Parallax is what we use to measure how far away planets are (I think). Can anyone provide me with a clear *** Dirt Simple*** (I’m no scientist) explanation of how it works? I’m really curious and all the explanations I found on the net went waaaay over my head. Thanks
It’s an extension of the way we figure out regular depth perception.
Look through one eye at a time. You’ll notice that things that are closer seem further left through your right eye. Substitute telescopes for eyes and miles for centimeters and you can figure out the relative distance of stars.
Think about walking down the road and looking out into a field. Out on the other side of the field is a windmill while in between you and the windmill is a broken down wagon. As you move down the road and look out at the windmill, the wagon’s apparent position changes with respect to the windmill. the change in position is known as the parallax.
Astrophysicists use this property to measure the distance to celestial objects. An image of the object against the background is taken. A later image from say six months later, when the Earth is in the opposite side of its orbit will be taken and compared to the original. The apparent change in position will allow you to measure the angle change which, when placed into the appropriate equations, can tell the distance for at least nearby objects out to approximately 500 light years.
Here’s a graphical treatment, along with equations.
Look at something at least ten feet away, like on the far side of the room (make it as far as possible). Hold your index finger up, about two feet from your face (or as close as you can get it and still focus on it), and right between you and the further object. Now, try alternately closing and opening your left and right eyes. Look at how far your finger appears to jump when you change eyes, then notice how the further object jumps much less when you change eyes. That difference in apparent distance that they jump is due to parallax, or the different points of view of your two eyes. If you know the distance to your finger and the apparent distance that each object jumps, the distance to the further object can be calculated by trigonometry, which I don’t think I can explain without drawing pictures.
Now, imagine that the moon is your finger, a nearby star is the further object, and two telescopes a thousand miles apart are your two eyes. By looking at the differences between the view from one telescope and the view from another, and knowing already the distance to the moon, you can calculate the distance to the star by using the same trigonometry. Astronomers have also used this principle by comparing pictures of stars taken when the Earth was on opposite sides of the sun.
Clear as muddy water, right?
Is that some sort of a slight, Bob? 
Hell, yes! Ever since you registered last month, I’ve been lurking here, grinding my teeth and bouncing up and down and muttering obscenities to myself, waiting for a thread about parallax to come up so that I could trash Larry Mudd!
Actually, I say that because I’m so bad at explaining things, even if I think I understand them. Half the time when I try to explain something here, someone who doesn’t understand what I said comes along and tell me I’m wrong, and then says the exact same thing I did, but simpler. I should be an elementary school math teacher.
FWIW, Bob I thought your explanation was the clearest and most complete thus far.
Just to add gas to this fire, I have an explanation of parallax with links on my own website.
You can try this little experiment… put your fingers with the tips together, about an inch or two in front of your eyes, then focus on the wall (at least 10 feet away).
Now pull your fingers apart a tiny bit…
You have a sausage floating in front of your eyes!!
</slight hijack>
Actually, that optical illusion is caused by parallax 
Glenoled
Almost, Bob. You don’t use a foreground object to determine the distace to the further objects, you use the distant objects to determine the distace to the closer ones. Something that’s far away will appear to move very little, and if it’s far enough away, you can approximate it as moving not at all. You then use that as your reference to see how much a closer object (like the Moon) appears to move, and then use that to determine the distace to the closer object. As Ankh_Too mentioned, stars only have a measureable parallax out to about a few hundred light years, so anything further away than that is far enough to use as a reference. If you want the distance to something further away than that, you either need to improve your parallax method (with greater angular resolution or a longer baseline), or use other methods.
Ha… just like to take the opportunity to concur with friedo, your explanation was beautifuly concise. It’s kind of funny, it’s a subject that’s pretty close to me-- (I’ve been playing with 3D photography for about 10 years, and have made a fun 3D “shadow projector” that uses polarizing filters. You face a wall, and two shadows are projected so that it appears as if your shadow is “behind” the wall, reaching back to you, like in a mirror,) but I seem to be pretty inarticulate on the subject.