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Muslim astronomers and astrologers extrapolated where in the sky planets would be on future dates.
They may have read an Arabic translation of Archimedes citing Aristarchus of Samos writings. Muslim astronomers disagreed over whether Aristotle or Ptolemy was correct.

Abū Rayhān al-Bīrūnī  (973 – 1048) learned the astronomy of India by interrogating Indian scholars whom the Muslims had imprisoned.

Nearly all Muslim astronomers agreed that the Earth does not rotate.

Muslim astronomers thought that the stars were not much further away than Jupiter..

Al-Zarqālī discovered that the apparent elongated path of Mercury did not agree with Ptolemy’s theory, but avoided admitting that the path was not composed of circles.

Al-Biruni’s data would later be used by Copernicus, who believed the Earth and other planets traveled around the sun.

Neither the Muslims nor Copernicus knew that planets sped up as they got nearer to the sun.
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the planet Mercury

Muslim astronomers and astrologers such as 8th century Ahmad Nahavandi and 10th century Ibn Yunus (950 – 1009) compiled tables of the rising and setting times of the moon, fixed stars, and planets (wandering stars), and in the positions of them relative to each other. From this they extrapolated where in the sky these objects would be on future dates.
Astronomers such as Abū Isḥāq Ibrāhīm ibn Yaḥyā al-Naqqāsh al-Zarqālī, also known as Al-Zarkali or Ibn Zarqala (1029–1087) published tables giving the weekly positions oïf the planets several years in advance.
Astronomers used this data to use astrology to predict future events.

The book of the 1001 Inventions exhibit falsely claims Muslim astronomers invented the concept of a planet moving in “linkages of constant length vectors rotating at constant angular velocities”. This is merely the epicycle model which Muslim astronomers copied from Ptolemy.

Abū Rayhān al-Bīrūnī (973 – 1048)  may have read an Arabic translation of Archimedes citing Aristarchus of Samos writing in the 3rd century BC that the Earth and other planets traveled around the sun, and correctly listing the planets in the sequence of their distance from the sun.
Abū Rayhān al-Bīrūnī  (973 – 1048)
learned the astronomy and other sciences of India by interrogating Indian scholars whom the Muslims had imprisoned, such as the Indian astronomer Brahmagupta.
In 1030 al-Biruni mentioned in a few pages the Indian heliocentric theories of Aryabhata, Brahmagupta and Varahamihira in his Ta’rikh al-Hind.”

Al-Sijzi (Sijzi) (c. 945 – c. 1020) agreed with such ancient Greek astronomers as Hicetas, Ecphantus and Heraclides Ponticus that the Earth rotates.

Al-Biruni firmly believed in the geocentric model, probably because he thought, like most astronomers, that the Earth could not be spinning because that would cause high winds or cause objects to fly off the spinning Earth.
One reason al-Biruni did not think that the Earth could be in an orbit around the sun is because [without a telescope] no seasonal parallax effect could be seen in the position of the stars. [Parallax is so small because stars are so far away. Muslim astronomers thought that the stars were not much further away than Jupiter.]

Abū Ishāq Ibrāhīm al-Zarqālī (1029–1087)  had a different model than Ptolemy for the motion of Mercury. They agreed in both of them using a deferent and epicycle for the motion of all planets. [This was needed because of the incorrect assumption that the planets circled the Earth when actually they circled the sun.] But the shape of the actual path of Mercury around the sun is a very elongated oval rather than the nearly circular paths of other planets circling the.sun. Both al-Zarqālī and Ptolemy were firm believers in the geocentric model, and neither of them ever considered the possibility that planets circled the sun. But something had to be done to make the model match the observation of astronomers. Ptolemy used an epicycle on an epicycle, but al-Zarqālī decided to instead use an oval and an epicycle.
Averroes (1126 –  1198) used mathematics to argue in favor of the unmodified astronomy model of Aristotle of concentric circular paths of the Sun and planets around the Earth.

In the 12th century, Gerard of Cremona (an Italian Catholic who came to Toledo Spain after Toledo’s reconquest by Christians) and others in Toledo began translating from Arabic to Latin scientific texts of Euclid, Ptolemy, Aristotle, Archimedes, Al-Zahrawi, and Al-Khwarizmi.

Kushyar ibn Labban (Kushyar Gilani) (971 – 1029) added improved values of the planetary apogees [the point at which an object is furthest from the Earth) observed by al-Battani (858 – 929)

Al-Biruni’s data and the data of several other Muslim astronomers would later be used in 1543 (and confirmed by observation) in the heliocentric theory of the Polish (Christian) scientist Copernicus.

Copernicus thought planets (including the Earth) moved in constant speed perfectly circular orbits around the sun. The need for most of the epicycles of the Ptolemy model was eliminated in the Copernican model because planets were not moving around the Earth.
But Copernicus needed to include at least 2 epicycles because, in the Copernican model,  the orbit of the Earth around the sun, and the orbit of the planet Mercury around the sun, were explained (incorrectly) as perfect circles of uniform speed, each around a point (a different point for each planet) that was approximately but not exactly the sun.
Copernicus, Ptolemy, and every Muslim model (except the weird Tusi couple model explained later) all were unable to explain the amount of variation in apparent size of the sun during a solar eclipse. [The weird Tusi couple model was not known to the Christian astronomers at the time of Copernicus, and is not mentioned much on Muslim websites.]

Johannes Kepler in 1596 explained that a planet moves in the path of an ellipse, with the sun at one of its 2 foci, with speed increasing as the planet approached the sun. [Later, Isaac Newton’s theory of gravity explained why Kepler was correct]
An analysis and comparison of the Ptolemy model (which Muslim astronomers followed but did not improve upon except to notice Mercury had 2 epicycles), the Copernicus model, and the Kepler model can be found at

Averroes was not the first to discover sunspots, which had been seen by ancient Chinese astronomers, and around 300 BC by a Greek named Theophrastus, and by other astronomers.

Aristarchus of Samos, in the 3rd century BC, had made measurements of what the angles were between the sun, moon, and Earth when the moon became exactly half lit. Using geometry, he calculated an estimate of how many times further away the sun was than the moon, but this estimate gave a value of 1/15 or 1/10 of what we now know the distance is. [Actually, his calculation gave the minimum distance, but he refused to consider the possibility that the distance was more than that.]

The astronomers Yaʿqūb ibn Ṭāriq (in the 8th century) and Sind ibn Ali (in the 9th century) copied from Hindu astronomers complete nonsense that the diameter of both Mars and the sun was 19 times the diameter of the Earth.

The planets appear to the eye as only specks of light, so without a telescope their angular diameters could not be measured.
Astronomers in India used the Aristarchus of Samos distance to the sun that was about 1/15 the actual value. In the 5th century, Indian astronomers looked at the fuzzy speck and wrongly calculated an angular diameter of Mars that was about 15 times the actual value. From these 2 pieces of erroneous data, the Indians accidentally calculated the diameter of Mars to be 89% of the correct value.
The claim that al-Farghani knew the “comparative sizes of the planets and the Earth” is false.
Even Venus, which at its closest to Earth extends to a width of 0.018  degrees, appears to the naked eye as only a point, not as something you can measure the diameter of and compare to the diameter of other planets.
Image by public domain
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