Literary Reading: Fatimid Astronomer’s Account of the Lunar Eclipse and Planetary Conjunctions

Diagram of a lunar eclipse. Source: space.com

Diagram of a lunar eclipse. Source: space.com

A total lunar eclipse occurs when a full moon passes through the earth’s shadow during its orbit. When the sun, Earth and moon are perfectly aligned, the Earth blocks the sun’s light. But the most spectacular view of the night is the moon’s eerie red and orange glow — caused by the sun’s indirect light being filtered through Earth’s atmosphere, trying to reach the moon. This is a breathtaking natural phenomenon that is worth seeing at least once in one’s life as it occurs in the sky.

Accounts of this eclipse have been transmitted from generation to generation and written down in scientific and popular texts.

Muslim scholars of the past recorded dozens of eclipses of the sun and the moon in different parts of the world, and one of the greatest astronomers in Islamic history is Ibn-Yunus who lived from 950-1009 CE in Cairo during the time of the Fatimid Imam Al-Hakim.

The Imam Al-Hakim gave him magnificent opportunities and facilities.

The Imam had established an Academy of Knowledge (Dar al-Hikma) and ensured that an observatory was housed in it. Ibn Yunus made excellent use of these exceptional facilities to measure more accurately the number of astronomical constants and to compile improved tables named after the Imam, the Hakemite Tables.

A thousand years ago, my forefathers, the Fatimid Imam-Caliphs of Egypt, founded al-Azhar University and the Academy of Knowledge in Cairo.

In the Islamic tradition, they viewed the discovery of knowledge as a way to understand, so as to serve better God’s creation, to apply knowledge and reason to build society and shape human aspirations.  His Highness the Aga Khan, 2004

Ibn Yunus’s major work, an astronomical handbook, was al-Zij al-Hakimi al-kabir.

‘Al-Kabir’  means ‘large’ which is apt and ‘al-Hakimi’  means that the work is dedicated to Imam-Caliph al-Hakim. The book is certainly large, containing 81 chapters. There are lists of observations made by Yunus and also observations made by his predecessors.

He describes 40 planetary conjunctions accurately and 30 lunar eclipses which were used by Simon Newcomb in his lunar theory. One example concerns a conjunction of the planets Venus and Mercury in Gemini, observed in the Western Sky.

Ibn Yunus noted:

“The two planets were in conjunction after sunset on the night [of Sunday 19 May 1000]. The time was approximately eight equinoctial hours after midday on Sunday…Mercury was north of Venus and their latitude difference was a third of a degree.”

Modern knowledge has confirmed the positions of the planets that Ibn Yunus described. Also the distance of one third of a degree that he gives is again exactly right.

Ibn Yunus also described an eclipse of the moon as follows:

“This lunar eclipse was [on 22 April 981]. We gathered to observe this eclipse at al-Qarafa, in the Mosque of Ibn Nasr al Maghribi. We perceived first contact when the altitude of the moon was approximately 21 degrees. About a quarter of the lunar diameter was eclipsed, and reemergence occurred about a quarter of an hour before sunrise.”

Lunar Eclipse - source Aip.org

Lunar Eclipse - source Aip.org

His other major contribution was in the field of calendars, which he improved upon. The first chapter of the Hakimi Zij gives calendar tables for Muslim, Coptic, Syrian and Persian calendars. Ibn Yunus gives tables to convert dates between these calendars and provides tables to compute the date of special religious observances, such as Easter.

Many other tables have been attriributed to Ibn Yunus. For example the author of a paper in the journal “Centaurus 18 (1973/74), 129-146, notes:

“(In this paper) I describe a set of tables for finding the longitude of the moon, attributed to the tenth century Egyptian astronomer Ibn Yunus. The underlying lunar theory is that of Ptolemy, but these (Ibn Yunus’s) tables are so devised that the user is spared the calculations which are associated with Ptolemy’s lunar tables.

Ibn Yunus’s tables…contain over 34000 entries ..The tables are of interest as the earliest attempt by a medieval scholar to solve the computational problem of the determination of the lunar position according to the sophisticated Ptolemaic theory.”

Other tables described the time since sunrise, the hour angle and the solar azimuth from the solar altitude. The high degree of accuracy displayed by these tables suggests that Ibn Yunus used systems of nonlinear interpolation.

He contributed his share to the development of trigonometry where trigonometric functions were given as arcs rather than angles. Spherical trigonometry reached a high level of sophistication in this work.

Finally in a strange way the scholar, who was in good health, predicted his death about seven days in advance.  During this time he tidied up his business affairs, locked himself in his house and spent time in prayers and reading the Quran until he died on the day he predicted.

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Adapted from Article by: J J O’Connor and E F Robertson on Ibn Yunus in School of Mathematics and Statistics Archives (http://www-history.mcs.st-andrews.ac.uk/), November 1999

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