History of Angles

It all starts with the circle.

In the circle below, a line segment starts at the center of the circle and ends on the edge.

This distance from the center to any point on the edge of the circle is the radius. Notice the little “r” above the line that stands for radius.

A circle with two radii lines has some space between the two lines. The measurement of that space is an angle.  It is measured in degrees.

Vocabulary: Read the definitions for the following words.

King Nebuchabnezzar of ancient Babylonia lived from 605-562 BCE.  During this time, the measurement of the circle was established to be 360 degrees.

One explanation for dividing the circle into 360 is that the Babylonians calculated the earth going around the sun in 360 days.

In modern times, angles may also be measured in radians.

We will only be practicing with degrees in this chapter.

Euclid wrote about angles.  He wrote about the following observations.

1.)  All right angles are congruent.

This is easy.

2.)  If two lines intersect a third line in such a way that the sum of the inner angles on one side is less than two right angles (180°), then the two lines must insect each other on that side if extended far enough.