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I wonder why Prof. Strassler does not consider star trails, like here, https://en.wikipedia.org/wiki/Star_trail https://en.wikipedia.org/wiki/Star_trail definit

by nyc111 5y ago

I wonder why Prof. Strassler does not consider star trails, like here, https://en.wikipedia.org/wiki/Star_trail https://en.wikipedia.org/wiki/Star_trail definitive proof of Earth's rotation. In historic times astronomers invented crystalline spheres [1] that carried stars to save the stationary earth doctrine. But now we know that crystalline spheres do not exist (stars have their own proper motions) so since stars do not rotate, the earth must rotate. What's wrong with this argument? [1] https://en.wikipedia.org/wiki/Celestial_spheres https://en.wikipedia.org/wiki/Celestial_spheres

KineticLensman 5y ago
> In historic times astronomers invented crystalline spheres [1] that carried stars to save the stationary earth doctrine. My understanding is that the ancients used the celestial spheres to explain the movements of the planets around the Earth - the spheres themselves moved relative to each other hence the different speeds and courses of the planets. The spheres could be made to work regardless of whether the Earth spins or not. If the Earth is static, then the spheres can still use their relative motions to explain the motion of the planets. In contrast, if the Earth is spinning, you add its spin to that of the spheres and an observer on Earth couldn't tell the difference.
NineStarPoint 5y ago
What's wrong with your argument is it assumes facts that themselves need to be proven. "Stars have their own proper motions" is not an easy fact to prove, and it took a very long time for people to prove that "stars do not rotate" (I presume you mean around the earth). The sort of thing being discussed here is the sort of thing you need to do to prove that crystalline spheres do not exist. There are of course other ways to prove that crystalline spheres do not exist, but they are things like the existence of black holes or the speed of light. And in general, all these sorts of things require a much broader knowledge base/trust in our current understanding to prove the idea that the earth rotates. This article is saying that if you can prove how a gyroscope works (itself requiring proof of the Conservation of Angular Momentum), you can show that the earth rotates. Which is ultimately a much simpler thing to prove than things about the nature of the stars themselves.
- nyc111 5y ago
  We know stars are distributed in various distances, this we know by observation. If not the earth but the stars rotate we must assume that billions of crystal spheres at different distances somehow rotate in exact coordination to trace the star trails that we observe. I don’t think there is such a huge “conspiracy” of crystal spheres to fool us.
  - codethief 5y ago
    > We know stars are distributed in various distances, this we know by observation. How do we know? Establishing the cosmic distance ladder requires far more elaborate tools than the ones discussed here (Special and General Relativity, for instance) and the necessary measurements and observations also presume in one way or another that Earth is rotating. Even if you're only talking about stars in our immediate neighborhood for which a distance measurement merely needs the parallax effect (instead of SR or GR): In this case the parallax effects literally rests on the rotation of Earth and its orbit around the sun.
    - nyc111 5y ago
      > the necessary measurements and observations also presume in one way or another that Earth is rotating. Yes, the lack of star parallaxes were the main argument offered by Tycho Brahe against a moving earth theory. When star parallaxes were observed the motion of earth was accepted. The way I understand it, to measure star parallaxes the earth must move. This is not an assumption, there is no circular reasoning. And we observe different parallaxes for different stars, meaning that stars are not all on the same distance from earth. I don't know if this is a trend like flat earthers to say that all stars are the same distance from the center of the universe? > In this case the parallax effects literally rests on the rotation of Earth and its orbit around the sun. So, this way you prove that the earth moves and stars have different parallaxes and are not all attached to a crystal sphere. I don't see a problem here.
      - sandworm101 5y ago
marcosdumay 5y ago
The author also seems think Foucault pendulums aren't conclusive. That one is much harder to argue against than star trails (there's no need to prove the absence of celestial spheres or other kinds of star movement). Honestly, I don't see much difference between a gyroscope experiment and the Foucault pendulum. Except that the later is much easier to do.
- dwohnitmok 5y ago
  As the article notes, the Foucault pendulum is slightly more work to prove. Nothing beyond high school math, but it's less obvious than "once you point this gyroscope at any star, it keeps pointing at that star no matter how that star moves, how could that be if the stars are moving but not the earth?"
  - sandworm101 5y ago
    >> how could that be if the stars are moving but not the earth? Because maybe the earth is fixed, the heavens all move, and the pendulum is reacting to the heavens. Maybe a tidal force from the stellar spheres is imparting a slight motion on the pendulum. Maybe only objects like gyroscopes are sensitive enough to feel this force.
    - dwohnitmok 5y ago
      Oh it's definitely possible to have alternative theories but they get increasingly outlandish as you make small modifications. E.g. "how does a gyroscope know which star to point at? What happens if the gyroscope points at the middle between two stars? What happens if you point it at a star that has not yet been discovered (e.g. more powerful telescopes reveal it in the future)? Do different stars have different effects?" Trying to answer these questions makes the stellar spheres theory require more and more ad-hoc additions to support.
cuteboy19 5y ago
Newtonian mechanics does not have absolute speed. We can only say A is moving at 5m/s relative to B. But Newton does have absolute acceleration. So we can say A is accelerating at 5m/s² without even thinking of B. Since a rotating body accelerates, we should be able to say that the earth rotates without even thinking of the sky. The proof does exactly this. Looking at a gyroscope is sufficient to prove that the earth rotates. No need to look at the sky
- nyc111 5y ago
  But the gyroscope looks at the sky, it points at the same star, right? This is how the proof works.
  - cuteboy19 5y ago
    Imagine that you live in a starless void. Could you still prove that the earth spins? Yes, simply point the gyroscope towards an arbitrary direction and repeat the same process as in the proof. If the earth doesn't spin, then the gyroscope would point at the same direction always. But if it does spin then it would show those circles.
moffkalast 5y ago
It doesn't account for the option that the Earth is stationary and the sky is spinning, much it like it would be implemented in a game or CGI scenes for example. Gyroscopes can rule this out as well since they're showing rotational change directly.
allturtles 5y ago
In this post and your subsequent replies you rely on the existing astronomical body of knowledge to show that the Earth spins. This misses the point of the OP, which is to try to provide a simple physical demonstration that anyone could perform or witness which shows that the Earth spins, in particular one that's more intuitive than Foucault's pendulum.

