Parallax - A Level Physics

[@kevinmhadley]

Now you can answer that question, “what will I ever do with geometry?”

[@purushottamkapote479]

Even after 10 years this explanation is unbeatable. Thank you Sir

[@DrPhysicsA]

In practice you cant do it that way. All you can do is observe where the star appears on the background of distant stars. Then 6 months later you do the same observation and note a slightly different position. The arc length distance enables you to calculate the angle. So you need both observations.

[@MoonWolfSotG]

One small typo at 8:11, 1 parsec is roughly 3.26 light year instead of 3.46 I must say an excellent video though, loved it!

[@DrPhysicsA]

Thanks. Quite right. I had thought that I had added an annotation to point this out. But I see it isn't there. And for some reason KZbin is not letting me add annotations to the video. But thanks for spotting the error.

[@steffenleo5997]

D=26 trillion miles instead of 26 billion miles the nearest star to sun(it"s a proxima centauri star you mean here)?....anyway thanks for you nice and informative video...👍👍👍

[@DrPhysicsA]

You are right that the measurement of θ is critical to getting an accurate assessment of the distance to the near star. In my diagram at 5:52 the angle at the star which forms a triangle with the earth at one point and six months later is 2θ. You can see from the diagram that the relevant angle for the purposes of calculating the tangent is 90-θ.

[@BrownPen_BluePen]

Uh, I am not smart and you explained this in such an easy to understand way I understood it right away. This was great, thanks for taking your time to make it.

[@DrPhysicsA]

You are looking at a nearby star against a background of distant stars at 2 points in the year (6 months apart). The nearby star will appear to have moved slightly compared with the background stars. You measure the angle (α) which that slight movement represents. Then d/x = tan α/2. d = earth to sun distance. x = distance to nearby star. Measurement is difficult. The earth must be in exactly the same position (relative to the background stars) each time. The angle is extremely small.

[@DrPhysicsA]

Good question. I suspect that in the space of 6 months the positional change due to these wider issues is negligible.

[@reto-fassbind]

Do your own parallax thing: stretch out your arm with your index finger pointed up. Look at it while alternately closing your left and right eye. Your finger will jump side to side compared to a distant object in the background. Your finger represents the observed star, your eyeballs the earth's March and September position, the eye distance the diameter of earth's orbit and the arm's length the to be calculated distance.

[@hmack23]

The way you explain your material is quite refreshing.

[@DrPhysicsA]

The calculation requires that the distant stars in the night sky are in the same position for both measurements in order to have a common background against which the parallax is measured.

[@brane7414]

So how the parallax is measured without knowing the distance?

[@danfulea3120]

The big question is how do you measure the angle theta? The rest is obvious...It seems nobody can give a good explanation how do you measure that stupid angle. If it is approximated by simple angle separation of two distant stars which is aligned with object star now and 6 month away, then this angle separation is not the parallax angle (by geometry)...just check it.

[@FalconFlurry]

Dan Fulea I'm a little late here, so maybe you already figured it out, but this only works if the star is directly overhead, if the star is off to one side you will get two different angles when you measure six months apart. Angle theta is measured as the apparent separation in the sky of the reference star and the target star. This can be determined with a telescope that is equipped to take those measurements. Arcseconds are a measurement of angle, but also give you information about the portion of the circumference (of the sky) that angle represents, same thing as radians. All the telescope is doing is measuring that portion of the circumference of the sky and reverse-engineering it to give you the measurement in arcseconds, or theta. In most cases the angle theta will not be equivalent in January and July because the star is off center, but the sum of those angles will equal the parallax angle and half of that will give you a line that intersects the sun. Showing the star directly centered over the sun is a bad example because it doesn't show you why you need to observe again six months later. That is my understanding of it, at least.

[@iTeerRex]

Yes you guys Dan, Falcon.. everyone does a very bad job of this. Although this is one of the better videos on this topic, still it doesn't go into how it is fully done. Everyone talks about the trees and the mountains to explain parallax, then they jump over and explain the tangent of an angle. Ok fine.. but the 3rd Crucial part.. how do you actually measure that stupid angle. It is not enough to say you just measure the angle.

