1. Seasons & Moon Motion

2. Two simple rotations around different axes and an observer oriented in a third direction
This explains a lot of features of the universe as observed in the sky!
Daily rising and setting
Different lengths of days (sun moving wrt cel.eq.)
Different altitude of sun at noon at different dates
Different altitude of sun at noon at different latitudes
Solar/sidereal day are different

3. Axis Tilt  Ecliptic
The Earth’s rotation axis is tilted 23½° with respect to the plane of its orbit around the sun
This means the path of the sun among the stars (called ecliptic) is a circle tilted 23½° wrt the celestial equator
Rotation axis pointing
to NCP, not SCP
Path around sun

4. Position of Ecliptic on the Celestial Sphere
Earth’s equator is tilted w.r.t. ecliptic by 23 ½ degrees
 Sun appears to be sometime above (e.g. summer solstice), sometimes below, and sometimes on the celestial equator
Skyglobe demo
7 visible “planets” incl. the sun and moon
Planet = “wanderer”
Days of the week named after the planets

5. The Seasons
Change of seasons is a result of the tilt of the Earth’s rotation axis with respect to the plane of the ecliptic
Sun, moon, planets run along the ecliptic
Equinoxes: day and night of equal length
Accounts for the N-S motion of the sun on the celestial sphere

6. The Seasons: Earth axis always points in the same direction
However, as Earth moves around the sun, the axis points sometimes towards and sometimes away from the sun.

8. The reason for the seasons

9. Daily path of the Sun in Winter and Spring

10. Daily path of the Sun in Summer and Fall

11. What changes?
Sun appears closer to the celestial north pole in the northern summer
Therefore it is higher in the sky
Therefore the sunlight hits us more directly
Therefore the energy deposit per unit area is higher
Therefore the temperature goes up
Therefore the sun is above the horizon longer
Therefore there is more sunlight per day

12. What does not change
The direction of the Earth’s axis (still points to the CNP)
The distance to the sun (there is a 1.7% change of the distance to the Sun, but we get closest to the Sun in early January!)
The fact that we are about an Earth radius closer to the Sun at noon in the summer is negligible (6300km compared to 150 mill.km is %)

13. Motion of the Moon
Moon shines not by its own light but by reflected light of Sun
 Origin of the phases of the moon
Moon revolves around the Earth
period of revolution = 1 month
Planets also shine by reflected light!

14. Phases of the Moon
The hemisphere of the moon facing the sun is ALWAYS bright
On Earth we see VARYING parts of this bright side

15. Phases of the Moon (cont’d)
Moon rotates around earth in one month
Moon rotates around itself in the same time
 always shows us the same side!
 “far side of the moon”
Unknown until 1960s

16. Does the Moon Rotate Around its Axis?

17. Motion of the Moon on the Celestial Sphere
Close to, but not quite on the ecliptic
The Moon’s orbit is inclined 5 degrees relative to the ecliptic, i.e. Earth’s (or Sun’s) orbit
Takes 1 month
(Sun: 1 year)

18. There are different definitions of “month”
Once around wrt what?
The stars (360, the real thing, sidereal month 27.3 days)
The sun (Full moon to full moon, synodic month 29.5 days)
Less relevant:
The spring point (tropical month)
Closest point to Earth (anomalistic month)
The ascending node (draconic month)

19. Eclipses Solar: Moon between Earth and Sun (S-M-E)
Shadow of Moon falls on Earth
Short (Minutes)!
Lunar: Earth between Sun and Moon
(S-E-M)
Shadow of Earth falls on Moon
Long (Hours)!