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Department
Astronomy & Astrophysics
Course
AST201H1
Professor
Michael Reid
Semester
Winter

Description
AST201H-­‐  Stars  and  Galaxies         Professor  Michael  Reid                                                                                                                            Lecture  Notes     January  15,  2013   Lecture  3:  Gravity  and  Light   Review:  Kepler’s  First  Two  Laws  of  Planet  Motion   • The  orbit  of  each  planet  around  the  Sun  is  an  ellipse  with  the  Sun  at  one  focus,   nothing  at  the  other  one.     o *not  orbiting  around  the  Sun,  but  orbiting  around  the  center  of   mass   • As  a  planet  moves  around  its  orbit,  its  sweeps  out  equal  areas  in  equal  times.   When  the  objects  are  closer,  the  force  of  gravity  is  stronger,  therefore,  planets   move  faster  when  it  is  closer  to  the  sun.   Kepler’s  Third  Law   • Consider  Merry-­‐Go-­‐Around,  solid  body  rotation  –  everyone  takes  the  same   length  of  time  to  complete  one  turn  whether  it  is  on  the  inside  or  outside     o Depending  on  the  orbital  period  and  orbital  semi-­‐major  axis     o Solid  Body  Rotation  –  means  every  object  takes  the  same  length  of   time  to  complete  one  orbit  turn   o   • Logically,  the  further  the  planet  is  from  the  Sun,  it  takes  longer  time  for  that   planet  to  complete  one  circle  around  the  Sun  –  but  Planet  Motion  is  NOT  solid   body  rotation     •                                                   AST201H-­‐  Stars  and  Galaxies         Professor  Michael  Reid                                                                                                                            Lecture  Notes     • *Kepler’s  Third  Law:  Orbi
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