1. How do we know the Earth is round?
Ships at sea
Stars change when you move north or south
Lunar eclipses
This was known in all civilizations
It is a myth that it was not known before Columbus

2. Does the Earth go around the Sun or the other way around?
Both are valid points of view
Geocentric reflects observations
Heliocentric can explain why

3. Orbit of the Earth around the Sun
Ecliptic plane
The axis is tilted 23.5° with respect to the perpendicular to the ecliptic plane
This is called the obliquity
If there was no tilt, the Earth would rotate straight up
The obliquity is not the angel between the axis and the plane (which is 66.5°)

4. Orbit of the Earth around the Sun 2

5. Orbit of the Earth around the Sun 3
Ellipse, but almost a circle
The Sun is not at the centre, but at one of the focuses
The eccentricity is a number between 0 and 1 that measures how elongated an ellipse is (e = 0 is a circle)
Ecliptic has nothing to do with the word ellipse, but comes from the word eclipse
Eclipses can only happen when the Moon crosses the ecliptic

6. Orbit of the Earth around the Sun 4
The Sun is not at the centre, but at a focus

7. Seasons Perihelion: The Earth is closest to the Sun around 4 January
Aphelion: The Earth is furthest from the Sun around 4 July
Perihelion and aphelion do not cause the seasons!
Amount of sunlight
Angle of the rays
Absorption from the atmosphere

8. Seasonal Markers Spring (or vernal) equinox 20-21 March
Summer solstice June
Fall (or autumnal) equinox September
Winter solstice December