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… and this is a biggie … 3) Precession of the Earth’s spin axis [text Sect.2.2] Since Earth’s shape is an oblate spheroid and it’s spin axis is tilted to the ecliptic (and also at ~5 O to the orbital plane of the moon) there are gravitational torques on the Earth due to the Sun & Moon (& the other planets) …eg. Sun trying to pull equatorial bulge into plane of the ecliptic … result is the direction of the spin axis moves around on a circular path every ~26,000 years “precession” (~50”/yr clockwise) Ian M. George PHYS 304 (2011 Fall) Meeting 04
Tilt angle stays (approx) constant as a result …. the North Celestial Pole moves in a circle (of “radius” ~23.45 O ) “around” the sky obviously the Celestial Equator also moves [Image Credit: Pearson Education] Ian M. George PHYS 304 (2011 Fall) Meeting 04
Page 1 and 2: Meeting #4 PHYS 304 Meeting 04
Page 3 and 4: [Image Credits: Stellarium] … rig
Page 5 and 6: …relationship between systems can
Page 7 and 8: … again its usually best to choos
Page 9 and 10: Origin: Sun “Special Direction #1
Page 11: … which corrections are really ne
Page 15 and 16: For the last few years… The preci
Page 17 and 18: … for ground-based observations
Page 19 and 20: A final problem for ground-based ob
Page 21 and 22: … even spin axis wobbles (v.sligh
Page 23 and 24: there are 24 hours in a “Day”,
Page 25 and 26: Additional Slide [not shown in clas
Page 27 and 28: Solar Day is longer than Sidereal D
Page 29 and 30: ut Solar Time is actually a bit tri

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