For many centuries, it was a very difficult task to measure accurately the geographical longitude.
In order to measure our own (local) longitude, we must use the Sun. Before doing so, however, we must discuss the concept of time indication that is known as "Local Noon".
"Local Noon" occurs when the Sun is seen exactly in the South direction.
Strictly speaking, "Local Noon" is the exact time when the Sun reaches its highest point in the sky. This is also the time that it crosses the vertical, imaginary line, that astronomers call the "meridian".
1. The above drawing shows the path of the Sun in the sky on a certain day and at a certain geographical location during wintertime. When does the Sun rise? When does the Sun set ?
2. The half-way point between sunrise and sunset indicates the middle of the day, that is "Local Noon". Calculate this average from the drawing above. What is the time of Local Noon in the situation above ?