1. the Night Sky

2. Historically, patterns in the sky helped people navigate and keep time.
Note: Egyptians got it right 365 days, Greeks (e.g Hipparcus knew their stuff pretty well too)
Roman was off by 10 months long, 304 days! Fixed 713 BC, 355
Still off: Julius Caesar (leap years) 46 BC true year a bit shorter
Fixed in Pope Gregory XIII 1582 – skipped 10 days – rioting because people thought 10 days subtracted from their lives
Egyptian skychart

3. The Sky
                                        
                                      
Orion
Stars appear in fixed patterns called constellations. (Planets, “wanderers,” appear to move.)

4. What we see is their projection onto a virtual “Celestial Sphere.”
Note that the stars that make up constellations are at different distances.
                                                                  
What we see is their projection onto a virtual “Celestial Sphere.”

5. The Earth’s rotation makes the sky appear to revolve around us.
                                             
                                       

6. Polaris: does not appear to move because it's aligned with the Earth's axis.

7. Seasonal Constellations
Seasonal Constellations: stars closer to the equator will rise and set and we will see different constellations through the year.

8.                                               
Axial Tilt: the Earth’s axis is tilted 23.5o from the plane of the Earth’s orbit. So the Sun and planets appear to move along a tilted line in the sky

9. The Seasons
And when our hemisphere is tilted toward the Sun, we have summer; away, we have winter. Summer Solstice: Longest day (Sun at highest point) Winter Solstice: Shortest day Equinoxes: Equal Night/Day

11. Precession: Long Term Changes
Over 26,000 years the Earth’s axis moves in a complete circle. (Polaris was not always the pole star.)
                                                               

12. The Motion of the Planets
The planets wander on the sky along the ecliptic: mostly West to East but occasionally backwards – retrograde motion

14. Ptolemy
Ptolemy’s model (~140 CE) used 80 circles. -this didn't explain retrograde motion.
                                                

15.                                           
Heliocentric Model: Copernicus felt a Sun-centred system could explain the motion more simply.

16. Heliocentrism and Retrograde motion
                                                                                          

17. Galileo Galilei
Galileo: did not invent the telescope but was the first to study astronomy with it (1609).