1. And The Celestial Sphere
The Night Sky
And The Celestial Sphere

2. The Celestial Sphere
The ancients thought the stars were stuck into a sphere, half of which could be seen as a dome overhead at any given time.
They called this the celestial sphere.

3. The celestial sphere does not actually exist, but it is still a useful way to think about the sky.

4. Diurnal Motion
The spinning of the Earth makes the celestial sphere appear to spin. As time goes by, all stars move, completing a circle every 24 hours.
“Diurnal Motion”

5. The Celestial Poles and Equator
The two points directly above poles do not appear to move.
The sphere spins around them.
They are called the North and South Celestial Poles.

6. Half-way between the poles, above the equator, lies the Celestial Equator.

7. Annual Motion
As the year passes, the direction to the Sun relative to the celestial sphere changes.
The stars in the direction of the Sun cannot be seen as they are up during the day.

8. As the year goes by, the stars that you can see at night change …
And the time that a star rises and sets changes as the year goes by, too.

9. The Ecliptic
Because of the approximate 23.5° tilt of the Earth’s axis, the path of the Sun in the sky does not follow the celestial equator. (Its path is at a 23.5° tilt from it.)
The Sun’s path is called the Ecliptic.

10. The Ecliptic crosses the Celestial Equator twice: On about September 22nd, also known as the Autumnal (or Fall) Equinox...

11. And on about March 21st, known as the Vernal (or Spring) Equinox.

12. From the Autumnal to the Vernal Equinox the Sun is south of the celestial equator.
From the Vernal to the Autumnal Equinox it is north of the celestial equator.