ASTR 1P01 Chapter Notes - Chapter 2: Precession, Thuban, Drawing Pin

• School: Brock University
• Department: Astronomy
• Course: ASTR 1P01
• Professor: Bozidar Mitrovic

Document Summary

Over 2000 years ago hipparchus compared a few of his star positions with other astronomers and realized that celestial poles and equator were slowly moving relative to the stars. Later they understood this motion was caused by a special motion of earth called precession. The slow change in orientation of earth"s axis rotation, one cycle taking. Earth spins like a giant top but does not spin upright relative to its orbit around the sun. Earth"s axis is tipped 23. 4 degrees from vertical - earth"s equator is tipped 23. 4 degrees relative to its orbit. Earth"s large mass and rapid rotation keeps it axis of rotation pointed towards the star polaris. Axis of rotation would not change direction if earth was a perfect sphere. Gravity of the sun and moon both pull on the bulge tending to twist earth"s axis upright relative to its orbit. Due to all these forces and earth"s rotation, earth"s axis precesses at a slower circular sweep.