<p>The new New York Times article <a href="https://www.nytimes.com/2019/05/28/science/solar-eclipse-einstein-physics.html" rel="nofollow noreferrer">The Eclipse That Made Einstein Famous</a> describes several events and discussions on the hundred year anniversary of the the 1919 solar eclipse and verification of the deflection of light by the Sun as predicted by General Relativity. The roles of Albert Einstein, Arthur Eddington, and others are discussed in the article, the play performed at the Royal Astronomical Society in London, in the Nature Physics article <a href="https://www.nature.com/articles/s41567-019-0519-3" rel="nofollow noreferrer">A hundred years of the first experimental test of general relativity</a> and the book <a href="https://press.princeton.edu/titles/17193.html" rel="nofollow noreferrer">No Shadow of a Doubt; The 1919 Eclipse That Confirmed Einstein's Theory of Relativity</a> by Daniel Kennefick.</p>
<p>The instruments used for the measurement are shown in the illustration and photograph below.</p>
<p>My question is about the astrometric technique used, and the calibration and verification.</p>
<p>The instruments are long and laying on their sides, viewing the eclipse and the stars near the Sun by reflection in coelostats (mirrors in front of telescopes, moved by clockwork mechanisms).</p>
<p>I can imagine several sources of instrumental and experimental errors due to variations in <em>field distortion</em> and <em>magnification</em>, presumably the measurement was done by comparing the eclipse images with similar images taken by the same instruments of the same field of stars in the same position in the field of view a few months later, at night, in the dark, in order to show that the tiny deflections were not due to variation from one instrument to another.</p>
<p>Is that actually how it was done? Was the experimental set-up left in-place between day and night measurements, or was it packed up and moved?</p>
<p>Was the flatness of the coelostat mirrors independently confirmed somehow?</p>
<hr />
<p>Images from the new New York Times article <a href="https://www.nytimes.com/2019/05/28/science/solar-eclipse-einstein-physics.html" rel="nofollow noreferrer">The Eclipse That Made Einstein Famous</a></p>
<p><a href="https://i.sstatic.net/tZbV7.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/tZbV7.jpg" alt="enter image description here" /></a></p>
<blockquote>
<p>Einstein’s theory suggested that starlight would bend near a massive body like the sun.CreditChronicle/Alamy</p>
<p>In Sobral, Brazil, two coelostats with movable mirrors were used to direct images of the eclipsed sun into a pair of horizontal telescopes.
Credit Royal Observatory, Greenwich/Science Museum Group</p>
</blockquote>
<p><a href="https://i.sstatic.net/E8f92.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/E8f92.jpg" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2019-05-29T04:00:24.040Z
It's not that bad. You can pack things up and take a shot in another place, and the results are going to be very repeatable. Field flatness affects mostly focus and aberration, not the location of the center of the diffraction figure. The size of the image is dictated by the focal length (if imaging in prime focus) and so it's fixed for a given instrument with very high precision. Ergo, taking images X months before or after should provide exact replicas of the eclipse shots, sans the GR effects. 1.7 arcsec is quite a lot of bending; the resolving power of just 100 mm of aperture is 1 arcsec.
Florin Andrei
CC BY-SA 4.0
2019-05-29T04:05:00.653Z
BTW, "magnification" is a bad word among the cognoscenti. Try to avoid it. ;) It's not relevant to this discussion anyway.
Florin Andrei
CC BY-SA 4.0
2019-05-29T04:39:03.017Z
[Dyson, Eddington, and Davidson 1920](https://doi.org/10.1098/rsta.1920.0009) may help.
Mike G
CC BY-SA 4.0
2019-05-29T04:43:54.123Z
@FlorinAndrei "It's not that bad" isn't really standard parlance for a modern astrometer, much less one trying to upend scientific theory with a few photographic plates. I'm just asking here about what was actually done, not what each user thinks might or might not have been necessary. However if you can demonstrate in an answer convincingly from first principles that this 100 year old equipment set would have the necessary repeatability after several setups and tear-downs, that would be great! re "field flatness", I mean field distortion, edited. Thanks!
uhoh
CC BY-SA 4.0
2019-05-29T06:58:28.237Z
@uhoh It seems neither of us is a true "astrometer" anyway, though I could be wrong about that. Regardless, as a matter of general advice, when you're receiving help, freely given, no matter the alleged or real quality of that help, it's always a good idea to pay attention to the tone of your discourse. Lest you turn goodwill into reluctance and distance. Have a nice day!
Florin Andrei
CC BY-SA 4.0
2019-05-29T08:26:34.793Z
@FlorinAndrei your comment is dismissive of my question, suggesting that I'm asking about a trivial concern, in a fairly authoritative tone, yet without any supporting facts. I don't find it helpful or positive. It's quite a challenge to move telescopes around the world in 1919 lay them on their sides, set up heliostats, expose glass photographic plates, make measurements, calibrate, confirm and produce results so reliable you can set the scientific world on its head.
uhoh
CC BY-SA 4.0
2019-05-29T08:32:46.023Z
@FlorinAndrei Stating that it's "not that bad" and "the results are going to be very repeatable" downplays the importance of my question and more importantly the work of Eddington and other astronomers and assistants to make this measurement in such a convincing way. I've measured my own plates with a "fancy" 1950's or 60's Mann measuring machine myself ([e.g.](https://underctskies.wordpress.com/tag/mann-measuring-machine/)), stars & spectra, and even with that and a research telescope this is a non-trivial task. So I've challenged the the content of your comment because I feel it's incorrect
uhoh
CC BY-SA 4.0
2019-05-29T08:38:08.160Z
@FlorinAndrei let's see if we can find out how good these turn-of-the-last-century instruments actually were, thermal stability of the mirrors and all, and how repeatable the measurements really were, and converge on a fact-based answer!
uhoh
CC BY-SA 4.0
2019-05-30T00:34:04.963Z
I suspect the answer is that you should probably read Kennefick’s book, but Mike G’s suggestion looks like a very good start. From a cursory read, it appears that in the two months between the eclipse observations and follow-up nighttime observations, they left everything in place except the mirrors, which they stored inside to keep them dust-free.
Peter Erwin
CC BY-SA 4.0
2019-05-30T00:48:29.690Z
@PeterErwin Thank you for your encouragement! I think this is a fascinating and really compelling story. I don't have access to the paper today (it seems to be paywalled, need to get to a library) and I'll see what I can do about finding a copy of Kennefick’s book where I live. In the mean time, it's possible someone besides me will also find this interesting enough to write up an answer!
uhoh
CC BY-SA 4.0
2019-05-30T05:10:17.283Z
@uhoh If Royal Society won't let you read a 99-year-old paper, try [JSTOR](https://www.jstor.org/stable/91137)?
Mike G
CC BY-SA 4.0
2019-05-30T05:19:39.503Z
@MikeG my mistake. I've been doing literature searches recently and each journal has its own layout. I looked in the "usual places" (orange) an miss the the two obvious ones (blue). It's (slowly) opening now... yay!!  https://i.sstatic.net/zJRoT.png
uhoh
CC BY-SA 4.0
2019-06-03T14:31:30.410Z
@MikeG if nobody writes an answer in another day or two based on that paper, I will. But let me know if you're planning on it and I'll hold off so you can put an answer together. It's certainly a wonderful read!
uhoh
CC BY-SA 4.0
2019-06-03T22:00:19.277Z
@uhoh Knock yourself out. :)
Mike G
<observational-astronomy><telescope><history><astrometry><instruments>
How was the astrometry of the famous 1919 solar eclipse confirmation of General Relativity calibrated and verified?
uhoh
# How was the astrometry of the famous 1919 solar eclipse confirmation of General Relativity calibrated and verified?

