4 ms·

Terrence Tao has an old excellent presentation on the “cosmic ladder” which shows how from this measurement you can build up to measure the largest distances in

by bryan0 3y ago

Terrence Tao has an old excellent presentation on the “cosmic ladder” which shows how from this measurement you can build up to measure the largest distances in the visible universe: https://terrytao.files.wordpress.com/2010/10/cosmic-distance-ladder.pdf https://terrytao.files.wordpress.com/2010/10/cosmic-distance...

082349872349872 3y ago
Wow, TIL the reason Aristarchus' contemporaries didn't accept his heliocentric model...
- nico 3y ago
  pages 156-158: “Ironically, when Aristarchus proposed the heliocentric model, his contemporaries dismissed it, on the grounds that they did not observe any parallax effects...” “ so the heliocentric model would have implied that the stars were an absurdly large distance away.” “[Which, of course, they are.]”
  - StackOverlord 3y ago
    http://homework.uoregon.edu/pub/emj/121/lectures/tycho121.html http://homework.uoregon.edu/pub/emj/121/lectures/tycho121.ht... > Tycho Brahe (1546-1601) proposed an experiment that would determine whether or not the earth goes around the sun. Basically, if the Earth orbits the sun, nearby stars should periodically "move" back and forth in their position with respect to more distant stars every 6 months. If the Earth was stationary (at the center of the Universe, this wouldn't occur.
    - DiogenesKynikos 3y ago
      Yes, but historically, scientists ended up accepting the heliocentric model long before the first stellar parallax was measured. The heliocentric model's parsimony (explaining many different phenomena, such as retrograde motion of the inner planets, with the fewest assumptions) and various supporting observations (such as Venus' phases, which rule out any strictly geocentric model), combined with a physical theory that grounded the model (Newton's laws) rendered any alternative implausible by the late 1600s at the latest. It would be nearly 200 years before parallax was first detected.
  - BurningFrog 3y ago
    Never thought of that. It is/was actually a pretty good argument.
  - Hayvok 3y ago
    I’m not familiar with the Ancient Greek objections here, but I remember Renaissance scientists being hung up on this same thing. They had measured the visible diameter of stars and without any parallax, would’ve also meant that the stars were absurdly large in addition to far away. I don’t think they had a problem with the distance, per se, but the implied size. (You have to treat stars as points - any perceived diameter is an illusion - but nobody knew that at the time.)
    - unwise-exe 3y ago
      Yeah, using basic geometry to calculate the size of a star from it's distance and the size of the dot it makes in your telescope, doesn't give sensible answers when the size of the dot is due to quantum diffraction (an Airy disc). And since that's named after a guy who lived in the 1800's, you can guess how much the ancients knew about it.
  - mcmoor 3y ago
    Knowing this makes me perplexed whenever someone treats geocentrists as idiots. Sometimes science really can get only as far as our "eyes" can see.
    - soraminazuki 3y ago
      If we had to prove which is right without access to prior wisdom, most of us would fail miserably. Our current understanding of the world can't be built in any single person's lifetime. But it's easier than ever to do your own research in the modern age. Heliocentrism vs. geocentrism is a well established topic. It doesn't take a genius to reach the right information. You can even conduct your own experiment if you want to. The only thing stopping people from learning besides poverty is their own free will. I think the reputation is well deserved.
      - pizzafeelsright 3y ago
        Try to correct people to say, "What a beautiful earth rotation" instead of "sunrise". Doing research means believing works written by old dead men who held power. Sometimes, they are proven false. Other times, they were right all along.
        soraminazuki 3y ago
        > Try to correct people to say, "What a beautiful earth rotation" instead of "sunrise". That's just figure of speech. I don't see how it's relevant. > Doing research means believing The thing about good science is that it's verifiable, no need to believe. In this case, anyone with a cheap telescope can conduct their own experiment. There's just no excuse for believing in geocentrism in 2023. As I mentioned, this is a well established scientific fact we're talking about here. People believing in geocentrism failed at multiple levels. For starters, they failed to distinguish trustworthy information from outright misinformation. They also failed to comprehend the details of the arguments being made, or haven't even bothered. On top of that, they also failed at critical thinking too. That's more than enough reason to draw conclusions about one's ability to learn. > works written by old dead men who held power People so powerful that they were persecuted for their work by the church. There are also countless people, currently alive, of various age and gender, independently verifying scientific facts and writing about it. Nobody gets their facts solely from "old dead men."
tgbugs 3y ago
Ah what a fantastic presentation. It answers a lingering question about the use of spectroscopy and apparent brightness to determine the distance to a star. The key step that I had missed was using the stars with distances measurable by parallax to calibrate the color/apparent brightness so that it could be used for stars that were beyond the distance for parallax. Obvious in hindsight, but wonderfully and clearly explained here.
labster 3y ago
The thing that stood out to me was the second rung of the ladder, the Moon. Having a large companion satellite is not a prerequisite for life (probably), so how would aliens without one bridge this gap? It seems like it would take a lot more physical theory beyond simple geometry to get there.
- defrost 3y ago
  If you can find a copy I can recommend The Tragedy of the Moon which covers such questions (from a 1973 perspective), also raising the advantages to land based life moving from the oceans with the assistance (or forcing) of the tides and tidal zones. As a sub thread bonus I've got a sneaking suspicion Terence Tao read this collection of essays about the same time he met Paul Erdős in Australia in the early 1980s. https://en.wikipedia.org/wiki/The_Tragedy_of_the_Moon https://en.wikipedia.org/wiki/The_Tragedy_of_the_Moon
- jaggederest 3y ago
  > Having a large companion satellite is not a prerequisite for life (probably) I don't know, there's a sibling comment to this one that brings up some proposed benefits to having a moon. It's possible that the foundation of life itself relied on having tidal variation of some kind, since AFAIK the equivalent of early amino acids were formed in tidal pools cut off from the main ocean and cooked under the stress of UV and evaporation. Obviously not something we'll know for sure until/unless we find other organic life, but it's interesting to think about something like having a large orbital influence being a requirement. Plug that into the fermi equation with observed planets with a moon >0.2% the size of the parent body and it has a definite damping effect on the probability of observing complex life. Edit: Also shocked I had to revise my numbers, the moon is only 1.2% of the weight of the earth. I thought it was something more like 10% - so you can probably put "tides too strong to allow life" under the other side of that fermi equation factor as well!
  - ahazred8ta 3y ago
    they may have found Darwin's 'warm little pond' - http://www.nature.com/nchem/journal/vaop/ncurrent/full/nchem.2202.html http://www.nature.com/nchem/journal/vaop/ncurrent/full/nchem... Common origins of RNA, protein and lipid precursors in a cyanosulfidic protometabolism
ljlolel 3y ago
This has changed my life thank you
ck2 3y ago
The way a few very very smart ancients figured out how things actually are in the solar system with just trivial observations and feeble measurements makes me realize that is exactly what modern astronomers are doing with the universe. Thousands of years from now someone will do a presentation on how a few of us "ancients" correctly estimated the distance to other solar systems with such poor instruments and how their modern measurements from actually going there show we were only 10% off!

