If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

# Stellar parallax clarification

## Video transcript

I got a comment on the video where we first introduced parallax especially relative to stars essentially asking how do we know that this angle and this angle is always the same or how do we know that we're always looking at an isosceles triangle where this side is equal to this side it worked out for this example that I drew right here but what if the star what if the star was over here what if the star was over here then if you just look at it this way if you take at this point the triangle looks not it is no longer it's clearly no longer an isosceles triangle it looks more like a scalene triangle I guess where all the sides are different and so a lot of that trigonometry won't apply because we won't be able to assume that this is a right triangle over here and what I want to make clear is is that that is true you would not be able to pick these two points during the year these two six-month these two points in our orbit six months apart in order to do the same math that we did in the last video in order to calculate this and still have an isosceles triangle what you want to do is pick two different points six months apart so what you want to do is if this is the Sun you want to pick two different points six months apart where it does form an isosceles triangle so if this is the distance from the Sun to this other star right over here you want to point you want to pick a point in Earth's orbit around the Sun here and then another or point in the orbit six months later which would put us which would put us right over here right over here and if you do that if you do that then we are now all of a sudden we are looking at two right triangles if we pick those if we pick those periods correctly and the best way to think about whether this is a perpendicular angle is you're going to try to find the maximum parallax from Center in each of these time periods here it's going to be maximally shifted in one direction and then when you go to this six months later it's going to be maximally shifted in the other direction so to answer that question the observation is right at exactly the middle of summer in the middle of winter all stars will not form and I an isosceles triangle with the Sun and the earth but you can pick other points in time on the year six months apart where any star will form an isosceles triangle hopefully you found that helpful