5 ms·

> The phenomenon of precession — in which Earth slowly wobbles on its axis by around one degree every 72 years — means that the position of the fixed stars slow

> The phenomenon of precession — in which Earth slowly wobbles on its axis by around one degree every 72 years — means that the position of the fixed stars slowly shifts in the sky. The researchers were able to use this to check when the ancient astronomer must have made his observations, and found that the coordinates fit roughly 129 bc — during the time when Hipparchus was working. That's amazing, using the discovered results themselves to step backwards and find out when the observations were made.

dylan604 4y ago
Back in the 90s, there was a documentary* on the pyramids at Giza that took the wobble into consideration. There are holes that in modern times do not appear to have much significance, but winding the clock back && adjusting for the wobble aligns said holes with significant stars. There's a foggy memory that this theory might have been debunked, but it was the first time I had heard of the wobble being used in such a manner. *I have no recollection of a title. Just recall watching it on VHS. One of those nights at Blockbuster when all of the "good" movies were unavailable so went to find something else from the middle of the floor
- ljf 4y ago
  Not the documentary but: https://en.wikipedia.org/wiki/Star_shaft https://en.wikipedia.org/wiki/Star_shaft And related: https://en.wikipedia.org/wiki/Orion_correlation_theory https://en.wikipedia.org/wiki/Orion_correlation_theory
  - dylan604 4y ago
    Yes, I should have remembered it being Orion. Just seeing your link reminded me how their theory said the pyramids were positioned along the Nile similar to how Orion is positioned to the Milky Way. Now I'm going to read the wiki link to see how my memory holds up. Edit: yup yup. The wiki link gave the name of the documentary, and of course it's on youtube: https://www.youtube.com/watch?v=cfgRF1e66_8 https://www.youtube.com/watch?v=cfgRF1e66_8
    - schrijver 4y ago
      My history teacher made us watch this in high school when it just came out! She was very into it, at the point where the scholar who works for the Egyptian state starts voicing his criticisms she told us this was the response of a man afraid to loose his power :)
      - dylan604 4y ago
        Yeah, all of the naysayers were a bit off putting in a weird way to me about this. I understand it's a "new" idea (at the time), but it just seemed so dismissive. It's very compelling information, and a very interesting idea. The arguments against were not nearly as compelling to me.
mikeyouse 4y ago
It’s a slightly different take on it but there’s a popular TikTok account (and I assume a related community) where people find old globes and then try to pinpoint when they were printed based on the labeling of disputed territories, renamed countries, redrawn lines etc. Its fascinating how specific they can get. https://www.tiktok.com/@jakie62/video/7140007175476235562 https://www.tiktok.com/@jakie62/video/7140007175476235562
- JoshTriplett 4y ago
  https://xkcd.com/1688/ https://xkcd.com/1688/
  - mikeyouse 4y ago
    Of course there's an XKCD for that. Amazing, thanks for sharing.
- jamesmaniscalco 4y ago
  This sounds like a kind of historical GeoGuessr.
- HeyLaughingBoy 4y ago
  I remember back in the early 90's seeing for the first time a post-WW2 map that just said "Germany." Not East Germany and West Germany, but just Germany. Stood and stared at it for while with a weird mix of emotions.
antognini 4y ago
Somewhat related, but the Earth's rate of rotation changes somewhat randomly over the centuries due to tidal friction, tectonic motion, and climate change. These random perturbations add up, so over the course of a few thousand years, the rotation angle of the Earth can be uncertain by up to 70 degrees or so. But because total solar eclipses occur at times that we can calculate and are only visible from specific locations on the Earth, geologists have been able to use ancient accounts of solar eclipses to constrain the long-term change in the Earth's rotation rate.
- pfortuny 4y ago
  Just a question: 70 degrees in just several thousand years? Is that a mistake?
  - antognini 4y ago
    Those were the numbers I saw when I was looking into this a few weeks back. If you think about it, a few millennia corresponds to ~1,000,000 days. So to know the rotation angle to within 10 30 degrees or so, you need to know the length of the day to better than one part in 10^7. But each year the length of the day changes by a few tenths of a millisecond, which is about 3 x 10^-8 of the length of a day. So you have to know the change in the length of the day reasonably well not only today, but at all times going back 2000 years. Those few tenths of a millisecond add up over thousands of years!
    - pfortuny 4y ago
      Ah, I see where your number comes from. However, those deviations should happen in both senses (adding and subtracting)? But thanks for the computations!
      - ls612 4y ago
        They do but in the long run because of the tidal forces of the moon days become inexorably longer.
doctoboggan 4y ago
The precession of the earths axis is around 26,000 years for 1 cycle. What's incredible is that the ancient babylonians were able to detect and measure this at the time.

