topics: |  history | science | astronomy | |

Interesting facts, but somewhat poorly written (or translated). Occasionally verbose.

I felt that the author was rather reluctant to name or credit Islamic figures. For example.

Can we speak, therefore, of a "Copernican revolution"? Indeed, there was an evolution in our thinking about the world, an evolution which was very rapid and far-reaching from the 14th to the 17th century. Instrumental in this evolution were many factors, some purely scientiflc, some political, some economic. During this period, translations of the great Greek works, through Arabic intermediate translators, permeated the Western world, where great translators were at work, notably in international centers such as Toledo. The work of Gerard of Cremona, Adelard of Bath (12th century), and Michel Scot (l3th century) had a considerable importance. p.4

While stating that there was a large change from 14th to the 17th century, it does not mention that the only astronomers of this period were all working in the Islamic world. In fact, the few people named later, all Christian names, are primarily known as translators of the Arab scholars. This style of writing gives the impression that the translators were the most important pieces in the puzzle; the people whose texts were being translated remain obscure, and appear to be peripheral. Yet it is the ideas of the Arabic astronomers that created the anti-Ptolemaic universe that Coperinicus came to inhabit.

This attitude is of course contradited by later parts of the text itself, so this sort of positioning seems quite uncalled-for.

Also on Book Excerptise

     

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	In the 1500s, medicine students had to learn "medical
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	[This may seem laughable today, but I wonder how many of today's
 	measures in medicine would appear as laughable in the
 	2500s.] 

* A More Perfect Heaven: How Copernicus Revolutionised the Cosmos
   by Dava Sobel (2011)

	Working at a remote outpost in Poland, often under the shadow of
	war from Albrecht of Prussia and others, Copernicus formulated the
	heliocentric theory that changed our vision of the universe. ...


* Indian astronomy: an introduction
   by S. Balachandra Rao (1994)

	Sometimes, the sun may remain in the same rAshi for more
	than a lunar month; in such situations, one has an extra month
	(adhikamAsa, chapters 5 & 6)

* Science and Civilisation in China: v.4
    Physics and Physical Technology, part I: Physics
    by Joseph Needham and Wang Ling and Kenneth G Robinson (1977)

	The Indian [trigonometry] was taken over by the Arabs and
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Excerpts

Aristotelean motion

We can consider paradigmatic of the pre-Ptolemaic doctrine the following
statements:  (p.3)

(1) The very nature of heavenly bodies (stars, planets) differs from that of
    the sublunary world. The sublunary world is subject to imperfections,
    fantasies, while the astral world is, on the contrary, perfect, and even
    eternal (aether-nal, to play on the "aetheral" nature of the astral
    world); 

    [disproved in 1572 by Tycho Brahe, who observed comets and a supernova,
    which exhibited non-circular (non-celestial) motions of great amplitude
    and variations in brightness.  Tycho demonstrated by accurate
    measurements that both objects were definitely more distant than the
    Moon; hence, they did not belong to the imperfect, sublunary world. ]

(2) Trajectories of planets cannot be anything but circular; 

    [held true by Copernicus and Tycho; Kepler was the first to abandon it.] 

(3) Motions of planets are uniform, i.e. of constant velocity on their
    trajectories; 

    [violated very early; e.g. Ptolemy's equans mechanism, center of
    "uniform" motion is different from center of sphere.

    We note that uniformity is also preserved by Kepler's second law, 
    which says that a constant area is swept in a given time interval 
    by the vector radius during the motion of a planet]

(4) Only a material body can be located at the center of all these
    trajectories; 

    [not followed in many ancient theories, such as the Pythagorian
     Philolaus, which envisioned the Earth and the "anti-Earth" turning
     around some immaterial central flre.  Also violated in the Ptolemaic
     system, which has an imaginary point (the "deferent") at the center of
     the sphere.)

(5) This material body is the Earth.

    [contrary suggestions include the heliocentricism of Aristarchus.  Even
    after Copernicus, the idea of geocentricism continued to hold, even in
    the ideas of Tycho Brahe, and continuing well after Galileo even. ]

Can we speak, therefore, of a "Copernican revolution"? Indeed, there
was an evolution in our thinking about the world, an evolution which
was very rapid and far-reaching from the 14th to the 17th century.
[by mostly Islamic / Arabic astronomers]

3.3 Arabic Contribution to Astronomical Knowledge

[The Alfonsine tables were part of] the last flowering of the Arabic-Jewish
school in Spain. King Alfonso the 10th (?-1284) gathered together several
astronomers under the leadership of Isaac ben Said to construct new
astronomical tables, the best possible ones. These were the so-called
Alfonsine Tables (1282), and they were indeed better than the Tables of
Toledo of al-Zarqali, already two centuries old.  

