How to determine your latitude by observing the stars.
The line NS is the Earth's axis: S is the South Pole, and the North Pole points toward N. E stands for either East or Equator. A is your position on the surface of the Earth. The Earth's axis points very close to Polaris, the North Star. If you look at Polaris, it is so far away that you are looking very nearly parallel to the Earth's axis. NE which is tangent to the circle at A represents the horizon, so angle NAP represents the angle above the horizon where one sees Polaris. That angle is always equal to the latitude at your position.Using this method, even the ancients could tell that the Earth is round. This explains how the Egyptians could have measured the circumference of the Earth very precisely.