Why the Sun and Moon go dark

This lesson asks you to conduct investigations that explain the causes of astronomical phenomena that can be observed from Earth. The museum offers one such phenomenon, the eclipse, as one medieval astronomer explained it and as people once saw it. The investigations themselves, with models and observations, are for your class. ¹

The expectation asks for investigations that explain causes. Think of a lamp, a ball for the Moon and another for the Earth, and of what you would move to make each kind of eclipse. Check whether the viewer's position changes what is seen.

Read one report as a chronicle gives it. Ibn al-Imad in Shadharat al-Dhahab relates that the sun was eclipsed in a great eclipse on the eighteenth of Shawwal 836 AH ². The eclipse extended from after the afternoon prayer to near sunset. The people performed the eclipse prayer and thought the sun had set eclipsed, until it cleared shortly before that ². Now set it beside al-Farghani's account: a solar eclipse occurs when the moon conjoins the sun at the node without latitude, hiding it from the observer ³, and parallax means an eclipse may appear partial from one place and total from another ⁴⁵. The record of the event adds that its Gregorian date is derived and needs to be checked against a modern source ².

The records preserve no sentence in which al-Farghani says why he wrote about eclipses, and no preface stating a religious purpose. What they show is how he works. Al-Farghani explains eclipses by conjunction and opposition, latitude and parallax, and draws the intervals between them from all of this according to the months ⁴. He estimates the length of the earth's shadow-cone in Ptolemy at about 268 earth-radii ⁶. The same records report that in 836 AH the people performed the eclipse prayer ²; they do not connect that prayer to al-Farghani's chapters, and the museum does not.

The two records stand on different footing. The report of 836 AH rests on Ibn al-Imad's Shadharat al-Dhahab, page 224, and its Gregorian date is derived and unchecked ². For al-Farghani the museum reads the Latin edition with Golius's notes, and the record flags its own doubts: some figures are OCR readings and are doubtful ⁶. In one of Golius's examples, an eclipse Ibn Yunus observed, the Diocletian year is printed as 69 where 694 is correct by the other reckonings, a correction flagged and not relied on without checking ⁷. A comparison of the intervals and eclipse lengths with observed eclipses and the Almagest is listed as not yet added.

Two observers in different places might describe the same eclipse differently, one calling it partial and one total. A sceptic would say that two different accounts mean at least one witness is wrong. Al-Farghani's account of parallax says an eclipse may appear partial from one place and total from another. Which reading would you apply to a chronicler's report such as Ibn al-Imad's, and what would you check before deciding, and why?

• Eclipses: lunar and solar eclipses, the nodes, parallax and the intervals between eclipses.
• The solar eclipse of 836 AH: the chronicle's report and the prayer the people performed.
• al-Farghani: the author, the Latin edition and Golius's notes.

Read about solar and lunar eclipses. Build a model with a lamp and two balls, and test which arrangement matches each case al-Farghani describes.