(Asked by: username_0 on 2019-05-29)

The new New York Times article [The Eclipse That Made Einstein Famous](https://www.nytimes.com/2019/05/28/science/solar-eclipse-einstein-physics.html) describes several events and discussions on the hundred year anniversary of the the 1919 solar eclipse and verification of the deflection of light by the Sun as predicted by General Relativity. The roles of Albert Einstein, Arthur Eddington, and others are discussed in the article, the play performed at the Royal Astronomical Society in London, in the Nature Physics article [A hundred years of the first experimental test of general relativity](https://www.nature.com/articles/s41567-019-0519-3) and the book [No Shadow of a Doubt; The 1919 Eclipse That Confirmed Einstein's Theory of Relativity](https://press.princeton.edu/titles/17193.html) by Daniel Kennefick.

The instruments used for the measurement are shown in the illustration and photograph below.

My question is about the astrometric technique used, and the calibration and verification.

The instruments are long and laying on their sides, viewing the eclipse and the stars near the Sun by reflection in coelostats (mirrors in front of telescopes, moved by clockwork mechanisms).

I can imagine several sources of instrumental and experimental errors due to variations in *field distortion* and *magnification*, presumably the measurement was done by comparing the eclipse images with similar images taken by the same instruments of the same field of stars in the same position in the field of view a few months later, at night, in the dark, in order to show that the tiny deflections were not due to variation from one instrument to another.

Is that actually how it was done? Was the experimental set-up left in-place between day and night measurements, or was it packed up and moved?

Was the flatness of the coelostat mirrors independently confirmed somehow?

