The article mentions the cosmic distance ladder, which is one of my favorite things in all of science. How do we know how far away the really far stuff is? It's non-trivial and I find the history fascinating.
It all started with knowing the distance from the earth to the sun. Nobody had a clue until Richer and Cassini got within 10% in 1672. Then we nailed it down in 1769 with James Cook's voyage to Tahiti, the primary purpose of which was to observe the transit of Venus from the other side of the world.
From there if you know basic geometry, you can observe the nearby stars shift a bit when the earth goes around the sun (parallax), but that only works to about 10k light years.
Then, we discovered a couple unbelievably convenient astrophysics hacks: Cepheid variables (Henrietta Swan Leavitt, 1908) and Type 1A supernovae (Subrahmanyan Chandrasekhar, 1935, the namesake of the Chandra X-Ray Observatory). These allowed us to move out a couple more rungs on the ladder.
From there, the relationship between redshift and distance becomes significant and that takes us to the edge.
In the 3rd century BC, Aristarchus calculated that the Sun was between 18 and 20 times farther away from the Earth than the Moon, and proposed the Heliocentric model as a result. The true value is instead approximately 400 times. But it's incredible given that he didn't have lenses, the value of Pi, and that the Geocentric model was considered correct until 1800 years after his death.
Yes. The fact that even this was a thought and he was charting the space bodies and trying to establish distances between them. The rules were invented. The players just were not famous.
His method was fundamentally sound though. He realized that moonlight is reflected sunlight, and that if you observe the angle between the sun and the moon in the sky at exactly half moon, you can calculate the distance to the sun relative to the distance to the moon.
I appreciate that he worked on it at least. Around the same era, someone else calculated the circumference of the earth (and that it was round) in a pretty accurate fashion (between −2.4% and +0.8% off) based on measuring shadows on equally sized posts at different locations on the same date. Googled, it was Eratosthenes, the cities were Alexandria and Syene/Assuan.
He almost certainly knew that it was closer to 400x, but could not believe it, or did not think others would believe it. Given the size of the Earth (which he knew), that would have made the distance to and the size of the Sun something hard to stomach.
Can we call it an estimation if it's a range (18 to 20 times further than the Moon) and yet it is incorrect?
If he said "between 2 and 1'000'000 times farther than Moon", it would be very imprecise, but not incorrect. If he said "20 times farther" - it would be an extremely inaccurate estimate.
> Then we nailed it down in 1769 with James Cook's voyage to Tahiti, the primary purpose of which was to observe the transit of Venus from the other side of the world.
He was captaining the Endeavour for that journey! Which is what that space shuttle orbiter was named after. And it's the reason why the shuttle uses that British spelling. It was also the source name for the command module of Apollo 15. And, the Crew Dragon (SpaceX) that got to the ISS last week is named Endeavour (after the shuttle orbiter). The shuttle is at the California Science Center and they recently "stacked" it along with the external tank and boosters. (and it means a few years before accessible again). It was way cooler when you could walk right under it.
When Hubble launched, an error in the production of its mirror ruined its vision.
It was Endeavour on mission STS-61 in 1993 that corrected it.
Great comment! Maybe you can help me with a book recommendation?
I was recently looking for a book which was basically your comment, but more in depth and covered the last couple thousand years. I wanted a to read about the history of astronomy - yknow, what was the state of the art in, say, 1350 or whatever. If you know of anything, I’d be super interested!
Unfortunately I don't have any books to recommend. I don't remember where I learned about Cepheid variables and type 1a supernovae (maybe science shows, maybe youtube, ...) but I learned about the transit of Venus stuff on a big Wikipedia rabbit hole one evening.
I think the pre-quantum mechanics era for physics and astronomy is super interesting. People figured out so much with such primitive tools, and it's all very accessible and easy to understand.
Cosmos by Carl Sagan covers similar stories including the history of astronomy and the mathematics invented to explain it - it is like a whistle stop tour of these subjects and more + ties them together conceptually.
You might be interested in "Unrewarded" by "Ben Moore" which has an interesting take by telling the history of astronomy through the lives of those that made these discoveries but were not awarded a Nobel Prize.
I've often thought about this myself. I'm sure scientists involved are aware of the compounding errors with each step and build that in, but I'd love to see an analysis that breaks that down. When I first saw it I thought the errors due to cephids must be a large component of uncertainty, but really I've no idea how well contained that is.
Error analysis of cosmic distance ladder is fiercely technical subject. https://arxiv.org/abs/1103.2976 Table 5 is titled "H0 Error Budget for Cepheid and SN Ia Distance Ladders". (It is old and the field is moving fast, but this is what I happened to remember.)
Among total error of 3.1%, Cepheid reddening is 1.4% and the second largest source of uncertainty. SN Ia statistics is the largest with 1.9%. Rarely discussed in popular treatment is anchor distance, the third largest source with 1.3%. It is uncertainty of bottom lungs of the ladder, eg the distance to Large Magellanic Cloud before Cepheid and SN Ia are involved.
> Rarely discussed in popular treatment is anchor distance
Yes, and I'm sad that it's so rare.
The Megamaser Cosmology Project is incredibly awesome; combining line of sight acceleration, velocity, velocity gradient, and observer angle on the sky is very "anchor".
Not sure that's convincing: Why would I multiply a unit of time (Yug) with unit of distance (Yojan) to arrive at the distance to the sun? Also note that per Wikipedia, the historical value of the Yogan can range from 3.5km (~2.2 miles) to 15km (~9.3 miles). How was the value of 8 miles chosen?
I'm sure your goal is to boost the intellectual reputation of ancient hindu philosophy, but you're mostly scuffing the intellectual reputation of hinduism.stackexchange.com
It all started with knowing the distance from the earth to the sun. Nobody had a clue until Richer and Cassini got within 10% in 1672. Then we nailed it down in 1769 with James Cook's voyage to Tahiti, the primary purpose of which was to observe the transit of Venus from the other side of the world.
From there if you know basic geometry, you can observe the nearby stars shift a bit when the earth goes around the sun (parallax), but that only works to about 10k light years.
Then, we discovered a couple unbelievably convenient astrophysics hacks: Cepheid variables (Henrietta Swan Leavitt, 1908) and Type 1A supernovae (Subrahmanyan Chandrasekhar, 1935, the namesake of the Chandra X-Ray Observatory). These allowed us to move out a couple more rungs on the ladder.
From there, the relationship between redshift and distance becomes significant and that takes us to the edge.
https://www.uwa.edu.au/science/-/media/Faculties/Science/Doc...