[@FalconFlurry]

@@iTeerRex Wel I did actually explain how to find that angle, you need a telescope that is capable of taking such measurements. Another option is you can calculate the field of view on the eyepieces of your telescope to use as a reference point -essentially determining the angle by scale, but that would give you more of an approximate answer.

[@iTeerRex]

@@FalconFlurry Yes, but I was talking about the videos.

[@FalconFlurry]

@@iTeerRex ah, ok. Sorry, my bad

[@DrPhysicsA]

Thanks for spotting the error. I have put in an annotation to point out the mistake.

[@DimzReacts]

*mistakes

[@robinx4231]

Watched this to understand Parellex using optic when shooting.. Explained a lot better than some shooting channels!

[@JocanMaveric]

2:40 HOW DO YOU MEASURE THE ANGLE?

[@Sokol10]

2000+ years ago they use a instrument called Astrolab. :))

[@cusid0]

You could have mentioned that the theta angle is also obtained from the vertex of the two right-angled triangles where the star is located, the distant point, in practical terms it would be more enlightening than pointing out the theta angle projecting from the vertex of the earth (90 - theta). They are equivalent, but you could have considered this detail, because in practical measurement, the theta angle will appear from the vertex where the star is.

[@37rainman]

I understand parallax distance measure system. My question is: when they started all this, why didn't they simply call an AU 186M miles, and use the ENTIRE angle displacement against the background as th angle instead of 1/2 of it. My question is simply an assertion that u need no 90 deg angle because for these tiny angles, sine= tan= th angle in rad measure.--which is true. (So in other words u take th entire displacement angle expressed in radian and divide into 186M mi--to get just as accurate a result) ie: the sin/tan/rad of a 2 sec angle ALL =0.000009696273622--thats 9 significant figures--u cannot measure ur angle to anywhere near that accuracy

[@yesok3562]

The concept of radian is credited to Roger Cotes, who developed and used it in his Logometria (1714). Galileo died~ 1642 and after this the system was created. There was probably window period between the parallax system and the creation of the concept of radians.

[@bontrom8]

dots on the circle: March would be to the left of center, foreground objects move opposite of your position in space!

[@easycooking686]

let me start by stating, the everyday things in life that are observed are so much clear when physics and math get involved. Thanks, DrPhysicsA for the video and explanation. I have one concern, now after watching ur video and other similar parallax videos, I have noticed an inherent assumption that the "transmitter on top of the mountain" in ur diagram is in line with the "tree". How is this ensured in real scenarios when measuring the distance of a relatively close star?

[@kaloomuwana9077]

On every single video ive seen they always draw the two angles as being the same, is this always the case? Surely the two angles will be different in certain cases?at 2:40

[@litewave3831]

Your videos have always been interesting and helpful. According to the diagram at 2:42 how could the telescopes ever see the star in the first place !!! At the night side of the earth we would not spot the star because it is not located in front of the night side of earth. At the day-side of earth we wouldn't see the star due to day light ?! Can anyone help explain please.

[@jaredronning3020]

I believe you meant 26 trillion miles. Alpha Centauri is 4.367 light years away. That's 2.6*10^13 miles. Billion is 10^9. Trillion is 10^12. So 26 trillion miles.

[@DrPhysicsA]

+Jared Ronning Yes. Slip of tongue.

[@donnymo3481]

@@DrPhysicsA ur cool

[@Joe48274]

Why the hell are you using miles?!

[@gretawilliams8799]

Joonatakine Because he wants HATERS to waste their time and energy

[@joshbeck9761]

Because you're a fuckwit

[@kankanarock3443]

Joonatakine So this can even work in an ellipse right?( The orbit of Earth around the sun )

[@loyertamara]

Because it fuckd you up.

[@hazirafakhrurroziamir1363]

Maybe he's an American.

[@mrboyban]

Wouldn't the fact that the universe is in constant expansion possibly make the parallax technique completely irrelevant in cosmology?