---

Images from the new New York Times article [The Eclipse That Made Einstein Famous](https://www.nytimes.com/2019/05/28/science/solar-eclipse-einstein-physics.html)

[![enter image description here](https://i.sstatic.net/tZbV7.jpg)](https://i.sstatic.net/tZbV7.jpg)

> Einstein’s theory suggested that starlight would bend near a massive body like the sun.CreditChronicle/Alamy
>
> In Sobral, Brazil, two coelostats with movable mirrors were used to direct images of the eclipsed sun into a pair of horizontal telescopes.
> Credit Royal Observatory, Greenwich/Science Museum Group

[![enter image description here](https://i.sstatic.net/E8f92.jpg)](https://i.sstatic.net/E8f92.jpg)

**Tags:** observational-astronomy, telescope, history, astrometry, instruments

> **Comments:**
> **username_1**: It's not that bad. You can pack things up and take a shot in another place, and the results are going to be very repeatable. Field flatness affects mostly focus and aberration, not the location of the center of the diffraction figure. The size of the image is dictated by the focal length (if imaging in prime focus) and so it's fixed for a given instrument with very high precision. Ergo, taking images X months before or after should provide exact replicas of the eclipse shots, sans the GR effects. 1.7 arcsec is quite a lot of bending; the resolving power of just 100 mm of aperture is 1 arcsec.  
> **username_1**: BTW, "magnification" is a bad word among the cognoscenti. Try to avoid it. ;) It's not relevant to this discussion anyway.  
> **username_2**: [Dyson, Eddington, and Davidson 1920](https://doi.org/10.1098/rsta.1920.0009) may help.  
> **username_0**: @FlorinAndrei "It's not that bad" isn't really standard parlance for a modern astrometer, much less one trying to upend scientific theory with a few photographic plates. I'm just asking here about what was actually done, not what each user thinks might or might not have been necessary. However if you can demonstrate in an answer convincingly from first principles that this 100 year old equipment set would have the necessary repeatability after several setups and tear-downs, that would be great! re "field flatness", I mean field distortion, edited. Thanks!  
> **username_1**: @username_0 It seems neither of us is a true "astrometer" anyway, though I could be wrong about that. Regardless, as a matter of general advice, when you're receiving help, freely given, no matter the alleged or real quality of that help, it's always a good idea to pay attention to the tone of your discourse. Lest you turn goodwill into reluctance and distance. Have a nice day!  
> **username_0**: @FlorinAndrei your comment is dismissive of my question, suggesting that I'm asking about a trivial concern, in a fairly authoritative tone, yet without any supporting facts. I don't find it helpful or positive. It's quite a challenge to move telescopes around the world in 1919 lay them on their sides, set up heliostats, expose glass photographic plates, make measurements, calibrate, confirm and produce results so reliable you can set the scientific world on its head.  
> **username_0**: @FlorinAndrei Stating that it's "not that bad" and "the results are going to be very repeatable" downplays the importance of my question and more importantly the work of Eddington and other astronomers and assistants to make this measurement in such a convincing way. I've measured my own plates with a "fancy" 1950's or 60's Mann measuring machine myself ([e.g.](https://underctskies.wordpress.com/tag/mann-measuring-machine/)), stars & spectra, and even with that and a research telescope this is a non-trivial task. So I've challenged the the content of your comment because I feel it's incorrect  
> **username_0**: @FlorinAndrei let's see if we can find out how good these turn-of-the-last-century instruments actually were, thermal stability of the mirrors and all, and how repeatable the measurements really were, and converge on a fact-based answer!  
> **username_3**: I suspect the answer is that you should probably read Kennefick’s book, but username_2’s suggestion looks like a very good start. From a cursory read, it appears that in the two months between the eclipse observations and follow-up nighttime observations, they left everything in place except the mirrors, which they stored inside to keep them dust-free.  
> **username_0**: @PeterErwin Thank you for your encouragement! I think this is a fascinating and really compelling story. I don't have access to the paper today (it seems to be paywalled, need to get to a library) and I'll see what I can do about finding a copy of Kennefick’s book where I live. In the mean time, it's possible someone besides me will also find this interesting enough to write up an answer!  
> **username_2**: @username_0 If Royal Society won't let you read a 99-year-old paper, try [JSTOR](https://www.jstor.org/stable/91137)?  
> **username_0**: @MikeG my mistake. I've been doing literature searches recently and each journal has its own layout. I looked in the "usual places" (orange) an miss the the two obvious ones (blue). It's (slowly) opening now... yay!!  https://i.sstatic.net/zJRoT.png  
> **username_0**: @MikeG if nobody writes an answer in another day or two based on that paper, I will. But let me know if you're planning on it and I'll hold off so you can put an answer together. It's certainly a wonderful read!  
> **username_2**: @username_0 Knock yourself out. :)  

---

## 0 answers