by nyc111 5y ago

KineticLensman 5y ago

> In historic times astronomers invented crystalline spheres [1] that carried stars to save the stationary earth doctrine. My understanding is that the ancients used the celestial spheres to explain the movements of the planets around the Earth - the spheres themselves moved relative to each other hence the different speeds and courses of the planets. The spheres could be made to work regardless of whether the Earth spins or not. If the Earth is static, then the spheres can still use their relative motions to explain the motion of the planets. In contrast, if the Earth is spinning, you add its spin to that of the spheres and an observer on Earth couldn't tell the difference.

NineStarPoint 5y ago

What's wrong with your argument is it assumes facts that themselves need to be proven. "Stars have their own proper motions" is not an easy fact to prove, and it took a very long time for people to prove that "stars do not rotate" (I presume you mean around the earth). The sort of thing being discussed here is the sort of thing you need to do to prove that crystalline spheres do not exist. There are of course other ways to prove that crystalline spheres do not exist, but they are things like the existence of black holes or the speed of light. And in general, all these sorts of things require a much broader knowledge base/trust in our current understanding to prove the idea that the earth rotates. This article is saying that if you can prove how a gyroscope works (itself requiring proof of the Conservation of Angular Momentum), you can show that the earth rotates. Which is ultimately a much simpler thing to prove than things about the nature of the stars themselves.

