How do we know the Earth is round

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 20-21 June
• Fall (or autumnal) equinox 22-23 September
• Winter solstice 21-22 December