# Kepler's Third Law Problem

#### MCFISHER

I have been stuck on this question for hours and have even tried doing it backwards by finding the answer and working backwards to find out what the radius is supposed to be but I still couldn't understand how to solve it. If someone could help me I would hugely appreciate it.
Use Kepler's third law to determine how many days it takes a spacecraft to travel in an elliptical orbit from a point 6 647 km from the Earth's center to the Moon, 385 000 km from the Earth's center.
I know the formula for keplers third law and that the radius or a in this case is supposed to be the semi-major axis, I just have failed to figure out how to get the semi-major axis.

#### topsquark

Forum Staff
I have been stuck on this question for hours and have even tried doing it backwards by finding the answer and working backwards to find out what the radius is supposed to be but I still couldn't understand how to solve it. If someone could help me I would hugely appreciate it.
Use Kepler's third law to determine how many days it takes a spacecraft to travel in an elliptical orbit from a point 6 647 km from the Earth's center to the Moon, 385 000 km from the Earth's center.
I know the formula for keplers third law and that the radius or a in this case is supposed to be the semi-major axis, I just have failed to figure out how to get the semi-major axis.
I'm not fond of the wording here, but if I'm reading it right 38500 km - 6647 km = 2a.

-Dan