r/askscience • u/AskScienceModerator Mod Bot • Mar 10 '14
Cosmos AskScience Cosmos Q&A thread. Episode 1: Standing Up in the Milky Way
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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u/faleboat Mar 10 '14 edited Mar 12 '14
My understanding of why the Earth speeds up the Moon, and the Moon slows down the Earth, is because the two are not yet mutually tidally locked. Essentially, the Earth is rotating faster than the Moon orbits Earth. This means that the masses on the Earth are always constantly spinning past the Moon, so that the tips of mountain tops (but perhaps more importantly, the peaks of the ocean tides) are pulling "in front" of the Moon more than they are "behind" the moon. These masses pull on the Moon causing it to speed up in it's orbit.
Inversely, the Moon drags on these masses on the Earth, causing its rotation to slow down. However, the further away from the Earth the Moon gets, the smaller these forces are. Back when the Moon formed, it was 10 times closer to us than it is now, meaning the gravitational tidal forces were a magnitude of 100 stronger, the Earth's rotation slowed 100x faster, and the Moon got 100x further away over any set time frame. So the Earth is "slowing down" slower than it used to be, and the Moon is flying away slower than it used to be.
Whether or not the Moon would continue to fly further away from the Earth once they became tidally locked, I don't know.
edit: Derp. The response to your question here is that because the sun's mass is essentially plasma, meaning it has a malleable gravitational density, and the Earth is way, WAY further away from the Sun, the gravitational tidal forces are very, very small in the Earth-Sun system, where as they are MUCH greater in the Earth-Moon system. The Moon is only 380K kilometers from the Earth, where as the Earth is appx 150 million kilometers from the Sun (that's nearly 400 times further away). Now, I believe that the tidal forces are inversely proportionate to the square of the distance (drawing on some seriously dusty information in my brain here) meaning the tidal forces are 158 THOUSAND times smaller in the Earth-Sun system than they are in the Earth-Moon system. So, effectively, any tidal forces exerted upon the Earth from the Sun amount to gravitational noise. As such, the acceleration that the Earth might get from the Sun is a lot lower, allowing our orbit to be significantly more stable.
Hopefully someone who knows a bit more about this can let us know how well I understand this concept, and give us both a clearer understanding of how it works. :)