Class 11 Physics (India)
- Scale of the large
- Angular measure 1
- Angular measure 1
- Angular Measure 2
- Angular Measure 2
- Intro to parallax
- Parallax: distance
- Parallax method
- Parallax in observing stars
- Stellar distance using parallax
- Stellar parallax clarification
- Scale of the small
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!(14 votes)
- 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)
- 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)
- 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.(7 votes)
- at4:40,sal says when sun is just setting,why not observe it as sun is just rising; like in the previous case?(7 votes)
- 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. At5:21, Sal says that at sunset 6 months later "straight up is the same direction".(7 votes)
- if you were to shine a torch in the night sky for about a minute how far would it go into space(4 votes)
- 1 light minute or about 18 million km. However, it would be so diffuse by that distance that it would most likely be undetectable.(8 votes)
- 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)
- The ancient Greeks have used this already. And before GPS kicked in it was a common technique in navigation at sea.(7 votes)
- what if you wait 3 month instead of 6 month?(4 votes)
- 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.(4 votes)
- What are some other examples of Parallax? Could someone please list some?(2 votes)
- 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.(5 votes)
- 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)
- 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.(3 votes)
- Does not the earth rotate west to east. Sal drew it opposite or am I wrong(1 vote)
- You are right that Earth rotates from west to east. But it can look like it is traveling the other way from a different perspective. If you look at the solar system from above, it looks like it is rotating from east to west. If you look at the solar system from below, it looks like it is rotating from west to east.(7 votes)
- If Sal assumed that the angle and the distance are the same in 2 parts of the isosceles triangle, then why have to wait for 6 months to observe another position of the star while the data seem enough to calculate?(2 votes)
- To measure the parallax you need to make 2 observations to know how much a star moves against the background, by doing this 6 months apart you get the maximum angle.(4 votes)
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?