[@berlinmae07]

I always thought about that too, im still searching for an answer though

[@JocanMaveric]

Hello, i'm trying to get my head around all this and was wondering how exactly they measure the "θ" angle, and if we can measure that, why cant we measure the angle with a star and our star (sun) straight off, or is it too close and bright?

[@sagnikmukherjee2028]

Have you found your answer? If so, could you share it with me? :)

[@사라-t9g]

2:30 what is the fixed point in space? That’s what’s confusing me

[@yayser]

Nice video. I think you have missed explaining the link between how much the star moves in the pictures due to parralax and the angle theta. How will I know theta if the star moves between pictures taken after 6 months?

[@timothyrenaud7348]

How do you measure an angle of say 1/2 arcsec ?

[@brane7414]

But you said that teta is the angle that we measured. How?? It seems that we cant know the angle without the distance and vice versa. So how do we even calculate this? How do we calculate the angle without knowing the distance?

[@purpledoo]

Does someone know how to measure the parallax angle from the apparent shift of the star from the background??? I can't find this info anywhere...

[@MrDdressler]

Hey great video, thank you for the simple explanations. Not only did you answer my question about star distance measuring techniques but also what the hell a parsec was.

[@cbob213]

Aaaaand it's a Star Trek Communication Badge!

[@Devasena_Harika]

1:58 it is a dead star

[@morthgar]

As we know Solar System moves through the Milky Way. Other stars move as well. What is the impact of this fact on measurement with this method?

[@hazirafakhrurroziamir1363]

Wouldn't it's wiser to measure the angle in January and July separately and not considering the angle of the stars, sun and earth would perfectly 90°? If it's 90° why would you wait until July? Given the fact that infinitesimal change of the angle would drastically changes the distance. CMIIW both for my grammar and my understanding to this concept.

[@juliakim1208]

Which is the brand of the pen you use, DrPhysicsA?

[@SeifSalamaKarem]

6:30 Is it trillion miles or billion?!

[@CuCulainn1]

the distance to the nearest star is a lot more than 26 billion miles!

[@DrPhysicsA]

+Brian Gorman (The Cake Is A Lie) Ooops. Yes slip of tongue. Should have said 26 trillion miles. One light year is about 6 trillion miles.

[@mauriskdamedov7518]

but the orbit of earth around the sun is elliptical...so how we could mesure after 6 months while the distance isn't the same... in other way you've worked on a circular orbit while the the orbit in real is elliptic...and thanks a lot for your work you are helping me a lot .

[@psnZYKRON]

i love your videos , they're really helpful :)

[@oliverA4936]

I've searched for hours. How did we figure out the distance of the sun WITHOUT any guessing?

[@anjanamalviya6411]

Wonderful explanation. Thank you!

[@7uc4s]

The lesser is theta, bigger is the distance 'D', right? Because when theta tends to zero the tangent tends to infinite... is that right?

[@Goultard-b5e]

But HOW do you measure angles if you don't have the distance of the star first?

[@dimitardimitrov131]

Great explanation ! Thanks !

[@chritophergaafele8922]

But how did we know or measure the angle theter

[@randomguyonthenet]

After watching many videos still not understanding I got it in this vid thx

[@selsebilx]

We measure at two point (after six month later) just because of accuracy problem, right?

[@DrPhysicsA]

Sapiens Sapiens Yes. It gives us the longest baseline for our calculation.

[@tscoffey1]

+DrPhysicsA Isn't it also because the line from Sun to the star must be a perpendicular bisector of the triangle formed by the two measurement points, and the star? (okay, after thinking more 3-dimensionally, the situation where measuring at shorter intervals fails would be when the Sun, Earth, and star are nearly colinear - ie, the star is on the ecliptic. For stars near the zenith (north, or south), then shorter measurement intervals work, but with less accuracy).

