# Thread: Need help with allegedly sneaky question

1. ## Need help with allegedly sneaky question

Hi,

I was given this problem by my teacher: "It's about 1.496x10^8 km from the Sun to the Earth, and about 384 000 km from the Earth to the Moon. So when there is an eclipse of the moon, how far is the Moon from the Sun?" Before this he says this is a sneaky one, but it seems simple. If the moon is eclipsed, it's behind the earth, so all I need to do is add the length between the Moon and the Earth with the distance between the Earth and the Sun. Is there something I'm missing?

Thanks!

2. ## Re: Need help with allegedly sneaky question

You need to add on the diameter of the earth too

3. ## Re: Need help with allegedly sneaky question

we generally should consider distances from the center of the spherical objects. Add the radii / diameter accordingly.

4. ## Re: Need help with allegedly sneaky question

I think the sneakiness of the question comes from rounding the result correctly. What subject and topic is this a question from: physics, algebra, scientific notation, floating point? Also, Kevin, what is your answer?

5. ## Re: Need help with allegedly sneaky question

I think you're right, this question was on a page with other question about significant digits. I suppose my answer would be rounded to 3 significant digits, as additions/subtractions are rounded to the least precise number (in this case there isn't anything to the right of the decimal) and the least accurate number is 3 significant digits.