DISCOVERY PASSPORT
✦ MATH STUDIO · SEVEN DISCOVERIES
Earth Detective: Measure an entire planet.
Travel to ancient Egypt. Use the Sun, two sticks and your own measurements to figure out how big Earth is. No answer is given before you experiment.
🧭 Explore by doing🌍 3D Earth🧪 No account needed
~ 240 BCEOne big question
How could Eratosthenes measure the world without ever travelling around it?
DISCOVERY 01
Explore our extraordinary planet
The globe and sun rays form an idealized geometry lesson. This simplified ancient setup is not a precise geographic survey.
Start by choosing both cities.
YOUR REAL EXPERIMENT
Catch the shadow
Move the ruler before recording. A measurement is your own estimate.
On this device
SCIENTIST'S NOTES
Why the method works
- Sunlight reaching the two cities is effectively parallel.
- A vertical stick points away from Earth's centre.
- At the June solstice at solar noon, the idealized Syene stick has no shadow. Alexandria's shadow angle matches the cities' central angle.
- One full circle contains 360 degrees. Multiply the distance between the cities by 360 divided by your measured angle.
Historical honesty and real-world limitations
The simulation intentionally places idealized Syene on the tropic and Alexandria 7.2° north on the same meridian. Real Aswan and Alexandria are not exactly on the same meridian, modern Syene is not exactly beneath the solstice Sun, and the exact length of an ancient stadion is disputed. Historical accounts report 5,000 stadia between the cities and circumferences of 250,000 or 252,000 stadia. The familiar 800 km and 40,000 km demonstration is a teaching approximation. The Sun's date/time model is simplified, not an astronomical ephemeris.
Try this outside with a stick and ruler only when supervised. Never look directly at the Sun or use glass to focus sunlight.