And 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 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”

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.

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 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

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

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.