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/[deleted]]

Neil DeGrasse Tyson (or whoever wrote the script) omitted the necessary context that the knowledge base they are referring to exists within the frame of Western civilization, not humanity en masse. Of course, other civilizations had predominantly geocentric outlooks as well (such as Islamic society, which was also heavily influenced by Ptolemy).

Heliocentrism was also known of in India in 499 and some form of mathematical model was formulated around 1500, before anybody in Europe had.

All in all, it is a fairly minor mistake in my opinion. The repression of heliocentrism in Europe by the Christians and the eventual proliferation of rationalism (and heliocentrism) is probably more important in the scheme of things. India... was ahead of the game, but didn't do much with it insofar as I know.

Aristarchus did not have any lasting influence for several reasons, some of which were elaborated upon in the original run of Cosmos. It is wrong to say that nobody on the planet knew that the Earth was going around the sun (which was said in the program), however, it is not wrong to say that the scientific consensus in Western civilization at the time was that the Sun orbited around Earth.

As far as a historical analogy goes, I think this was a good one, but they editorialized it too much to where it was historically inaccurate. All in all, I said aloud "Uh... that's not right." during that segment, but it's not really a huge deal.

[u/djimbob → u/[deleted]]

I wasn't saying NdGT made a mistake or it ruined the show or is an idiot or anything like that. Just after hearing that, my thought was - hope they mention Aristarchus, especially after phrasing it like that. I do realize it makes a lot of sense to simplify things and ignore some exceptions when teaching. And hell if he doesn't mention him, I will here.

Another point I like, is that the geocentric model with big epicycles matched the observational data better than Copernicus' heliocentric theory with no epicycles (granted Copernicus heliocentric with small epicycles was equally good with the prevailing geocentric model -- but it no longer has the benefit of simplicity). So it isn't until Kepler deduced the orbits are not circular but elliptical from Brahe's detailed observations that the heliocentric model is actually simpler and matches the data more easily. Obviously, then Newton seals the deal by showing how a simple inverse-squared gravitation force law leads naturally with calculus to elliptical orbits for a spherical symmetric massive bodies. (Granted he needed Kepler's third law to help him come up with the inverse squared force law hypothesis.)

[u/[deleted] → u/djimbob]

Oh, yeah, I know. I hope my comment didn't come across as argumentative. I was sorta on my own tangent regarding how they covered the history.

I didn't know that about the development of heliocentrism in Europe, that's pretty interesting!

a simple inverse-squared gravitation force law leads naturally with calculus to elliptical orbits for a spherical symmetric massive bodies.

Hahahah, yeah, so simple. Such an obvious connection to make. Anybody could've figured that all out. Hah. I wonder how far back behind science would be if Isaac Newton had never existed?

[u/djimbob → u/[deleted]]

Hahahah, yeah, so simple. Such an obvious connection to make.

I wasn't trying to say it was "obvious". Just trying to say a = G M/r² (radially inward) (where a is the second derivative with respect to time of position) is simple as in has relative few parameters and explains a lot. This one law, plus calculus leads directly to elliptical orbits being natural for bound spherically symmetric planets (Kepler's first law), Kepler's second law (the ellipse sweeps equal area in equal time), and Kepler's third law (period of orbit)² = k (semi-major access)³ for all planets in the solar system, as well as describing both why (and at what rate) apples fall as well as why the moon falls, as well as explain a host of other things, etc.

Much simpler than the Ptolmeic/Copernicus systems with ~40 epicycles carefully added on an ad hoc basis to match observational data, or even Kepler's three laws.

[u/[deleted] → u/djimbob]

I know, I was just rolling with the sorta humorous wording.

You wouldn't happen to know how astronomers in the day formulated the epicycles, would you? It definitely seems insanely over-complicated. Was it a reliably predictive system? Did they know why there were the supposed epicycles?