# 7/Historical Note

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## Historical Note on $7$

The most obvious contemporary social significance of the number **$7$** is the number of days in the week, which may have originated from the fact that it corresponds approximately with the number of days between specific phases of the moon.

### Rational Diagonal

The number $7$ (seven) was referred to by the ancient Greeks as the **rational diagonal** of the $5 \times 5$ square.

This was on account of the fact that:

- $5^2 + 5^2 = 50 \approx 7^2 = 49$

## Sources

- 1986: David Wells:
*Curious and Interesting Numbers*... (previous) ... (next): $7$ - 1997: David Wells:
*Curious and Interesting Numbers*(2nd ed.) ... (previous) ... (next): $7$