nyc111 5y ago
We know stars are distributed in various distances, this we know by observation. If not the earth but the stars rotate we must assume that billions of crystal spheres at different distances somehow rotate in exact coordination to trace the star trails that we observe. I don’t think there is such a huge “conspiracy” of crystal spheres to fool us.
- codethief 5y ago
  > We know stars are distributed in various distances, this we know by observation. How do we know? Establishing the cosmic distance ladder requires far more elaborate tools than the ones discussed here (Special and General Relativity, for instance) and the necessary measurements and observations also presume in one way or another that Earth is rotating. Even if you're only talking about stars in our immediate neighborhood for which a distance measurement merely needs the parallax effect (instead of SR or GR): In this case the parallax effects literally rests on the rotation of Earth and its orbit around the sun.
  - nyc111 5y ago
    > the necessary measurements and observations also presume in one way or another that Earth is rotating. Yes, the lack of star parallaxes were the main argument offered by Tycho Brahe against a moving earth theory. When star parallaxes were observed the motion of earth was accepted. The way I understand it, to measure star parallaxes the earth must move. This is not an assumption, there is no circular reasoning. And we observe different parallaxes for different stars, meaning that stars are not all on the same distance from earth. I don't know if this is a trend like flat earthers to say that all stars are the same distance from the center of the universe? > In this case the parallax effects literally rests on the rotation of Earth and its orbit around the sun. So, this way you prove that the earth moves and stars have different parallaxes and are not all attached to a crystal sphere. I don't see a problem here.
    - sandworm101 5y ago

marcosdumay 5y ago

The author also seems think Foucault pendulums aren't conclusive. That one is much harder to argue against than star trails (there's no need to prove the absence of celestial spheres or other kinds of star movement). Honestly, I don't see much difference between a gyroscope experiment and the Foucault pendulum. Except that the later is much easier to do.

dwohnitmok 5y ago
As the article notes, the Foucault pendulum is slightly more work to prove. Nothing beyond high school math, but it's less obvious than "once you point this gyroscope at any star, it keeps pointing at that star no matter how that star moves, how could that be if the stars are moving but not the earth?"
- sandworm101 5y ago
  >> how could that be if the stars are moving but not the earth? Because maybe the earth is fixed, the heavens all move, and the pendulum is reacting to the heavens. Maybe a tidal force from the stellar spheres is imparting a slight motion on the pendulum. Maybe only objects like gyroscopes are sensitive enough to feel this force.
  - dwohnitmok 5y ago
    Oh it's definitely possible to have alternative theories but they get increasingly outlandish as you make small modifications. E.g. "how does a gyroscope know which star to point at? What happens if the gyroscope points at the middle between two stars? What happens if you point it at a star that has not yet been discovered (e.g. more powerful telescopes reveal it in the future)? Do different stars have different effects?" Trying to answer these questions makes the stellar spheres theory require more and more ad-hoc additions to support.

cuteboy19 5y ago

Newtonian mechanics does not have absolute speed. We can only say A is moving at 5m/s relative to B. But Newton does have absolute acceleration. So we can say A is accelerating at 5m/s² without even thinking of B. Since a rotating body accelerates, we should be able to say that the earth rotates without even thinking of the sky. The proof does exactly this. Looking at a gyroscope is sufficient to prove that the earth rotates. No need to look at the sky

nyc111 5y ago
But the gyroscope looks at the sky, it points at the same star, right? This is how the proof works.
- cuteboy19 5y ago
  Imagine that you live in a starless void. Could you still prove that the earth spins? Yes, simply point the gyroscope towards an arbitrary direction and repeat the same process as in the proof. If the earth doesn't spin, then the gyroscope would point at the same direction always. But if it does spin then it would show those circles.

moffkalast 5y ago

It doesn't account for the option that the Earth is stationary and the sky is spinning, much it like it would be implemented in a game or CGI scenes for example. Gyroscopes can rule this out as well since they're showing rotational change directly.

allturtles 5y ago

In this post and your subsequent replies you rely on the existing astronomical body of knowledge to show that the Earth spins. This misses the point of the OP, which is to try to provide a simple physical demonstration that anyone could perform or witness which shows that the Earth spins, in particular one that's more intuitive than Foucault's pendulum.