[@jondeveaugh615]

Drphysic this is good video well done. I do not have a problem with physics math for parallax. Have you ever tried to use the proper motion data of stars like Barnard's star or Proxima Centauri, and verify data to actually be a representation of parallax? It would appear the motion of the stars does not represent parallax as defined. Could you possibly verify and get back to me. It would be a great help. Thank you.

[@manatole_]

Hello, i'm confused as to why you can't just take the measurement of one angle of θ and use that since, in your calculations, all you use the angle for is to calculate tan(90-θ). What purpose does the second angle have?

[@cadkls]

Isn't it 24 trillion miles? Since alpha centauri is 4 light years away and a light year is approximately 5.9 trillion miles, and 4 x 6 is 24. 4 x 5.9 is even less.

[@DrPhysicsA]

Well spotted. I thought I had put an annotation to explain this. But I have now. Thanks.

[@dr.anonymous7479]

Can someone simplify 5:52 please it’s very hard for me to understand

[@redmoon1515]

Main point is missing here, how to measure angle first to calculate earth distance from star?

[@selfcaretoyou]

I hope next time you use metric system of measurement ... For international students

[@JohnVKaravitis]

Limit is about 1,000 parsecs. It's 206,265 AUs, or 3.26 light years. Also, is he using the British definition of a billion?

[@ad2894]

You measure two angles, what do we do with the other one??

[@sangeetadiwakar6982]

Just to take the average to get most probable value

[@Esra132]

that was very well explained thank you.

[@kanecarey8163]

Who decided 1 arc second would be suffice for measuring angles from stars? What if you actually had to break it down even more or less?

[@kanecarey8163]

Surely you would need a rough measurement to start with to know how far you could break a degree down?

[@achinraizada9805]

Still didn't got how did we arrive at the numeric value of parallax angle at the first place...That is Theta....

[@sea5205]

Amazing video!

[@debankurbssak6872]

How to find the distance between earth and moon

[@EvilNecroid]

u explained things well but how the hell to u work out that other angle? sorry my maths teacher was stupid

[@BHFJohnny]

Stellarium says the paralax angle of the closest known star Proxima Centauri is 0,77233" (that gives us 4.22ly). That's a hell of a precision. Let's google HOW the hack they measure it! :D

[@atmf3524]

I think you meant trillion. you should really use parsec though. Seeing as PAR = parallax and SEC = arcsecond.

[@aripapagianis8269]

How do you find the angle theta in the first place?

[@yeetspageet5679]

I imagine it's rather complicated. But something like pointing the telescope at the same angle, at the same time of day, after the earth has moved a sufficient amount. Then adjusting the telescope so it is looking at the star again. You can find out how far the earth has moved (I imagine this is the complicated part, our sun moving round the galaxy and that) and you have how much you needed to turn the telescope by. Doing this you can do some trigonometry to work out the parallax angle. This is a really messy explanation I know, but I hope I somehow helped with my limited understanding. I've only done this with moving a telescope alone a straight line to work out how far a lamppost is. You may not even need to do any trig as long as you know the straight line distance the earth has moved, and how much the telescope turned by to look at the star again.

[@jinlee3574]

Mathematical correct but your equations are based on hypothesis , assumptions to begin with What happened if your assumptions is wrong?

[@dheeshanya]

This was very helpful...thank you :D

[@isurudissanayaka6194]

why don't we measure distant to nearby star from our earth? here 'D' is the distant from the sun.

[@wessel624]

Hello, I had a question, I expect that the angle θ and the angle θ 6 months later has to be the same? Otherwise the sun will not have an angle of 90 degrees with Earth and the star, or is that negligible aswell? And why is it 90 - θ. Because i think that the angle will never be 90 degrees, it will be something less always isn't it? and a very little diffrence in angle will make a huge diffrence in length. I hope you can answer my questions, and sorry for my bad English, Wessel

[@felipehime]

What a great video !

[@pufalupagus]

Thanks man keep the videos coming

[@FruitoftheSpirit-0909]

How could they have measured 1/3600th of a degree with no computers or electronics?!

[@moyrml]

very informative, thank you very much.

[@aztonmartine]

very nice .......

