Re: How many stars can fit behind a dime?

First it is necessary to estimate the number of stars one can see. There are two previous answers in our archives that are relevant (Which I found by using the Google side of our search engine to search on "how many stars", including the quotation marks.):

one

two

So let's say that on the very best viewing night in a location where you can see the sky really well you can see 4000 stars.

Now, what fraction of the sky can you hide with a dime? In mathematics we say that half the sky (The Earth gets in the way of seeing the entire sphere of the sky!) is 2*PI steradians in solid angle. (Don't get lost in this part; I'll re-connect with "reality" in a short while!) A dime is 17.91mm in diameter, and at your age "an arm's length" is roughly 400mm, so we can compute that a dime at arm's length is about 0.00157 steradians in solid angle. Dividing that by the solid angle of half the sky (2*PI) we can compute that the dime covers the fraction 0.00025 of the sky (0.025%).

Assuming that we can see 4000 stars in our part of the sky and the dime covers 0.00025 of that sky, we multiply 4000 * 0.00025 and obtain the number of stars that the dime covers: one !

This assumes a number of things:
a) That you can see 4000 stars in the sky.
b) That the stars are evenly distributed across the sky. This is only roughly true.
c) That your arm, with a dime held in your hand, is 400mm long.

Obviously in parts of the sky where the stars are a bit more crowded than average you might be able to cover more than one star, but it won't be much more than one!

John Link, MadSci Physicist