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In the 11th century, a brilliant Persian scholar named Abu Rayhan Al-Biruni (973–1048 AD) made one of the most astonishing scientific achievements of his time—calculating the circumference of Earth with remarkable accuracy. Unlike the Greek mathematician Eratosthenes, who estimated Earth's size using shadows, Al-Biruni used an entirely different approach based on trigonometry and a clever method involving a mountain.
The Experiment on a Mountain
Al-Biruni's method involved the following steps:
- Finding a High Mountain: He first climbed a mountain near Nandana (present-day Pakistan) to gain a clear view of the horizon.
- Measuring the Angle of the Horizon: He used a quadrant (a measuring instrument) to determine the dip of the horizon—the angle between the true horizontal line and the visible horizon.
- Applying Trigonometry: Using this angle, he applied trigonometric calculations to determine the radius of Earth. He assumed the Earth was a perfect sphere and used a formula based on the law of sines.
- Calculating Earth’s Circumference: Once he had the radius, he applied the standard formula for a sphere’s circumference: His result was about 39,721 km, which is very close to modern measurements (40,075 km)—an astonishing achievement considering he used only basic tools and no satellites!
Why Is This Important?
Al-Biruni’s work was revolutionary because:
- He was the first to measure Earth's size using a purely mathematical and observational approach.
- His method was independent of assumptions like flat Earth or mythical explanations.
- His accuracy was unmatched for centuries.
Modern Measurements of Earth’s Size
How Scientists Measure Earth’s Size Today?
Today, scientists use advanced technology such as:
✅ Satellite Geodesy – Using satellites like GPS, GRACE, and GOCE, scientists can measure the Earth’s shape and size with millimeter precision.
✅ Laser Ranging – Scientists fire laser beams at satellites and measure the time taken for the beam to return, allowing precise distance calculations.
✅ Astronomical Observations – Space probes and observations from the Moon or Mars help refine Earth’s size calculations.
Modern Earth Measurements:
- Equatorial Circumference: 40,075 km
- Polar Circumference: 40,008 km (Earth is slightly flattened at the poles due to rotation)
- Average Diameter: 12,742 km
- Radius:
- Equatorial radius: 6,378 km
- Polar radius: 6,357 km
Interesting Facts About Earth’s Size
🌍 Earth is Not a Perfect Sphere – It’s an oblate spheroid (wider at the equator than at the poles).
🌍 Fastest Spinning at the Equator – The equator moves at 1,670 km/h due to Earth’s rotation!
🌍 The Moon Helps Us Measure Earth’s Size – Scientists used lunar eclipses to determine Earth's diameter by observing its shadow on the Moon.
🌍 Earth’s Size Changes Slightly – Due to gravitational forces, tectonic movements, and climate change, Earth’s shape shifts over time.
🌍 If You Drive Around Earth – If you could drive nonstop at 100 km/h, it would take about 17 days to circle Earth’s equator.
🌍 Earth Compared to Other Planets:
- Earth is the 5th largest planet in the Solar System.
- Jupiter is 11 times bigger in diameter.
- Mars is only half the size of Earth.
Final Thoughts
Al-Biruni’s method, using simple trigonometry from a mountain, was one of the most remarkable scientific achievements of the medieval Islamic world. His results were only 0.9% off from modern values, showing how advanced his thinking was.
Today, satellites and space missions have refined our knowledge, but the foundation of Earth’s size measurement was laid over 1,000 years ago by scholars like Al-Biruni. His contributions are a testament to the power of observation, mathematics, and human curiosity.
