Celestial Sphere Revisited

The CS is an ancient concept - it's a crystal sphere that was imagined
to rotate around the earth. All of the stars were imagined to be fixed
it's surface. In this model, which is of course wrong, the sun travels
around - it traces an apparent path (the ecliptic). The path of the
sun goes higher after the winter solstice, following a direct
East-West path only on the day of the spring equinox. After that, it
keeps rising higher and higher until the summer solstice - longest day
of the year. And then, the path starts to become lower.

Mainly, the CS idea is useful because it allows us to visualize the
universe as a sphere - it's convenient, even though it's not correct.
We can imagine a "celestial equator," a circle that mimics the Earth's
equator. Plus, we can imagine the ecliptic - the path that the Sun
appears to take. These 2 circles intersect at two points, the
equinoxes. The ecliptic circle/path has its highest point at the
summer solstice and lowest point at the winter solstice.

So, even though the CS idea is false, it's super useful for
visualizing how weird the motion of the Sun (and planets, which move
along the ecliptic path) is, when viewed from around the Earth.