Before phones, before wristwatches, and long before the printed prayer calendar on the masjid wall, a person who wanted to know the exact time at night could hold up a flat brass disc, sight a bright star, turn a few plates and read the hour. That instrument was the astrolabe. It was not invented in the Muslim world, but scholars and craftsmen there used it, wrote about it and refined it for centuries, and some of the most beautiful examples ever made carry Arabic and Persian inscriptions.

You do not have to fly to Istanbul or Oxford to think about one. Harvard's Collection of Historical Scientific Instruments, in the Science Center in Cambridge, holds a Persian planispheric astrolabe made around 1620. Here is what an astrolabe does, why it mattered so much in Muslim civilization, and how to plan a visit.

What an astrolabe actually is

The most common kind, the planispheric astrolabe, is a model of the sky pressed flat. It has a few main parts:

  • The body (the "mother"), a round brass plate with a raised rim marked in degrees and hours.
  • Plates for each latitude. Because the sky looks different from Isfahan than from Cairo, makers engraved separate plates for different latitudes, showing the horizon and lines of altitude for that place.
  • The rete, a lacy cut-out disc on top. Each pointed tip marks a bright star, and a ring on the rete shows the path of the sun through the year.
  • The alidade, a sighting rule on the back used to measure how high the sun or a star stands above the horizon.

To use it, you measure the altitude of the sun by day, or a known star by night, then turn the rete until that star sits at the matching altitude line on the plate. The instrument then shows you the time, what is rising and setting, and much more. The Metropolitan Museum of Art describes the astrolabe as a tool "used to determine the time by measuring the altitude of the sun or any visible stellar object."

Why it mattered for Muslim life

Astronomy flourished in the Muslim world between the eighth and eleventh centuries, with Baghdad a major center under the Abbasid caliphs, and other cities such as Cairo, Rayy and Isfahan also supporting research. Scholars translated Greek, Sanskrit and Persian works into Arabic and then built on them.

Part of the drive was practical worship. As the Met's essay on astronomy in the medieval Islamic world puts it, the religion "required the ability to correctly determine the time and direction of Mecca for prayer, the moment of sunrise and sunset for fasting during Ramadan, and for fixing the appearance of the moon that marked the start of a new month." That need, the essay says, "led to the refinement of scientific instruments, an improvement in methods for making observations, and the creation of new calendrical systems."

The Qur'an itself invites people to notice that order. It says the sun and the moon move "by precise calculation" (Surah Ar-Rahman 55:5), and that God made the sun a radiance and the moon a light and set its phases "so that you may know the number of years and the calculation of time" (Surah Yunus 10:5). These verses are not a manual for building instruments, and nobody needs to read modern astronomy into them. But they help explain why generations of Muslim scholars treated watching the sky carefully as worthwhile work.

A woman astrolabe maker in Aleppo

One of the names that comes down to us from the tenth century is al-'Ijliyyah, the daughter of an astrolabe maker known as al-'Ijli. According to the bibliographer Ibn al-Nadim, she and her father learned the craft from a Baghdad instrument maker, and she worked at the court of Sayf al-Dawla, the first emir of Aleppo, who ruled from 944 to 967. Beyond that, very little is known about her life. Popular articles often call her "Mariam al-Asturlabi," but that first name does not appear in the only early source. In 1990 an asteroid discovered at Palomar Observatory was later named 7060 Al-'Ijliya in her honor.

Other famous scholars of the same era also worked closely with the sky. 'Abd al-Rahman al-Sufi (903-986) wrote the Book of the Images of the Fixed Stars from new observations in Isfahan, and al-Biruni (973-1048) wrote a major work on solar, lunar and planetary motion. (We wrote about al-Biruni measuring the Earth from a hilltop and al-Sufi's "little cloud," the Andromeda Galaxy earlier this month.)

A Persian astrolabe in Cambridge

Harvard has been collecting scientific instruments for teaching and research since 1672, and its Collection of Historical Scientific Instruments cares for more than 25,000 objects. Among them is a Persian planispheric astrolabe, inventory number DW0596, dated to about 1620. In 2017 it was the centerpiece of a Harvard exhibition called Syntax of an Astrolabe, which built large models of the geometry hidden inside the instrument.

The collection's permanent gallery, the Putnam Gallery, shows instruments that tell the story of science at Harvard, from sundials to 18th-century electrical apparatus. What is on display changes over time, so if you are going specifically to see an astrolabe, it is worth emailing the collection first to ask what is out. You can also look up objects in the collection's online catalog, Waywiser, before you go.

Planning a visit

  • Where: Harvard Science Center, 1 Oxford Street, Cambridge. The galleries are on the east side of the building, near the corner of Oxford and Kirkland Streets. The closest T stop is Harvard on the Red Line.
  • When: Sunday through Friday, 11 a.m. to 4 p.m. The collection lists closures for Nov. 25 and 26 (Thanksgiving), Dec. 24 to 27, and Jan. 1.
  • Cost: The Harvard Museums of Science & Culture list general admission to the collection as free.
  • Good to know: The galleries hold a maximum of 20 visitors at a time, and groups should register at least two weeks ahead. There is no eating or drinking inside. Personal photos are allowed, but no flash, tripods or selfie sticks.
  • Questions: chsi@fas.harvard.edu or 617-495-2779.

The gallery is small, which makes it a manageable outing with kids. Visits on Friday can be planned around Jumu'ah, since the gallery opens at 11.

Try it at home

You can feel the basic idea of an astrolabe without any brass. On a clear evening, pick a bright star and estimate how high it is with your fist held at arm's length (one fist is roughly 10 degrees). Note the time. An hour later, check again. The star will have moved by a steady amount, because the sky turns at a steady rate. That steady turning is exactly what astrolabe makers captured in metal, and what let people read the hour, and the approach of prayer times, from the sky.