More information on the figure.

Added the path of the sun on the ecliptic, along with important
positions such as solstices and equinoxes.
Figure for Chapter 2.16.

Made a figure illustrating parallax measurements for a section in
chapter 2.
Figure for showing spherical triangles.
New figure.

Showing different circles on a sphere, such as great, small and vertical
circles.
Updated with new figures.
Added some figures and wrote some text.

Added a figure for the ecliptic, and did some changes in the text. Also
added a figure for parallax angles and added a small amount of text.
Added a few figures, otherwise small changes.