1. Sun-Earth System

2. Daily Motions of the Earth
Precession is a gravity- induced, slow and continuous change in the orientation of a Earth’s rotational axis
The Earth's rotation axis is not fixed in space. Like a rotating toy top, the direction of the rotation axis executes a slow precession with a period of 26,000 years

3. Annual Motions
We see weather changes, day length changes, temperature changes as result of Earth’s motion around the Sun
The plane in which Earth orbits the Sun is called the ecliptic

4. Effects of Earth’s Tilt
Earth’s axis tilted to the ecliptic at degrees
As a result of the tilt of Earth’s axis and Earth’s motion around the Sun, the Sun changes its altitude in the sky
The Earth’s tilt is most directly responsible for the seasons that we see every year

5. As Earth moves around Sun, the altitude of the Sun decreases in the northern hemisphere
Summer solstice-Sun is at highest altitude in the sky (June 21); number of daylight hours at maximum
Summer solstice-Sun is directly overhead at Tropic of Cancer (23.5N latitude)
Winter solstice-Sun is at lowest altitude in the sky (Dec 21);number of daylight hours at minimum; Sun is directly overhead at Tropic of Capricorn (23.5S latitude)
Solstices

6. Equinoxes
Two positions Earth is not pointed at Sun; as result both hemispheres receive same amount of sunlight and Sun is directly overhead at the equator
Called autumnal and vernal equinox (equinox means “equal nights”)