dylan604 4y ago

Back in the 90s, there was a documentary* on the pyramids at Giza that took the wobble into consideration. There are holes that in modern times do not appear to have much significance, but winding the clock back && adjusting for the wobble aligns said holes with significant stars. There's a foggy memory that this theory might have been debunked, but it was the first time I had heard of the wobble being used in such a manner. *I have no recollection of a title. Just recall watching it on VHS. One of those nights at Blockbuster when all of the "good" movies were unavailable so went to find something else from the middle of the floor

ljf 4y ago
Not the documentary but: https://en.wikipedia.org/wiki/Star_shaft https://en.wikipedia.org/wiki/Star_shaft And related: https://en.wikipedia.org/wiki/Orion_correlation_theory https://en.wikipedia.org/wiki/Orion_correlation_theory
- dylan604 4y ago
  Yes, I should have remembered it being Orion. Just seeing your link reminded me how their theory said the pyramids were positioned along the Nile similar to how Orion is positioned to the Milky Way. Now I'm going to read the wiki link to see how my memory holds up. Edit: yup yup. The wiki link gave the name of the documentary, and of course it's on youtube: https://www.youtube.com/watch?v=cfgRF1e66_8 https://www.youtube.com/watch?v=cfgRF1e66_8
  - schrijver 4y ago
    My history teacher made us watch this in high school when it just came out! She was very into it, at the point where the scholar who works for the Egyptian state starts voicing his criticisms she told us this was the response of a man afraid to loose his power :)
    - dylan604 4y ago
      Yeah, all of the naysayers were a bit off putting in a weird way to me about this. I understand it's a "new" idea (at the time), but it just seemed so dismissive. It's very compelling information, and a very interesting idea. The arguments against were not nearly as compelling to me.

mikeyouse 4y ago

It’s a slightly different take on it but there’s a popular TikTok account (and I assume a related community) where people find old globes and then try to pinpoint when they were printed based on the labeling of disputed territories, renamed countries, redrawn lines etc. Its fascinating how specific they can get. https://www.tiktok.com/@jakie62/video/7140007175476235562 https://www.tiktok.com/@jakie62/video/7140007175476235562

JoshTriplett 4y ago
https://xkcd.com/1688/ https://xkcd.com/1688/
- mikeyouse 4y ago
  Of course there's an XKCD for that. Amazing, thanks for sharing.
jamesmaniscalco 4y ago
This sounds like a kind of historical GeoGuessr.
HeyLaughingBoy 4y ago
I remember back in the early 90's seeing for the first time a post-WW2 map that just said "Germany." Not East Germany and West Germany, but just Germany. Stood and stared at it for while with a weird mix of emotions.

antognini 4y ago

Somewhat related, but the Earth's rate of rotation changes somewhat randomly over the centuries due to tidal friction, tectonic motion, and climate change. These random perturbations add up, so over the course of a few thousand years, the rotation angle of the Earth can be uncertain by up to 70 degrees or so. But because total solar eclipses occur at times that we can calculate and are only visible from specific locations on the Earth, geologists have been able to use ancient accounts of solar eclipses to constrain the long-term change in the Earth's rotation rate.

pfortuny 4y ago
Just a question: 70 degrees in just several thousand years? Is that a mistake?
- antognini 4y ago
  Those were the numbers I saw when I was looking into this a few weeks back. If you think about it, a few millennia corresponds to ~1,000,000 days. So to know the rotation angle to within 10 30 degrees or so, you need to know the length of the day to better than one part in 10^7. But each year the length of the day changes by a few tenths of a millisecond, which is about 3 x 10^-8 of the length of a day. So you have to know the change in the length of the day reasonably well not only today, but at all times going back 2000 years. Those few tenths of a millisecond add up over thousands of years!
  - pfortuny 4y ago
    Ah, I see where your number comes from. However, those deviations should happen in both senses (adding and subtracting)? But thanks for the computations!
    - ls612 4y ago
      They do but in the long run because of the tidal forces of the moon days become inexorably longer.

doctoboggan 4y ago

The precession of the earths axis is around 26,000 years for 1 cycle. What's incredible is that the ancient babylonians were able to detect and measure this at the time.