by bryan0 3y ago

082349872349872 3y ago

Wow, TIL the reason Aristarchus' contemporaries didn't accept his heliocentric model...

nico 3y ago
pages 156-158: “Ironically, when Aristarchus proposed the heliocentric model, his contemporaries dismissed it, on the grounds that they did not observe any parallax effects...” “ so the heliocentric model would have implied that the stars were an absurdly large distance away.” “[Which, of course, they are.]”
- StackOverlord 3y ago
  http://homework.uoregon.edu/pub/emj/121/lectures/tycho121.html http://homework.uoregon.edu/pub/emj/121/lectures/tycho121.ht... > Tycho Brahe (1546-1601) proposed an experiment that would determine whether or not the earth goes around the sun. Basically, if the Earth orbits the sun, nearby stars should periodically "move" back and forth in their position with respect to more distant stars every 6 months. If the Earth was stationary (at the center of the Universe, this wouldn't occur.
  - DiogenesKynikos 3y ago
    Yes, but historically, scientists ended up accepting the heliocentric model long before the first stellar parallax was measured. The heliocentric model's parsimony (explaining many different phenomena, such as retrograde motion of the inner planets, with the fewest assumptions) and various supporting observations (such as Venus' phases, which rule out any strictly geocentric model), combined with a physical theory that grounded the model (Newton's laws) rendered any alternative implausible by the late 1600s at the latest. It would be nearly 200 years before parallax was first detected.
- BurningFrog 3y ago
  Never thought of that. It is/was actually a pretty good argument.
- Hayvok 3y ago
  I’m not familiar with the Ancient Greek objections here, but I remember Renaissance scientists being hung up on this same thing. They had measured the visible diameter of stars and without any parallax, would’ve also meant that the stars were absurdly large in addition to far away. I don’t think they had a problem with the distance, per se, but the implied size. (You have to treat stars as points - any perceived diameter is an illusion - but nobody knew that at the time.)
  - unwise-exe 3y ago
    Yeah, using basic geometry to calculate the size of a star from it's distance and the size of the dot it makes in your telescope, doesn't give sensible answers when the size of the dot is due to quantum diffraction (an Airy disc). And since that's named after a guy who lived in the 1800's, you can guess how much the ancients knew about it.
- mcmoor 3y ago
  Knowing this makes me perplexed whenever someone treats geocentrists as idiots. Sometimes science really can get only as far as our "eyes" can see.
  - soraminazuki 3y ago
    If we had to prove which is right without access to prior wisdom, most of us would fail miserably. Our current understanding of the world can't be built in any single person's lifetime. But it's easier than ever to do your own research in the modern age. Heliocentrism vs. geocentrism is a well established topic. It doesn't take a genius to reach the right information. You can even conduct your own experiment if you want to. The only thing stopping people from learning besides poverty is their own free will. I think the reputation is well deserved.
    - pizzafeelsright 3y ago
      Try to correct people to say, "What a beautiful earth rotation" instead of "sunrise". Doing research means believing works written by old dead men who held power. Sometimes, they are proven false. Other times, they were right all along.
      - soraminazuki 3y ago
        > Try to correct people to say, "What a beautiful earth rotation" instead of "sunrise". That's just figure of speech. I don't see how it's relevant. > Doing research means believing The thing about good science is that it's verifiable, no need to believe. In this case, anyone with a cheap telescope can conduct their own experiment. There's just no excuse for believing in geocentrism in 2023. As I mentioned, this is a well established scientific fact we're talking about here. People believing in geocentrism failed at multiple levels. For starters, they failed to distinguish trustworthy information from outright misinformation. They also failed to comprehend the details of the arguments being made, or haven't even bothered. On top of that, they also failed at critical thinking too. That's more than enough reason to draw conclusions about one's ability to learn. > works written by old dead men who held power People so powerful that they were persecuted for their work by the church. There are also countless people, currently alive, of various age and gender, independently verifying scientific facts and writing about it. Nobody gets their facts solely from "old dead men."

