• It’s a massive, imaginary sphere where all of the objects in the sky reside, with Earth
located at its center.
• When we peer into the night sky, the stars of constellations appear to be very close to one
another. In reality, they may be very far apart from each other and at greatly varying
distances from Earth.
• The stars are so far away, that as a result we lack depth perception when looking at them
in the night sky.
• The ancient Greeks mistook this illusion of proximity to be reality. They conceived that the
stars and constellations lied on a great celestial sphere that surrounds Earth.
• While only an illusion, this model is useful because it allows us to map the sky as we see
it from Earth.
North celestial pole
North celestial pole - The point
directly over Earth’s North Pole.
South celestial pole – The point
directly over Earth’s South Pole.
Celestial equator – A projection of
Earth’s equator into space,
completing circling the celestial
Ecliptic – The path the Sun follows
as it appears to circle around the
celestial sphere once each year.
South celestial pole