On this episode of Our Island Universe: Find out how the day to day movement of the sun, and the shadows it casts, can prove that the Earth is round and even tell you how big it is.
Shanil Virani, Director of the John C. Wells Planetarium Harrisonburg, VA.
Follow on Twitter as shanilv
Transcript:
The Sun reaches its highest point in the sky at noon. Hence the saying “high noon”. But to us here in the Valley, the Sun is never directly overhead at the zenith which is 90 degrees above the horizon! On the summer solstice, the Sun reaches about 80 degrees above the horizon. On the winter solstice, the Sun reaches about 25 degrees above the horizon. In fact, the Tropic of Cancer and the Tropic of Capricorn are defined to be those imaginary lines of latitude running across the surface of the Earth where the Sun is directly overhead at noon on the summer solstice and winter solstice, respectively. The equator, at zero degrees latitude and midway between the Tropics, is where the Sun is directly overheard at noon on the 2 equinoxes.
But that’s just an astronomical detail that isn’t really that important. Right? Well, at local noon, there is no shadow cast by an object when the Sun is directly overhead. Eratosthenes, a Greek who lived more than 2,000 years ago and invented the discipline of geography, used this simple observation to not only conclude that the Earth must be round, he was also able to estimate the size of our planet to better than a few percent accuracy! He did this because he knew that at local noon on the summer solstice in the ancient Egyptian city of Syene, located on the Tropic of Cancer, the Sun would appear directly overhead at the zenith. On the same day, at the same time, he measured the angle of elevation in Alexandria (~800km away) and found it to be about 7degrees south of the zenith. That is, on the same day and at the same time that there is no shadow in Syene, there is a substantial shadow in Alexandria! How could this be? Eratosthenes reasoned that the Earth could not be flat — it must be round. Moreover, by knowing the distance between these two locations, and the angular difference of 7 degrees, he calculated that that the Earth’s circumference must be approximately 40,000 km which is correct!
All because he paid attention to the Sun’s day-to-day motion. The shadows don’t lie.