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

2. The Celestial Sphere
The ancients thought the stars were stuck in a sphere
half of which could be seen as a dome overhead at any given time
This they called the celestial sphere.
The celestial sphere does not actually exist
But it is still a useful way to think about the sky.

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

4. 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.
Half-way between the poles, above the Equator, lies the Celestial Equator
it spins the most.

5. 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.
Thus 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.

6. 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.
It is tilted 23.5° from it. The Sun’s path is called the Ecliptic

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

8. From the Autumnal to the Vernal Equinox it is South of the celestial equator
From the Vernal to the Autumnal Equinox it is North of the celestial equator.