Fixed stars and wandering lights

This lesson asks you to describe the major components of the solar system and the universe and to compare their characteristics. The museum offers one medieval astronomer's account of two kinds of heavenly bodies, the fixed stars and the sun, moon and planets, and of how each moves. Other components, such as comets or galaxies, and the comparison with today's knowledge are for your class. ¹

The expectation asks for a comparison. Choose two or three features to compare, such as how a body moves, whether its place among the others changes, and how many of them there are, and set them out in a table.

Read how al-Farghani explains the word "fixed". Al-Farghani says the fixed stars are so called because all their motions from west to east are equal, so their figures and the distances between them remain fixed ². This follows Ptolemy's measurements, as he notes in chapter thirteen ³. He adds a slow motion on the poles of the zodiac of one part in every hundred years, a full circuit in thirty-six thousand years ²³. The sun and planets are different: the second motion is their own motion along the zodiac from west to east, opposite in direction to the first ⁴. Al-Farghani shows that the sun covers the zodiac by its own motion in a solar year, returning to its point in about three hundred and sixty-five days and a quarter ⁵.

The records read here preserve no sentence in which al-Farghani says why he wrote on the heavens, and they report no preface stating a religious purpose. What they show is a setting and a debt. Al-Mas'udi counts him among those who engaged in the sciences of the heavens ⁶. The chapters show al-Farghani estimating the earth's circumference from a degree measured by the cubit in al-Ma'mun's days ⁷. Golius places his time in al-Ma'mun's era and notes that the author mentions al-Ma'mun's observatories in chapters five and eight ⁸. The record also says that the division of motions into a first and a second is inherited from the astronomers before him.

Two questions about the author are answered differently in the record, and the museum decides neither. On his name, Golius records that the old Latin translation calls the author Ahmad, while the manuscript gives Muhammad ibn Kathir, and that the two names belong to one person ⁸. On his figures for the fourth clime, Golius defends them, holding that the error attributed to him goes back to Abu'l-Fida's copy ⁹. Golius also concedes that the date of the author's death is unknown, recorded without a final judgement ⁸.

The museum reads al-Farghani in the Latin edition printed at Amsterdam in 1669, with Golius's notes ¹⁰. The index lists thirty chapters ¹¹. The star counts show why a reader must check. One passage gives three hundred and sixty stars north of the zodiac, and elsewhere the text gives two hundred and sixty north, three hundred and forty on the zodiac and three hundred and sixteen south, figures that do not reach the total of one thousand and twenty-two, so the number is doubtful ^{12 13}. Two of the magnitude counts are OCR readings ¹². A comparison with other sources is listed as not yet added.

In one old book the same count of stars north of the zodiac appears as three hundred and sixty on one page and two hundred and sixty on another, and the second set of figures does not reach the stated total. Which figure would you trust more, and why? What could you check to tell a copying slip from a real difference?

• The fixed stars: their motion, the count in six magnitudes and the stars that never set.
• The second motion: the sun, moon and planets along the zodiac.
• al-Farghani: the author, the Latin edition and Golius's notes.

Read about the fixed stars and the second motion. Make a two-column table that compares how each kind of body moves.