tgbugs 3y ago

Ah what a fantastic presentation. It answers a lingering question about the use of spectroscopy and apparent brightness to determine the distance to a star. The key step that I had missed was using the stars with distances measurable by parallax to calibrate the color/apparent brightness so that it could be used for stars that were beyond the distance for parallax. Obvious in hindsight, but wonderfully and clearly explained here.

labster 3y ago

The thing that stood out to me was the second rung of the ladder, the Moon. Having a large companion satellite is not a prerequisite for life (probably), so how would aliens without one bridge this gap? It seems like it would take a lot more physical theory beyond simple geometry to get there.

defrost 3y ago
If you can find a copy I can recommend The Tragedy of the Moon which covers such questions (from a 1973 perspective), also raising the advantages to land based life moving from the oceans with the assistance (or forcing) of the tides and tidal zones. As a sub thread bonus I've got a sneaking suspicion Terence Tao read this collection of essays about the same time he met Paul Erdős in Australia in the early 1980s. https://en.wikipedia.org/wiki/The_Tragedy_of_the_Moon https://en.wikipedia.org/wiki/The_Tragedy_of_the_Moon
jaggederest 3y ago
> Having a large companion satellite is not a prerequisite for life (probably) I don't know, there's a sibling comment to this one that brings up some proposed benefits to having a moon. It's possible that the foundation of life itself relied on having tidal variation of some kind, since AFAIK the equivalent of early amino acids were formed in tidal pools cut off from the main ocean and cooked under the stress of UV and evaporation. Obviously not something we'll know for sure until/unless we find other organic life, but it's interesting to think about something like having a large orbital influence being a requirement. Plug that into the fermi equation with observed planets with a moon >0.2% the size of the parent body and it has a definite damping effect on the probability of observing complex life. Edit: Also shocked I had to revise my numbers, the moon is only 1.2% of the weight of the earth. I thought it was something more like 10% - so you can probably put "tides too strong to allow life" under the other side of that fermi equation factor as well!
- ahazred8ta 3y ago
  they may have found Darwin's 'warm little pond' - http://www.nature.com/nchem/journal/vaop/ncurrent/full/nchem.2202.html http://www.nature.com/nchem/journal/vaop/ncurrent/full/nchem... Common origins of RNA, protein and lipid precursors in a cyanosulfidic protometabolism

ljlolel 3y ago

This has changed my life thank you

ck2 3y ago

The way a few very very smart ancients figured out how things actually are in the solar system with just trivial observations and feeble measurements makes me realize that is exactly what modern astronomers are doing with the universe. Thousands of years from now someone will do a presentation on how a few of us "ancients" correctly estimated the distance to other solar systems with such poor instruments and how their modern measurements from actually going there show we were only 10% off!