Secret Codes and Measuring the Earth — al-Kindi + al-Biruni

What you'll learn

• al-Kindi (about 801–873) worked in Baghdad. His Manuscript on Deciphering Cryptographic Messages (about 850) is the oldest known written description of breaking codes by counting letters (frequency analysis).
• A simple code swaps every letter for another (here every letter moves 4 places: THE MESSAGE → XLI QIWWEKI). In English, E is the most common letter, so the most common letter in the code is probably E. Swap it back, then the next most common, and the message appears.
• Today's apps use maths with huge secret numbers (modern encryption), so counting letters can't crack them.
• al-Biruni (973 – about 1050) was born in Khwarezm (now Uzbekistan). He found a mountain's height from two spots on the plain, then from the top measured how far the horizon dips below a level line (about half a degree).
• Because the Earth is round, the dip and the height give the Earth's size: his radius ≈ 6,340 km, today's 6,371 km — within about 1 %.
• Your phone's GPS turns satellite signals into your spot on the map using the Earth's exact size and shape.
• Episode 3's challenge: each pin spot has 2 choices, so 2 × 2 × 2 = 8 patterns.