Transcript The Celestial Sphere

The Night Sky
And The Celestial Sphere
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.
Diurnal Motion, Celestial Poles and
The Celestial Equator
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”
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 spins the most.
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.
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.
It is tilted 23.5° from it.
The Sun’s path is called the Ecliptic
It crosses the Celestial Equator twice:
On about September 22nd,the Autumnal or Fall
Equinox
And on about March 21st, the Vernal or Spring
Equinox
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.