Unit 1 Lesson 1: The Celestial Sphere

From Earth, the sky looks like a 2D dome, not a 3D starscape

The Celestial Sphere is the name of the imaginary dome of the sky that completely surrounds the Earth

We can only see half of the dome at one time because the horizon blocks the rest

Horizon: point where the earth and sky meet

Constellations: stars that form fixed patterns on the celestial sphere

All stars move, but as seen from Earth, it is very slowly

It can take 10s to 1000s of years to make a noticeable shift

The names of most constellations are unknown in origin (no idea who named them or why)

Polaris, the North Star, sits quite near the celestial north pole

Polaris was not always the North Star and will eventually be replaced as the stars move across the sky

Rotation: One complete spin around the axis (24 hours for Earth)

Revolution: One complete trip around the star (365.25 days for Earth)

If we were to mark on the celestial sphere the path that the Sun takes as it moves through the constellations we would get a line around the celestial sphere, this line is called the ecliptic

 The region of the sky surrounding the ecliptic passes through 12 constellations called the zodiac