Calculating The Speed Of A Moon?

[Beale]

And this is meant to run on a mobile? Very nice. I've done my share of programming in my life but I've never wrote code for mobile, and frankly I've been wanting to learn. Do you think you could share your code? Just for personal, educational use (for me)?

I'd be happy to! I'll send a PM.

Fun fact: the Sun exerts twice as much gravitational force on the Moon as the Earth does. Go ahead and compute it if you want to check. It's the only natural satellite in the solar system for which that's true.

Hence, some people pushing to re-classify Earth-Moon system as a double planet.

I didn't know that. For this reason would it be difficult for Venus or Mercury to have a Moon?

[Meithan]

I'd be happy to! I'll send a PM.

Thanks for the PM!

I didn't know that. For this reason would it be difficult for Venus or Mercury to have a Moon?

Nope, they can still have moons in principle. It's a bit counterintuitive. Why does the Moon stick around the Earth if the Sun's pull is twice as big as the Earth's? The explanation has to do with calculating the resulting accelerations in the correct frame of reference, but in a nutshell it's what defines the Hill sphere: if a body is inside the Hill sphere of a planet (and doesn't have escape velocity), it will remain in orbit of that planet even if the Sun has a stronger gravitational pull.

But even though they could in principle have moons (Venus' Hill sphere is comparable to that of Earth), proximity to the Sun makes capture events less likely (since bodies move faster closer to the Sun) and moons formed in-system when the planets formed will tend to have unstable orbits over long periods of time due to the Sun's perturbations. There's also the Roche limit: if a moon orbits too close t the planet, it will get destroyed by tidal forces. All these facts conspire to make moons of Mercury or Venus unlikely.

[Beale]

Nope, they can still have moons in principle. It's a bit counterintuitive. Why does the Moon stick around the Earth if the Sun's pull is twice as big as the Earth's? The explanation has to do with calculating the resulting accelerations in the correct frame of reference, but in a nutshell it's what defines the Hill sphere: if a body is inside the Hill sphere of a planet (and doesn't have escape velocity), it will remain in orbit of that planet even if the Sun has a stronger gravitational pull.

But even though they could in principle have moons (Venus' Hill sphere is comparable to that of Earth), proximity to the Sun makes capture events less likely (since bodies move faster closer to the Sun) and moons formed in-system when the planets formed will tend to have unstable orbits over long periods of time due to the Sun's perturbations. There's also the Roche limit: if a moon orbits too close t the planet, it will get destroyed by tidal forces. All these facts conspire to make moons of Mercury or Venus unlikely.

Aaaah, thank you for the explanation, that makes a lot of sense.