# Orbit of Earth and Light

1. Feb 3, 2009

### ahrog

1. The problem statement, all variables and given/known data
Ole Roemer found that the maximum increased delay in the appearance of Io (a moon of Jupiter, I believe) from one orbit to the next was 14 seconds.
a) How far does light travel in 14 seconds?
b) Each orbit of Io is 42.5 h. Earth traveled the distance calculated above in 42.5 h. Find the speed of Earth in km/s.
c) See if your answer for part b is reasonable. Calculate Earth's speed in orbit using the orbital radius, 1.5 x 10^8 km and the period, one year.

2. Relevant equations
v=d/t

3. The attempt at a solution
a) I got it to be 4.2 x 10^9 m and I'm pretty confident about that answer, so that isn't where I need help.
b) If earth traveled the same distance as in a) in 42.5 h, I'm assuming I just go V=d/t where the answer is 27451 m/s...
c) For this, I just can't think of the steps. What formula for orbital motion do I need?

2. Feb 4, 2009

### mplayer

C) I think they want you to assume the earth is moving along a perfectly circular orbit around the sun (its really slightly elliptical, but I think you can ignore that for this question). Using the given radius (distance from earth to sun), you can calculate the circumference of that circle (orbit). This is the total distance the earth travels over the given period of a year. You'll need to convert year into its equivalent amount of seconds.

Now you can use: v = d / t