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

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

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°)

Orbit of the Earth around the Sun 2

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

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

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

Seasonal Markers Spring (or vernal) equinox 20-21 March Summer solstice 20-21 June Fall (or autumnal) equinox 22-23 September Winter solstice 21-22 December