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

2. The Celestial Sphere
The ancients thought the stars were attached to the side of a sphere, half of which could be seen as a dome overhead at any given time.

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

4. If we think about the way the night sky LOOKS to us, we can understand how ancient people assumed that earth was the center of our Solar System. We can use also this idea to better understand how astronomers map out our own, modern “Celestial Sphere.”

5. Diurnal Motion, Celestial Poles and The Celestial Equator
The spinning of the Earth makes the celestial sphere appear to spin, so as time goes by, all stars “move,” completing a circle every 24 hours…

6. Diurnal Motion They are called the North and South Celestial Poles.
…except the points directly above the north (and south) poles which do not appear to move. The sphere spins around them.
They are called the North and South Celestial Poles.
Half-way between the poles, above the equator, lies the Celestial Equator. It appears to spin the most.

7. The ecliptic is the slanted blue circle, while the celestial equator is the circle parallel to the earth’s equator.

8. Annual Motion
As the year passes, the direction to the Sun relative to the celestial sphere changes.
(Obviously, the stars in the direction of the sun cannot be seen as they are up during the day).
(Milky Way Timelapse video timelapse: desktop)

9. As the year goes by, the stars you see at night change.
The times that a star rises and sets changes as the year progresses, as well.

10. The Ecliptic
Because of the 23.5° tilt of the Earth’s axis, the path of the Sun in the sky does not follow the celestial equator. Instead, it is tilted 23.5° from it.
The Sun’s path is called the Ecliptic.

11. It crosses the Celestial Equator twice:
On about September 22nd, aka
the Autumnal or Fall Equinox
And on about March 21st, aka
the Vernal or Spring Equinox

12. From the Autumnal to the Vernal Equinox, the Ecliptic is south of the celestial equator…
… and from the Vernal to the Autumnal Equinox it is north of the celestial equator.