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athymetry is the measurement of depth of water in oceans, seas or lakes. That strange-sounding science — bathymetry — is very important to your daily life, whether you are a lawyer, economist or construction worker. (Don’t worry, there’s a historical perspective here too, so stay tuned.)

For example, say you ordered an item on Amazon’s website. Your package may be stowed in a cargo ship that hunkers across the Atlantic Ocean, en route to you, and the boat will eventually need to dock at a port. Bathymetrists study water to make sure that mariners can dock safely at ports and that they don’t crash into icebergs like the Titanic did.

Back in the 12th century, no one was thinking about how Amazon would deliver their packages across a vast ocean. Yet, bathymetry experts (as well as recipients of Amazon packages) owe much to Muhammad al-Idrisi — often referred to as al-Idrisi — even if they’ve never heard of him.

An Arab-Muslim geographer, al-Idrisi, lived in Sicily during the 12th century.

While little is known about his childhood, al-Idrisi created the most accurate map of the known world of his day when he completed it in 1154. The geographer compiled a set of maps known as Tabula Rogeriana — translated from Latin as “The Book of Roger” — for Roger II, who was king of Sicily at the time.

Tabula Regeriana incorporates both cultural and political commentaries on the seven climate zones it represents — and does it in a way that hits the key cartographic points’ bulls-eye. To be sure, al-Idrisi did his research.

Al-Idrisi drew information for the maps he created from the geographic knowledge of traveling merchants and explorers, as well as his own knowledge. It took al-Idrisi 15 years to complete Tabula Regeriana — but the impact of his work echoed across the known world.

Christopher Columbus and Vasco da Gama — among others — referred to al-Idrisi’s works before they set sail. What made al-Idrisi’s work so valuable to geography wolverines?

For one thing, al-Idrisi was among the first to claim the world was round. According to al-Idrisi, waters adhered to its surface and were surrounded by a blanket of air. He even calculated the circumference of the Earth to be 22,900 miles, which was only off by eight percent from modern calculations. (The Earth’s circumference is actually 24,901 miles.)

He also corrected earlier mistakes about the location of the lakes from where the Nile started its journey, and its path. Originally, al-Idrisi visualized the cartographic knowledge he gained on a silver disk that measured 80-inches in diameter, weighed almost 300 pounds and had legends written in Arabic.

While the original disk-map seems to have been lost to history, al-Idrisi’s ideas have been preserved in Tabula Rogeriana. It didn’t take long for geography-hungry travelers to have Tabula Regeriana translated. And who could blame them? The volume consists of 70 small maps that were much better than any other maps of the time.

The original Tabula Regeriana has been copied many times. The most complete one — made in Cairo in 1553 — is at the Bodleian Library in Oxford, United Kingdom.

A few more words should be said about the importance of water, as recorded in Tabula Regeriana — and which were noted at the start of this article. In the volume, the word “water” appears 18 times. The sheer number of references to water in the book are intriguing, given that Roger II — the man who commissioned al-Idrisi to create the map for him — was primarily concerned with unifying the Norman territories (present-day southern Italy), primarily by making war.

But then, even warfighters need water.

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