The Celestial Sphere

1. The Celestial Sphere (The sphere should really be rotating, not Earth)

2. An Ancient Greek Idea • Imagine the universe to be a hollow sphere with the Earth at the center • The Celestial Sphere • Idea based upon how things _________ to be • The pattern of stars in the sky ________________ • Stars ‘embedded’ on the _________ • Pretend the stars are painted on a giant ‘bubble’ around Earth • Geocentric (________________) model appear does not change inside Earth centered

3. Celestial Sphere: Extension of the Earth’s Coordinate System • Celestial sphere • N/S celestial poles • ____________ • Celestial equator • ____________ • Declination • ____________ • Right Ascension • ____________ N/S Poles Equator Latitude Longitude Like a clear bowl over your head!

4. The Celestial Sphere NCP Earth’s spin axis The celestial sphere Earth Celestial equator SCP

5. NCP Celestial Sphere SS AE Path of Sun during the year Earth e Celestial equator SE WS Ecliptic SCP Ecliptic = apparent annual path of Sun on the CS Ecliptic (2)= _____________________________________________

6. Ecliptic Sun’s Path Celestial Equator Declination (degrees) Right Ascension (hours) North East South

7. Key terms • Celestial sphere: • _________ sphere of the sky (what it looks like) • Celestial poles joined by line that runs through the Earth’s north and south poles (this line is, in fact, the Earth’s spin axis) • NCP is directly above _______________ • Point about which the stars appear to rotate • Celestial equator: • ___________ of Earth’s equator onto the CS Apparent Earth’s North Pole Projection

9. Apparent motion: Earth’s rotation • As Earth ________, it looks as if the celestial sphere is _________ around us • In the north, the center of the rotation is the _______________________________ • In the south, the center of the rotation is the _______________________________ • The celestial sphere appears to rotate once every _________ rotates rotating North Celestial Pole - Polaris South Celestial Pole – No pole star 24 hours

10. Where on Earth • Definition: • Latitude (l): The angle between the Observer, the center of the Earth and the Earth’s equator (Measured in degrees) l = 0o (Equator) l = 90o (North pole) l = -90o (South pole)

11. (l = 90o) North pole zenith observer horizon l Equator (l = 0o) Earth South pole (l = -90o)

12. The latitude of an observer is equal to the altitude of the visible celestial pole To NCP • latitude of observer = l • altitude of NCP = A zenith B A l B = 90o - l To CE A + B = 90o l horizon Earth

13. The final steps:(hold tight) • By definition: angle A + angle B = 90o • But, angle B = 90o - l, so • A + 90o - l = 90o And hence, A = l

14. The Celestial Sphere

15. Precession In addition to its rotation and revolution, the earth’s axis also__________(wobbles) like a top. The angle between the ecliptic and the equator remains at 23.5°, but the direction changes. The period of thisprecession of the equinoxesis about ____________. precesses 26,000 years

16. Because of precession: • Polaris won’t always be the North Star • Orion won’t always be a winter constellation • Horoscopes are all wrong!