Alexandrian lighthouse and off axis parabolic mirror - an antique time mirror telescope?

[caracal]

Alexandrian lighthouse and off axis parabolic mirror - an antique time mirror telescope?

hi all,

I have not intented this as historical speculation, but more as technical and scientific speculation could certain kind of optical system, off axis parabolic mirror what i describe below later, have been used as telescope and to beam light in alexandrian lighthouse similarly as fresnel lens. I dont know why anyone haven’t thought this idea more.

I show here three sources, two of them is a take from medieval time chinese book and arabian book from the description of alexandria, third one is a single picture from Kitab al-bulhan or ”Book of Wonders”. Both of them are very short descriptions, first one and second one are just few sentences and third one is only a badly drawn picture.

I first mention that greek mathematican Diocles has studied parabolic concentrators in 2nd or 3rd century BC. There is not long way from these burning mirror to invention of telescope.

https://en.wikipedia.org/wiki/Diocles_(mathematician)

The First source:

Chau Ju-Kua: his work on the Chinese and Arab trade:

https://archive.org/details/cu31924023289345/page/n161/mode/2up

page 162 or 146 in the book – Alexandria lines 15-22:

” On the summit there was a wondrous great mirror; if war-ships of other countries made a sudden attack, the mirror detected them beforehand, and the troops were ready in the time for duty.

In recent years there came a foreigner, who asked to be given work in the guard-house of the tower; he was employed to sprinkle and sweep. For years no one entertained any suspicion of him, when suddenly one day he found an opportunity to steal the mirror and throw it into the sea, after which he made off”

The second paragraph is likely just invented story. The description is short but gives accurate description of some kind of magnifying mirror that can be used as telescope.

The second source:

I didn’t manage to find ebook for this next writing that is english translation that i took from one web page. Abu Hamid Al-Gharnati, who visited Alexandria in 12th century describes the lighthouse by following:

“The first tier is a square built on a platform. The second is octagonal and the third is round. All are built of hewn stone. On the top was a mirror of Chinese iron of seven cubits wide (364 cm) used to watch the movement of ships on the other side of the Mediterranean. If the ships were those of enemies, then watchmen in the Lighthouse waited until they came close to Alexandria, and when the sun started to set, they moved the mirror to face the sun and directed it onto the enemy ships to burn them in the sea. In the lower part of the Lighthouse is a gate about 20 cubits above the ground level; one climbs to it through an archway ramp of hewn stone”.

In the first part of the description the mirror again is told to be used as telescope. It is also exaggeration how far it can see, thought there might be mirages in mediterranean sea that makes possible to see over horizon. The second part of this text, what is about the mirrors ability to burn ships, is likely story of imagination.

I wonder why anyone haven’t thought this following concept. What i speculate is that this mirror has been off-axial parabolic mirror, that reflects light sideways into one point, maybe in 90 degree angle. The angular size of the mirror by looking from the focal point could have been as much as 2 x 17.5 = 45 degrees or even 22.5 + 35 = 57.5 degrees, before the viewer starts to be in front of reflecting area or the furthest part of the mirror becomes too elongated. With these values of angles the mirror is close to flat mirror that is in 45 degree angle from the viewer. If someone looks sideways this kind of mirror behind the focal point with eyepiece, it works as telescope. This kind of mirror can also beam light but it might not be as good as fresnel lens. During daytime it works similarly as parabolic solar cooker. It also can work as magnificator. The eyepiece of the telescope may have been another, smaller off-axial parabolic mirror or a lense. In principle using another off axis parabolic mirror behind focal point it is possible to make constant beam of light.

i have drawn schematic picture in the attachment file.

Here is also a link to schematic picture of off-axial parabolic scheffler reflector that reflects light in 90 degree angle. this is close to concept that i have in mind:

https://ars.els-cdn.com/content/image/1-s2.0-S0038092X1930430X-gr1.jpg

(This is a link that is not public web page)

The scheffler concentrator, that is used in solar cooking, is somewhat close to this kind of mirror. This concentrator is however so badly made that it cannot be used as objective mirror in telescope.

https://en.wikipedia.org/wiki/Wolfgang_Scheffler_(inventor)

Maybe the mirror in pharos lighthouse didn’t get much attention and became forgotten or something happened to books that describe it. It may have been a secret at that time also.

Why there is no technical description of such telescopes in antique time texts? The arabs may have kept this antique invention as secret, when they translated antique time books in ninth and tenth century. The reason for this would be that they think it is benefit in war.

what do you think of this?

 

The third source:

Kitab Al Bulhan by Abd al-Hasan AL-Isfahani:

https://archive.org/details/KitabAlBulhan/page/n17/mode/2up

page 84

What this picture shows, is that picture maker had clearly in mind concept of mirror for sightseeing. And the posture of the woman, that she is looking into mirror sideways, may tell that she is looking to the off axis mirror telescope that reflects light in 90 degree at its center. The mirror is also black what is like in magnifying mirror when there is no lightsource near focal point. What is missing is the eyepiece. This is not very accurate drawing.

I dont know if i am just imaging things here, but from the same book, for example pages 18 and 28 show strange looking man that seem to have strange magnified face. This may be a hint – they are looking through round mirror that way that their face is magnified. This hint tells that reader should not talk to anyone in detail about magnifying mirrors. Thought these faces and that rider that holds sword and someones head may just be weird illustrations. What is certain is that i can’t read the text in this book, its arabian.

 

 

Edited March 10, 2021 by caracal
detail

[caracal]

No one yet commented in this topic? How about more generally was there "ancient telescopes"? Well, maybe this is not a big thing and nothing new. As i seach from internet there are some writings about ancient time and ancient greek telescopes. So maybe there were some early grudely made ones around in ancient time. Maybe also this kind of reflector.  The antikythera mechanism tells on the other hand that they may have been well made if they existed. Sorry about my bad english by the way.

Edited March 12, 2021 by caracal
i added last two sentences.

[studiot]

15 minutes ago, caracal said:

No one yet commented in this topic? How about more generally was there "ancient telescopes"? Well, maybe this is not a big thing and nothing new. As i seach from internet there are some writings about ancient time and ancient greek telescopes. So maybe there were some early grudely made ones around in ancient time. Maybe also this kind of reflector.  

It could simply be that no one here knows anything about the ancient history of mirrors.

I definitely don't.

However you don't seem to be a crank and your posts seem genuine as is the proposed subject.

I suggest you email Brown University who seem to have some experts in the matter.

Good luck and I at least would be interested in their answer if you were kind enough to post it or a precis here.

+1 for your patience

https://www.brown.edu/Departments/Joukowsky_Institute/courses/13things/7306.html

Edited March 12, 2021 by studiot

[caracal]

...that would be good idea to contact or ask an expert like astronomer or historian about this subject. I try to do something and i think i update also this thread if i get answer from them. thank you for your comment