Levi ben Gerson describes the Jacob’s staff, transforming how humans measure the sky

Story: 1320 C.E.  |  Last updated: January 1, 1320

In the rolling hills of Provence, a French-Jewish mathematician and philosopher sat down to write a book about astronomy, philosophy, and theology. Somewhere in those pages, Levi ben Gerson — known to medieval scholars as Gersonides — described a simple, elegant instrument that would change navigation and astronomy for centuries. He called it, in Hebrew, the “Revealer of Profundities.” His Christian contemporaries called it the Jacob’s staff.

What the evidence shows

• Jacob’s staff: Levi ben Gerson first described the instrument in his major work, Sefer Milhamot Hashem (“Book of the Wars of the Lord”), written in Provence in the early 14th century C.E. — making it one of the most significant scientific contributions of the medieval Jewish scholarly world.
• Levi ben Gerson: A polymath who worked in mathematics, astronomy, philosophy, and biblical commentary, he designed the staff to make precise angular measurements of stars and celestial bodies — a problem that had challenged astronomers for generations.
• Cross-staff origins: The invention was likely shaped in part by fellow Provençal Jewish astronomer Jacob ben Makir, and possible conceptual ancestors appear in Chinese and Chaldean sources — meaning the “Revealer of Profundities” emerged from a broader, cross-cultural current of scientific inquiry.

How the instrument worked

The Jacob’s staff, in its original form, was beautifully simple. A user placed one end of a long pole against their cheek, just below the eye. A shorter crosspiece — called a transom — could slide up and down the main staff. By aligning the top and bottom of the transom with two objects (say, the horizon and a star), the user could read the angle between them directly from the staff’s scale.

For astronomers, this meant measuring the angular distance between celestial bodies with unprecedented portability and ease. For navigators, it eventually meant determining a ship’s latitude by measuring the angle between the horizon and Polaris, the North Star, or the sun.

Ben Gerson’s version was designed for astronomical use. The idea that it could guide sailors at sea came later — the earliest confirmed maritime use appeared in João de Lisboa’s Treatise on the Nautical Needle of 1514 C.E., nearly two centuries after ben Gerson put stylus to parchment.

A tool born from a community of scholars

It matters that ben Gerson did not work in isolation. Medieval Provence was home to a vibrant community of Jewish scholars who translated Arabic and Greek scientific texts into Hebrew and Latin, passing ancient knowledge into European hands. Jacob ben Makir — also known as Profatius Judaeus — worked in the same intellectual milieu and may have contributed foundational ideas that ben Gerson refined.

Further afield, the British sinologist Joseph Needham argued that Song dynasty Chinese scientist Shen Kuo described a similar instrument in his Dream Pool Essays of 1088 C.E. — more than two centuries before ben Gerson. Needham noted that Shen examined an ancient crossbow-like device unearthed in Jiangsu province and recognized its graduated sight could measure the heights of distant mountains using the same geometric principles.

Whether this constitutes an independent parallel invention, a precursor, or something else entirely remains a matter of genuine scholarly discussion. What is clear is that the desire to measure the sky — precisely, reliably, and with simple tools — emerged in multiple cultures across centuries, driven by needs in agriculture, religion, and travel that all humans share.

Lasting impact

The Jacob’s staff seeded a navigational revolution. As European maritime exploration expanded in the 15th and 16th centuries C.E., the cross-staff became standard equipment on ships venturing into open ocean. It helped sailors track latitude far from shore — a capability that underpinned the so-called Age of Discoveries and reshaped global trade routes.

The instrument’s core geometric logic — using a sliding crosspiece on a calibrated rod to measure angles — fed directly into the development of more sophisticated tools. The sextant, which replaced the cross-staff and its successors by the 18th century C.E., uses the same underlying principle of measuring the angle between two objects, just with mirrors and a curved arc instead of a sliding transom. Modern GPS may have retired the sextant from routine use, but the mathematical logic runs in a direct line from ben Gerson’s “Revealer of Profundities.”

In surveying, the name “Jacob’s staff” survives to this day — now referring to the simple pole used to support a surveyor’s compass or theodolite in the field. The name outlasted the instrument’s original form by more than 600 years.

Ben Gerson’s contributions to astronomy went further still. He catalogued star positions, devised improvements in astronomical tables, and made careful observations of the moon. His work was read and cited by later European astronomers, including Copernicus and Kepler, whose own revolutions in understanding the solar system built on a long chain of prior observation.

Blindspots and limits

The original Jacob’s staff was genuinely difficult to use with precision. Looking directly at the sun to measure its altitude was uncomfortable and potentially damaging to a navigator’s eyesight — a limitation that prompted sailors to mount smoked glass on the transom ends. Despite its elegance, the instrument was eventually superseded because the backstaff and later the octant were simply easier and safer to handle at sea, even if some researchers found the cross-staff more accurate under ideal conditions.

And the record is patchy. We know what ben Gerson wrote, but the full story of how the instrument spread, who improved it, and how much was lost or reinvented along the way is harder to reconstruct. The contributions of oral and practical traditions — the sailors and surveyors who adapted the staff through daily use — went largely unrecorded.

Read more

For more on this story, see: Wikipedia — Jacob’s staff

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