They then measured the differences between the tunes.

然后他们测量了曲调之间的差异。

If you measured the difference in fluid height directly, you'd get what's known as gauge pressure

如果直接测量高度差， 你会得到所谓的大气压。

The sun was not directly overhead, it was a slight angle, and all that Eratosthenes did was he measured the difference in the angle of the sun.

这是因为太阳不是在人们的正上方，它偏离了一个微小的角度。艾尔托斯蒂尼做的就是测量了各地太阳的不同角度。