[@joshuatanase3718]

You actually did not explain how you calculate the theta angle. How does the foreground (star) relate to the background (galaxies) from two different viewpoints? And how do you calculate the angle from that?

[@NadimAJ]

Beautiful method but in reality the measuring of stellar distances is a fudge.

[@justicesterne9843]

You are assuming the distance to the sun.

[@wessel624]

Ah thanks, That is very helpfull. Thank you.

[@Anku-xi5yz]

thanks mate

[@priyajanswongamikha3294]

how did 90-0 come?? help me

[@sebastianbroncano7643]

not zero, its theta its basically 90 minus the other missing angle (not the right angle)

[@ptathameshkhandare7512]

sir nice video but you have taken the value of small d in big D

[@KitKat-ve9fn]

Thanx🤫

[@37rainman]

May I remind u that the distance to th nearest star is not 26 billion miles -- it is 1000 times that far! It is 26 trillion miles. (-;

[@DrPhysicsA]

Thanks. Well spotted. I had made the elementary error of assuming that one light year is 6,000,000,000 miles, whereas as you point out it is 1000 times more than that.

[@Emzo99]

thanks you are a babe. Astro ib test tomorrow

[@DrPhysicsA]

Thanks for your kind comment. I hope the test went well.

[@flailingbranch8551]

Can someone pls make an fucking example instead of using d D? Believe it or not it would help me a lot

[@isaacphysicsanswers6794]

Shame you didn't use SI units or a close relative, like kilometres. Otherwise vey good, clear and simple.

[@lexi3398]

Haha! Im the only 12 yr old who gets this i bet! Good vid though

[@Elitea20]

Lexi Kelly noooooooo I’m in uni and I’m confused dammit

[@keifer7]

cool

[@lfcblaze]

HELLWO!

[@toniozezeca5240]

Man you just saved my life... thanks

[@lillililo]

really helpful thank you!

[@rem4441]

Assuming the sun is 93,000,000 mi. away, which it is not.

[@7uc4s]

How is that? Will you say that Earth is flat?

[@aoibheannainebradley9336]

I Do Consider It Would Be Worthwhile To Utiliize The Creative Faculty To Choose Another Term For Astronomical Units, Ræther Than Allowing Its Signification To Be Synonymous With The Term For Gold On The Periodic Table. Perhaps "Æ", "æ" 🐑 .

[@TamNguyen-he9jv]

you make mistakes every 30 sec, why can't u just rerecord the video

[@BrownPen_BluePen]

Tam Nguyen I mean this video already has views so I wouldn’t remake the vid if it was me. I was watching this to understand the concept of a parallax not this specific calculation of whatever Star he’s measuring... so for me and probably most people watching this, we are trying to better understand the concept of a parallax which makes the actual mathematical values of the parts of the formula he is using irrelevant anyways

[@DimzReacts]

yesss agreed

[@crawford323]

Both units of measurements being either miles or meters are equally worthless in my opinion because they are earth based. As a meter is 1/ten millionth of a quadrant and miles are the result of some human body measurement. Well I suppose they need something but to me a measurement of the most common element in the universe would be more useful. Base a new system on the wavelength of hydrogen. You could keep the force of gravity earth centric and water being as almost universal as hydrogen. Keep the base ten and discard the rest and start over.

[@SuperCuteBabyCat]

This is bullshit! They say earth moving and spinning so fast than a jet plane, and stars and planets have its own orbit and movement to the left right down up back forward, do you know how fast the stars and planets movement?? The answer is no, you dont even know the speed of object you are measuring!, so how can you fit your calkulation if you dont have perfect branchmark!? JUST 1 cm of misscalculation the result wouldbe super missleading people! Things on earth are very difrent with things in space! , things on space floathing and moving and super duper far without ground floor ! How can you get perfect clue to began your math calculation! ? The sun? How do you know the distance between sun and earth!? Oll of that is just math assumption!

[@komalbhardwaj7216]

it is good

[@chritophergaafele8922]

But how did we know or measure the angle theter