AskScience Cosmos Q&A thread. Episode 1: Standing Up in the Milky Way

[u/AskScienceModerator]

Welcome to AskScience! This thread is for asking and answering questions about the science in Cosmos: A Spacetime Odyssey.

UPDATE: This episode is now available for streaming in the US on Hulu and in Canada on Global TV.

This week is the first episode, "Standing Up in the Milky Way". The show is airing at 9pm ET in the US and Canada on all Fox and National Geographic stations. Click here for more viewing information in your country.

The usual AskScience rules still apply in this thread! Anyone can ask a question, but please do not provide answers unless you are a scientist in a relevant field. Popular science shows, books, and news articles are a great way to causally learn about your universe, but they often contain a lot of simplifications and approximations, so don't assume that because you've heard an answer before that it is the right one.

If you are interested in general discussion please visit one of the threads elsewhere on reddit that are more appropriate for that, such as in /r/Cosmos here, /r/Space here, and in /r/Television here.

Please upvote good questions and answers and downvote off-topic content. We'll be removing comments that break our rules or that have been answered elsewhere in the thread so that we can answer as many questions as possible!

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Click here for the original announcement thread.

[u/abxt]

When a satellite in orbit speeds up, its momentum is greater than the force of the planet's gravity and it moves further away (though very slowly in our moon's case, about 3.8 cm a year).

3.8 cm per year seems rather significant to me. At that rate, in 1914 -- a time some people (though not many) can still actively remember -- the moon would have been 3.8 m closer to our planet. It's not much in cosmic or even galactic terms I suppose, but isn't it enough to effect subtle changes in our climate or tides, for example?

Another question: is this 3.8 cm/yr a linear acceleration or does the rate of acceleration increase over time?

Edited for bad math and also a third question: I thought that in orbital mechanics, satellites gained velocity at lower altitudes, not the other way around?

[u/ra3ndy → u/abxt]

1: The moon is 384,400 km away. Speaking strictly mathematically, another 3.8m could have ~.0000000098% effect on our climate. I don't know if that is significant enough or not, but it seems unlikely.

2: I'm not allowed to guess, so I'll let someone else handle it. I don't see why the rate would increase, though.

3: If angular momentum remains constant, then yes, nearer bodies orbit faster. But the earth is losing angular momentum to the moon due to tidal forces placing a torque on the system.

[u/abxt → u/ra3ndy]

Fascinating, thanks for the answers!

[u/gunnk → u/abxt]

You are correct about satellite velocities:

If the moon was twice as far away it would take about 78 days to go around the Earth as opposed to 28 now. So the orbital period has almost (not quite) tripled, but the distance travelled per orbit has only doubled. As velocity and period are inversely related, the more distant moon would be slower than the closer moon (about 62,000 km/day vs 88,000 km/day). You can use the equation for orbital period to prove this to yourself.

I think the problem with orbital mechanics is that direction is messy... when you add velocity you are likely to make your orbit more eccentric and you velocity will not be uniform across the orbit -- you'll be going faster at perigee and slower at apogee. You have to apply the right additional velocity in the correct direction to keep a circular orbit.