Tech Story #14 · Golden Age G7: al-Biruni measured the Earth from a mountain
About the 1020s, al-Biruni measured the Earth's size with angles from a mountain top, within about 1 % of today's 6,371 km radius — the maths behind maps and GPS.
How do you measure the whole Earth… from one mountain? 🌍⛰️
📜 Meet al-Biruni (973 – about 1050): born in Khwarezm (today Uzbekistan), worked in Ghazni.
📐 Step 1: he found how tall a hill was by sighting its top from two spots on the plain.
🔭 Step 2: from the top he measured how far the horizon DIPS below the level line — only about half a degree!
🧮 Height + that tiny angle + some trigonometry = the size of the Earth.
🤔 How close did he get?
✅ About 6,340 km for Earth's radius. Today: 6,371 km. Within about 1 %!
🛰️ Maps and GPS still need Earth's size today.
Read the voiceover (transcript)
- How do you measure the whole Earth?
- Al-Biruni did it, about a thousand years ago!
- First, he worked out how tall a mountain was.
- Then, from the top, he measured how much the horizon dips.
- How close did he get?
- Within one percent of today's value!
- Maps and GPS still need that number.
- Next: the woman who built star computers. Follow for more!
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