# Ṭūsī and Copernicus: The Question of the Earth’s Motion.

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Ṭūsī and Copernicus: The Question of the Earth’s Motion

Mathematical Connections between Islamic Astronomy and Copernicus ► Copernicus cites a number of Islamic authors in his works ► Since 1950s, evidence has emerged that many of his mathematical models were from Islamic sources (e.g. the Ṭūsī couple)

Possible Connections Regarding Physics ► Previous connections are mainly mathematical ► Also evidence of connections regarding new concepts of physics that would allow for the motion of the Earth ► Note similarities in the following passages between Ṭūsī and Copernicus

COPERNICUS, DE REVOLUTIONIBUS, I.8 Then what should we say about the clouds and other things suspended in the air in whatever way or things that fall down or conversely things that rise up to the upper regions? [We would say] that not only the earth with the watery element conjoined with it moves in this way, but also not a small part of the air and whatever in the same way has a natural connection (cognatio) to the earth. Either the nearby air, mixed with the matter of earth or water, should conform to (sequatur) the same nature as the earth, or the motion of the air, which has been acquired by the contiguity of the earth, participates in a perpetual rotation without resistance. On the other hand, it is no less remarkable that the upper region of the air conforms to (sequi) the motion of the heavens, which is indicated by those suddenly-appearing stars that the Greeks call “comets” and “bearded stars.” It is maintained that they are generated in that place and, furthermore, like the other stars, they rise and set. We can say that that part of the air is unaffected by the terrestrial motion on account of its great distance from the earth. The air closest to the earth, and the things suspended in it, will appear still unless they are moved about by the wind or some other impulse (impetus). For what else is the wind in the air but a wave in the sea? Then what should we say about the clouds and other things suspended in the air in whatever way or things that fall down or conversely things that rise up to the upper regions? [We would say] that not only the earth with the watery element conjoined with it moves in this way, but also not a small part of the air and whatever in the same way has a natural connection (cognatio) to the earth. Either the nearby air, mixed with the matter of earth or water, should conform to (sequatur) the same nature as the earth, or the motion of the air, which has been acquired by the contiguity of the earth, participates in a perpetual rotation without resistance. On the other hand, it is no less remarkable that the upper region of the air conforms to (sequi) the motion of the heavens, which is indicated by those suddenly-appearing stars that the Greeks call “comets” and “bearded stars.” It is maintained that they are generated in that place and, furthermore, like the other stars, they rise and set. We can say that that part of the air is unaffected by the terrestrial motion on account of its great distance from the earth. The air closest to the earth, and the things suspended in it, will appear still unless they are moved about by the wind or some other impulse (impetus). For what else is the wind in the air but a wave in the sea?

NAṢĪR AL-DĪN AL-ṬŪSĪ, AL-TADHKIRA, II.1 It is not possible to attribute the primary motion to the Earth. This is not, however, because of what has been maintained, namely that this would cause an object thrown up in the air not to fall to its original position but instead it would necessarily fall to the west of it, or that this would cause the motion of whatever leaves the [Earth], such as an arrow or a bird, in the direction of the [Earth’s] motion to be slower, while in the direction opposite to it to be faster. For the part of the air adjacent to the [Earth] could conceivably conform (yushāyi`u) to the Earth’s motion along with whatever is joined to it, just as the aether [(here) = upper level of air] conforms (yushāyi`u) to the orb as evidenced by the comets, which move with its motion. Rather, it is on account of the [Earth] having a principle of rectilinear inclination that it is precluded from moving naturally with a circular motion. It is not possible to attribute the primary motion to the Earth. This is not, however, because of what has been maintained, namely that this would cause an object thrown up in the air not to fall to its original position but instead it would necessarily fall to the west of it, or that this would cause the motion of whatever leaves the [Earth], such as an arrow or a bird, in the direction of the [Earth’s] motion to be slower, while in the direction opposite to it to be faster. For the part of the air adjacent to the [Earth] could conceivably conform (yushāyi`u) to the Earth’s motion along with whatever is joined to it, just as the aether [(here) = upper level of air] conforms (yushāyi`u) to the orb as evidenced by the comets, which move with its motion. Rather, it is on account of the [Earth] having a principle of rectilinear inclination that it is precluded from moving naturally with a circular motion.

