# How High is the Sky?

Q:
How we can measure the height of the Sky?
Pakistan
A:

That's a nice question, but the answer is a little fuzzy.

If by "height of the sky" you mean height of the atmosphere, the air doesn't just stay the same density up to some height and then stop. The air density gradually gets smaller as you go up. There are many ways of measuring how that density falls off. Here's a simple one. Send up a balloon with some density and se how high it goes. It goes to the height where the air density equals the total density of the balloon.

If by "height of the sky" you mean the distance to stars and planets, each one is different. Distance to the closer stars can be measured by "stellar parallax". As the Earth goes around the Sun, that changes what direction you have to look to see a star. That's for the same reason that you have to change which way you look to see a building as you walk by. The closer you are, the more you have to change which way you look as you move. So that parallax allows measurements of distances to many stars. Measuring dIstances  to farther objects (distant galaxies) require methods that involve more complicated physics.

Mike W.

(published on 12/17/2015)

## Follow-Up #1: Why is sky blue?

Q:
First of all, thanks for your reply! When I tried to find the answer of a fascinating question, Why the sky is blue? I thought that since everything is made of atoms, so is the case with the Sky above us. And Atoms or molecules of the Sky must have reflect blue and absorb all other frequencies! So, I was wondering to know just distance between the atoms or constituents of the Sky and our Sea level, not the distance between that stars and earth!
Pakistan
A:

Actually, the sky absorbs very little visible sunlight. It does absorb a lot of infrared, and that absorption interferes more with energy leaving the Earth than it does with energy coming in as sunlight. That's what gives the famous greenhouse effect.

The blueness of the sky just comes from scattering some of the blue light, so it comes in from all directions. The reddish part of the light mostly comes straight in without scattering. That's why in the morning and evening the Sun looks reddish yellow.

You may want to have a look at our old answer:  for more on this.

Mike W.

(published on 12/20/2015)

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