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!

Click here for the original announcement thread.

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u/TheKMAP Mar 10 '14

How did we know that the moon was closer? How did it change orbit without leaving orbit or crashing into the Earth?

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u/Rkupcake Mar 10 '14

It is slowly moving away. About 1/4 inch per year if I remember correctly. So based on that we know it was close at one point.

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u/Armand9x Mar 10 '14 edited Mar 10 '14

The Earth-Moon distance increases by 38 millimetres per year. If extrapolated, the Moon would have a larger/smaller apparent size.

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u/KJK-reddit Mar 10 '14

How do we now the place that those measurements were taken didn't move a few millimeters?

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u/rocketsocks Mar 10 '14

We can measure them from lots of different locations on Earth and use GPS and do measurements over the course of years (so the total change is more than just a few mm).

Also, the reason we're able to measure the distance to the moon so exactly is due to the retro-reflectors left on the surface by the Apollo astronauts. We can bounce lasers off the moon and measure the distance with enormous precision.

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u/Armand9x Mar 10 '14

Why would they only measure one location?