximum distances the Earth is from the Sun- , so the cycle of day and night does not apply in such a time interval of 24 hours in a day, in the sense that you would want to measure parallax. You would have to wait a much longer time, so, six months as said.

      Interesting question though, but think of it - during daytime your eyes can't see any stars, neither can optical telescopes! :)
      (2 votes)

Video transcript

What I want to do in this video is explain what parallax actually is, and then try to visualize what the parallax would be like in the context of observing relatively nearby stars. And then in the next video, we're going to think about how we can use the parallax of nearby stars to figure out how far they actually are away from us. So parallax really is just the apparent change in position of something based on a different line of sight. So when you see that, while you're looking out at a car, you're going to see that depending on how far different things are from you, it looks like they're moving relative to each other. Right now I'm looking at my computer monitor. And if I move my head around or shake my head around, it looks like the wall behind the computer monitor is moving relative to the computer monitor. We've all experienced this. But let's think about what parallax means when looking at stars. So let me draw the sun here. And obviously none of this is drawn to scale. So let me draw the sun here. And let me draw the earth at some point in its orbit around the sun. And we're going to pretend like we're looking from above the solar system. So the earth will be rotating in this direction right over here. And let's say the star that we care about is right over here. Obviously, clearly not drawn to scale. And what we're going to do is we're going to wait to the point in the year, so the point in our orbit around the earth, so that right at dawn-- so we're sitting right here on the surface of the earth. And to simplify things, we're on the equator. And let's say that this star is roughly in the plane of our solar system. So we're sitting right here on the equator. And then right at dawn, right when the first light of the sun begins to reach me-- remember, right now the sun is lighting up this side of the earth-- so right when the first light of the sun is reaching me, I'm looking straight up. So if I look straight up right when the first light of the sun is reaching me, and I look straight up like that, I will be looking in that direction. Now, so let's say that that direction that I'm looking at is this direction right over here. And then let me make it clear, this is a separate part of the diagram here. Maybe I'll do it over here. So if the night sky looks like this, the sun is just beginning to rise on the horizon. If I look straight up, I'm looking in this direction. So where would this star be relative to straight up? Well, straight up is going to be like that. The sun is, the way I drew it right here, right to my left. Straight up is just like that. The sun is just coming over the horizon. This star right here, the apparent position of this star relative to straight up is going to be at some angle to the left of straight up. It's going to be right over there. And obviously the star won't be that big relative to your entire field of vision, but you get the idea. Maybe I'll draw it a little bit smaller, just like that. So there's going to be some angle here. And this angle, whatever it is, let's just call it Theta, that's going to be the same angle as this. And when I talk about the angle, I'm talking about if you measure from one side of the horizon to the other side of the horizon, you're essentially looking halfway around the earth. That would be 180 degrees. So you could literally measure what this angle is right over here. Now let's say we waited six months. What's going to happen? Six months, we're going to be on this side of the sun. We're assuming that our distance is relatively constant at one astronomical unit. Now what happens? Remember, the earth is rotating like this. So if we wait, right at sunset, right when the last glimpse of the sun has just gone away-- because you can remember, right now, the sun is illuminating this side of the earth. The sun is going to be illuminating that side of the earth. So if we're sitting right at the equator right over there, right when the sun is just setting, we look straight up. Let me do that in the same color. We look straight up. So six months later when we look straight up, where is the star relative to straight up? Well now the star will be to the right. It'll be in the direction. So if this is our field of vision six months later, now the sun is setting all the way to the right, on the right horizon. And if we look straight up, this star now is going to be to the right of straight up. So what just happened here? Well, it looks like relative to straight up-- and we're looking at the exact kind of position of the earth. We're making sure that we're picking times of year and times of day where straight up is the same direction. We're looking in the same direction of the universe. It looks like the position of that star has actually shifted. And let's say that this is the middle of summer, and that this is the middle of winter. Doesn't have to be. It could be any other two points in time six months apart. Then when we look at this star in the summer, it's going to be over here. Summer, it's going to be right over there. And when we look at the star in the winter, it is going to be over here. And, in general, for any star, especially stars that are in the same plane as the solar system, you can find two points in the year where that star is at a kind of a maximum distance from center. And those are the two distances, those are the two times of year that you'll want to care the most about, because it'll be most interesting to measure this angle. And I want to be clear, this angle here is going to be the same thing as this angle there. You can see it's symmetric this way. Whatever this angle is going to be, and you could look at this. This is an isosceles triangle. Whatever this distance is from here to here, is going to be the same as this distance from here to here. And so this angle is going to be equal to that angle, and that angle is going to be equal to that angle. What I want to do in the next video is think about if we're able to precisely measure these angles, either one of them or both of them. And let me be clear, if this angle in the night sky is Theta, and this angle right here is Theta, the difference over here is two Theta. So one option, if you want to kind of make sure that your number's reasonably good, you could measure just the total difference that it is around the center and then divide by two. But in the next video, what I want to do is if you are able to measure the apparent change in angle here, if you were to be able to measure that, how would you be able to use that information to actually figure out the distance to this star?