How deep is your well ?

Eratosthenes knew that there was a deep well in Syene in Egypt.
People walked down circular steps to get into the well to get water.
It was very dark on the steps.
But on one day a year (June 21st),
the sunlight at noon shone all the way down to the bottom of the well.

Eratosthenes went to another city further north in Egypt — Alexandria.
On that same day of the year, sunlight did not reach the bottom of wells.
And he noticed that his shadow was “longer.”

Eratosthenes observed the shadow of a tall obelisk in Alexandria.
He measured the angle of the shadow from the top of the obelisk.
The angle was 7.12 degrees.
The angle of shadows was 0 degrees in Syene.
Eratosthenes divided the degrees difference in a whole circle (360 degrees).
He found there were 50 pieces of “pie.”
The distance between Syene and Alexandria was about 1/50th of a circle.
1/50 of the circumference of the earth.

The migration of holes is also depicted in the cartoon shown in Figure 3.



Here, the chairs represent one-electron bonds,
and the students represent the additional electrons
that "migrate" to complete unsatisfied one-electron bonds.