# Orbital Radius of a Jovian Moon

• A
Forcino
TL;DR Summary
How did the first scientists figure out the orbital radius of a moon of Jupiter? How can observations lead to a calculation of the orbital radius of a moon of another planet?
How did the first scientists figure out the orbital radius of a moon of Jupiter? How can observations lead to a calculation of the orbital radius of a moon of another planet?

Forcino
Thank you.

So would you find the orbital radius of a Jovian moon by using a telescope to view Jupiter at opposition during a time when the moon is farthest from Jupiter? Then you could find the arcseconds of angle from the center of Jupiter to the center of the moon? Then you could use the equation D = θ d / 206,265? Then D would be the orbital radius of the moon?

Gold Member
Yeah, although:
1. it doesn't really have to be at opposition
2. you'd need to measure the angular distance of the moon over several/many orbits to get a good estimate of the maximum orbital radius.

• Drakkith
snorkack
How did the scientists figure out distance to Jupiter back in 17th century? Römer measured speed of light by 1676, but he got the speed about 30% too slow. How much of it was the error in travel time, how much the error in distance to Jupiter?
How much was Römer´s distance in 1676 better than Galilei´s distance, if any, in 1610?