من التذكرة لنصير الدين الطوسي وثبات جميع ما ذكرنا من الدلائل يدل على ثبات تلك الأجرام على الهيئة المذكورة. ولا يمكن إسناد الحركة الأولى إلى الأرض -- لا لِما قيل من أنّ ذلك يوجب أن لا يقع المرمي في الهواء على موضعه الأول بل يجب أن يقع في الجانب الغربي منه أو يوجب أن الحركة لِما انفصل منها كالسهم والطائر إلى جهة حركتها أبطأ وفي خلافها أسرع ، فإنّ المتصل بها من الهواء يمكن أن يشايعها بما يتصل بها كما يشايع الأثير الفلك بدلالة حركات ذوات الأذناب بحركته -- بل لكونها ذات مبدأ ميل مستقيم ، فيمتنع أن تتحرك على الاستدارة بالطبع

Islamic Context of the Discussion of the Rotation of the Earth ► Several astronomers and natural philosophers discussed this possibility before Ṭ ūsī (e.g. Bīrūnī and Ibn Sīnā ) ► Many more discussed it after Ṭ ūsī  Quṭb al-Dīn al-Shīrāzī  Al-Sayyid al-Sharīf al-Jurjānī  `Alī Qūshjī  `Abd al-`Alī al-Birjandī ► Questions involved  Location of comets and relevance for Earth’s rotation  Whether observational tests can settle this question  Dependence of Astronomy on Aristotelian Natural Philosophy

Relevance of Comets for Earth’s Motion ► Aristotle claimed comets were in the atmosphere, not the celestial realm ► But their daily motion (like the heavens) indicated some connection with the celestial realm ► Thus Ṭūsī claimed that their motion indicated that they moved in the upper atmosphere with the moon’s orb ► Disputed by Shīrāzī who noted that comets moved north-south as well as east-west

Can Observational Tests Settle the Question of the Earth’s Motion? ► Ptolemy insisted this was possible; otherwise a rock thrown straight up would not return to the place from which thrown ► Ṭūsī disputed this (as in quotation above) ► Shīrāzī disagreed with Ṭūsī and insisted observational tests were decisive; he claimed rocks of two sizes would follow the Earth’s rotation at different speeds

Do Astronomers Need Aristotelian Natural Philosophy to Settle Question of Earth’s Rotation? ► Ṭūsī, Ibn Sīnā, Nīsābūrī, Jurjānī and Birjandī say “yes” ► Ptolemy, Shīrāzī, Qūshjī, and Copernicus say “no” ► Note that a positive answer here usually implies a negative answer to the observational tests question (next slide)

DO ASTRONOMERS NEED THE (PERIPATETIC) NATURAL PHILOSOPHERS TO SETTLE THE MATTER? CAN OBSERVATIONAL TESTS SETTLE THE MATTER? Ptolemy Bīrūnī (sometimes) Shīrāzī Qūshjī Copernicus Ṭ ūsī Bīrūnī (sometimes) Ibn Sīnā (probably yes) Nīsābūrī Jurjānī Birjandī Ṭ ūsī Bīrūnī (sometimes) Ibn Sīnā (probably no) Nīsābūrī Jurjānī Qūshjī Birjandī Copernicus Oresme Ptolemy Bīrūnī (sometimes) Shīrāzī Buridan NOYESNOYES Irrelevant for Oresme and Buridan

Interpretation of Chart ► Note a reverse correlation between the observational test question and the natural philosophy question ► Bīrūnī changed positions from an early work ( Istī`āb ) to his later Qānūn ► Qūshjī and Copernicus are exceptions, in that both are negative regarding observational tests but are still not willing to depend on Aristotelian natural philosophy

“Coincidence” or Transmission Between Copernicus and Islamic Astronomy? ► Use of Comets in both is strong evidence of transmission; not used in earlier European sources ► The long and deep tradition in Islamic sources of discussing the Earth’s motion in the context of the relation of natural philosophy and mathematical astronomy has no precedent in Europe before Copernicus ► Use by Copernicus of Islamic mathematical models also argues for transmission

Conclusions ► Within Theoretical Astronomy in Islam (hay'a), there was an attempt to develop new, non-Aristotelian physical ideas ► Copernicus is much more in the tradition of Islamic hay'a from both the mathematical and physical points of view than in the tradition of medieval Latin science

Conclusions (continued) ► Islamic scientists were moving toward mathematical, philosophical and metaphysical views that are usually characterized as specifically early modern European ► Transmission between Islam and Europe in the period 1450-1650 must be seen as a dynamic, ongoing and interactive: the scarcity of textual evidence and the abundance of similar scientific products bespeaks a much wider range of contact than hitherto admitted