Al-Biruni Measures the Earth's Radius

Date

c. 1017 CE

Category

Science, technology & invention

Region

Punjab (present-day Pakistan), South Asia / Central Asia

Era

Post-Classical / Medieval

Significance

While residing at the fortress of Nandana in Pind Dadan Khan (in the Punjab region of present-day Pakistan), the Persian polymath Abu Rayhan al-Biruni devised an innovative method to determine the radius of the Earth using a single mountain rather than the traditional approach of measuring distances between two distant points.

He first calculated the height of a hill by sighting its peak from two ground positions and applying trigonometry, then climbed to the summit and measured the angle of dip of the horizon below the horizontal. From these values he computed the Earth's radius at roughly 6,340 kilometres, remarkably close to the modern mean value of about 6,371 kilometres. The result, later recorded in his astronomical encyclopedia Qanun al-Masudi (the Codex Masudicus), stands as a landmark in medieval geodesy, though scholars note that his neglect of atmospheric refraction makes the apparent accuracy partly fortuitous. The technique exemplified the empirical, instrument-based science of the Islamic Golden Age and influenced later geographic and astronomical scholarship.

Empires active in c. 1017 CE

• Abbasid Caliphate
  750 CE – 1258 CE
• Song Dynasty
  960 CE – 1279 CE
• Byzantine Empire
  330 CE – 1453 CE
• Kanem-Bornu Empire
  700 CE – 1900 CE
• Liao Dynasty (Khitan Empire)
  916 CE – 1125 CE
• Fatimid Caliphate
  909 CE – 1171 CE

Related science, technology & invention events

• Ibn al-Haytham Completes the Book of Optics
  c. 1021 CE
• Ibn Sina completes the Canon of Medicine
  c. 1025 CE
• Al-Zahrawi completes the Kitab al-Tasrif, foundational surgical encyclopedia
  c. 1000 CE
• Legendary origin of smallpox variolation (inoculation) in East Asia
  c. 1000 CE
• Bi Sheng invents movable-type printing in Song China
  c. 1040–1048 CE
• Wujing Zongyao records the first gunpowder formulas
  1044 CE

Further reading

A Little History of Science

The wider story

A Little History of Science

William Bynum

Warm, jargon-free tour of discovery from Babylonian stargazers to the genome.

Get it on Amazon →

As an Amazon Associate, Human Saga earns from qualifying purchases.