1. The Night Sky…last time, The Horizon System
The Sun rises in the east, transits, sets in the west.
The Moon….
The stars ….
The moon changes position against the stars from night to night
The sky changes with the seasons (“parade of the constellations”) ?

2. “Different Seasons, Different Constellations”
The Earth orbits the Sun
We see the Sun projected against background stars
Earth moves in a plane, Sun projected onto a line on the sky

3. The Moon and the major planets also move along the celestial line (ecliptic or zodiac)
This demonstrates and important fact: major solar system bodies are orbiting the Sun in the same plane
Astronomical basis for the origin of astrology

4. Seasonal Variations in the Position of the Sun (back in horizon system)

5. Explanation of Seasonal Variations: tilt of the Earth’s axis: obliquity of the ecliptic

6. Obliquity of the Ecliptic and the Altitude Angle of the Sun

7. Two Lines on the Sky
The ecliptic
The celestial equator
See Figure 2.4

8. Conclusion: seasons are due to tilt of Earth’s axis and motion of Earth about the Sun
Not changing distance between the Earth and Sun

9. A New Coordinate System: Celestial Coordinates
The stars “stick together” and define their own reference system. The planets move with respect to them
Celestial coordinates are Right Ascension and Declination
Right Ascension ….. Longitude
Declination ….latitude
See SC1 chart for illustration

10. Map of the Sky