These tables were, so to speak, the epilogue in the assault of the
[...] Arabic world against Ptolemaic doctrine in the name of Aristotle. ...
the attraction to the idea of the "solid spheres;' and the associated
rejection of the concept of the epicycle-deferent mechanisms, were directed
against Ptolemy.

Ibn Rushd (Averroes, 1126-1198) reflects the Aristotelian view:

    If the center of the Universe is not the center of the revolution, then
    another Earth must exist, and this contradicts the basic principles of
    Physics.

Averroes was looking for a system both consistent with the "principles of
physics" (i.e. Aristotelian physics) and the observed motions of the stars
and planets (his effort to save the phenomena). This eliminated the
Ptolemaic descriptions and the systems derived from them.

On the other hand, [Mūsā ibn Maymūn or] Maimonides (1l35-1208), a Jewish
philosopher who moved from Cordoba to Cairo, adopted a skeptic attitude
towards mathematical astronomy.  His position was that the sky belongs to
God; the Earth belongs to the sons of Adam. We can only understand sublunary
physics; hence, his involvement with the study of the properties of the four
basic elements and their mixtures and his lack of interest in the sky, which
man could not hope to understand.



Mūsā ibn Maymūn (Maimonides, 1135-1204) and Ibn Rushd (Averroes,
1126-1198).  Both were born in Islamic Spain and wrote in Arabic. 
Translated into Latin in the 13th c., both were widely influential
in the European renaissance.  The work of Ibn Rushd helped 
popularize Aristotle, and Maimonides was influential in interpreting
Jewish texts.


Ibn Rushd, however, was more optimistic about the possibility of describing
the sky.

Al-Bitruji

The hopes expressed by Ibn Rushd were more or less fulfilled by a real
astronomer, al-Bitruji (Alpetragius) who was probably of Christian origin,
a convert to Islam. Along with Averroes, al-Bitruji criticized Ptolemaic
ideas, finding them contrary to strict Aristotelian principles. He built a
system of his own (diagram below). p.152



Al-Bitruji's eight spherical shells, showing only the
relative position of each sphere - the actual
mechanism proposed was much more elaborate, in order
to reflect the observed phenomena.  The ninth sphere
is the one that sets the others in motion.  The center
of the system need not be the earth.


As in Maimonides, al-Bitruji initially described the spheres of earth,
water, fire and air. In his thought, there are then eight spherical sheIIs,
contiguous to one another, and centered at the center of the Universe
(not necessarily the Earth!). The eighth sphere contains the Milky Way
and the fixed stars. The seven internal spheres are, from bottom to top
the Moon, Mercury, Sun, Venus, Mars, Jupiter, and Saturn. FinaIIy, there
is a ninth sphere, without any celestial body. It moves, but although it 
communicates its motion to the other spheres, its motion is independent
of them. This is diurnal motion; its period is the sidereal day.

This model is not complex enough to save the phenomena, but it contains
some rather astute ideas, in the direction of complexity, and in order,
of course, to save the phenomena. For instance, al-Bitruji noted that
each of the eight spheres "desires" to follow the perfect and simple motion
of the ninth one, but cannot do it perfectly. In the same way, a stone
is willing to follow the impetus which initiated its motion, but this tendency
gradually fades until finally the projectile falls, when it is sufficiently
far away from the motor which gave it its impetus! 

Each sphere, therefore, has a particular polar axis. These poles are mobile
and describe small circ1es around the ec1iptic pole. Some planets (Mars,
Mercury) are located on their sphere, but not on the equator of these
spheres. Unfortunately, al-Bitruji completely failed to explain why the
distance Moon-Earth, or Venus-Earth changes during one revolution of either
Moon or Venus. Let us note also that, along with Aristotle, al-Bitruji
linked the motions on Earth (motions of seas, of continents) to the motion
of the eighth sphere.

Up to the time of Copernicus, al-Bitruji's book was the new gospel
and it was an anti-Ptolemaic one. It supported the arguments of all of
the adversaries of Ptolemy and this continuing dispute made room without 
a doubt for the new and original thinking of Copernicus two centuries
later. 154

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