Constellations

Celestial Sphere

Our Point of View

Angular Size and Distance

Relevant Geography

Earth’s Rotation

• 23 hours and 56 minutes relative to background stars (sidereal period)

• Spin axis of Earth defines points on the celestial sphere (using north and south poles) – North Celestial Pole

– South Celestial Pole

– Celestial Equator

• Sky appears to rotate east to west about the celestial poles because Earth rotates west to east.

Celestial Sphere

Motion on the Sky

Motion Depends on Declination

Circumpolar Constellations

• Circumpolar constellations never set.

• Circumpolar constellations change with latitude… sky changes with latitude

Tilt of Spin Axis

The Ecliptic

Ecliptic on the Celestial Sphere

The Equinoxes and Solstices

• Tilt of the spin axis is related to the seasons

• Effect of tilt on climate is not instantaneous

Precession of the Earth’s Axis

North Celestial Pole Changes