# How Do People Measure Planets & Suns?

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When people measure the planets and suns, they are traditionally measuring distance. Find out about how people measure planets and suns with help from an experienced education professional in this free video clip.

Part of the Video Series: About Astrophysics & Outer Space
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## Video Transcript

Hi, my name is Eylene Pirez, and I'm an astrophysicist and this is how do people measure planets and suns. So there is a lot of measuring techniques for distances especially. This is an endless list and it also depends on the distance that the objects are at, so depending how close or how far it is or what information can you obtain from the object, the method completely changes. I will go over some of the main ones to give you an idea of how the process works. So let's say if we have a planet with a moon, that's an easy one to figure out the mass of the planet and one of the ways to figure out the mass of the planet is from this equation and 2Pi squared divided by the gravitational form and this is the distance to the moon and this is the mass of the planet. And this is going to isolate the mass and you'll be able to calculate the mass of the actual planet by just looking at the period of the moon going around the planet. The other one is just the visual one, so okay we look at something in the sky and we see an object and we can figure out the diameter of it. So if we divide the diameter by 2, this gives us D which is the distance directly from the surface to the object and then the sign of the angle in the sky divided by 2 and this can give us the distance to that object. That's one way to measure distance visually. Now another way that is very popular to measure distance visually is the stellar parallax and stellar parallax is a little more complicated in the sense that you cannot look at stars that are very far and I explain why. If you are looking, let's say you are standing right here on Earth and you take a measurement of this star, you plot the star to really be over here. You wait to be on the other side of the orbit or on the other side of the planet and then you take the measurement of the same star and you see that the star has moved. This is equivalent if you put, if you look at it like the tip of a pen and then you cover your eye, one eye at a time you can see the tip of the pen move. This is the same effect as parallax. Now with this distance we can actually figure out how far it is by using simple geometry and this is the stellar parallax. Now things get a little complicated when the star is a few light years away. That way if you have something very far away even if you cover your eyes, you're not going to be able to see the shift any more because the star is too far away, it's just a point of light. This is when we have to resort to a lot of things and at this point I'm going to list them. We sometimes look at things that move in clusters. So it turns out there are a lot of stars, like to be in groups of a lot of stars moving together and we can generally look at the motion and if you see a lot of motion you know that the star is closer to us. If you barely see any motion, you can kind of know if the star is far and by having a lot of stars to measure from you are most likely to get the actual distance. The other one that is pretty famous is the period luminosity and this is for special stars. The stars that pulsate. So some stars just have this flash and depending on how the period of flashing is, we can actually relate that to the period luminosity of that star. Once we know the luminosity of the star we can figure out the brightness and with that we can actually figure out the distance. So as you can see, all these things are very convoluted in like what information can give us at certain distances. So there are many more techniques to measure stars and to measure planets and they also keep growing these techniques. My name is Eylene Pirez, and I'm an astrophysicist and this is ways that we measure planets and stars.

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