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### Course: Cosmology and astronomy>Unit 2

Lesson 3: Stellar parallax

# Stellar parallax clarification

Stellar Parallax Clarification. Created by Sal Khan.

## Want to join the conversation?

• Correct me if I'm wrong, but why does this have to be an isosceles triangle?
Can't you just measure the offset angle of the star based on the position of the sun, then in six months, do it again to form (logically) one big triangle? Which you could then use the knowledge of the two degrees to figure out the third and then use trigonometry to figure out the distance that way? I think that would be a lot easier than first trying to figure out how to line it up at a 90deg angle from the sun.
• You are correct, it would work, but the measurements with an isosceles triangle are relatively easy because two of the sides are the same length.
• I'm curious as to how an astronomer would track an individual star or object throughout the year to make that decision of maximum parallax. How does one identify a specific star to study from night to night when, at least to the layman, the vast majority of them seem very similar? Is there some way to "fingerprint" an individual star?
• There are several ways to fingerprint a star, the first one is the relative position to the other farther away stars in the field. You could produce a diagram of the nearby stars and use it to approximate the displacement of the star you are looking at. Another hint is the relative magnitudes of the stars around. Yet another fingerprint is the spectrum, or the relation between colors among the stars, for example spectral lines which yield information about temperature and composition. Say for example your star is red and very bright you could notice a very bright star in the same field of view but displaced.
• when dealing with such huge distances i'm curious how we know that we are at a right angle. and when finding the parallax of the star the example was 1.something arc seconds. what if the star is above the plane of our orbit? even just a minuscule amount like 1 or 2 arc seconds?
• If one could observe, measure and record the same star everyday for a year, comparing the dates to greatest readings farthest apart should give you all you need to find your starting points for measuring the angles.
• Is there any way we know where exactly we are in orbit at the time when this triangle becomes isosceles? I'm pretty sure they have some pretty crazy technology now to allow us to know these kinds of things, I was just wondering...
• Check the date and you will know where we are in our orbit.
• The sun's rays are refracted by the earth's atmosphere, so we don't see the sun rise and set on the horizon exactly when our position on the earth rotates into and out of the direct rays of sunlight. Given that the angle we are measuring is so small, doesn't the fact that we are not facing exactly where we think we are in relation to the base of the triangle established by the visible sun when it rises and sets on the horizon introduce an inaccuracy into this minuscule angular measurement?
• Whether or not we are facing exactly where we think we should has no bearing on the accuracy of the measurement.
• i don't get it. you're saying that every star out there can form an isosceles triangle? how is that possible?
• Sure. Try drawing it. The earth goes around the sun. For every star, there is a triangle that has two corners somewhere in earth's orbit, and one corner at the star.
• What if the star is moving away?
• That is an excellent question, that is called radial velocity and it also gets measured as another component of the velocity. That one does not affect parallax but can be seen in the star spectra.