1. How do we Know the Earth is Round?

2. Apollo Photos In 1972 Apollo 17 astronauts took a photo of the earth.
These photos, as well as orbiting astronauts, satellites, space stations, global exploration, understanding of astrophysics, gravity, etc., make it very clear that the earth is round.
This followed a similar photo taken by Apollo 8 astronauts in 1968 showing a round earth rising beyond the moon

3. Evidence that the Earth is Round
Eratosthenes experiment
Ships and buildings on horizon
Seeing farther from higher
Lunar eclipses
Different constellations
Everyone else is round...
Circumnavigation & exploration

4. ‘the heck does that mean?
The Earth is Curved!
On a flat model (or flat earth), both sticks can show no shadow at the same time.
“Parsimonious”, the principle of parsimony or Occam’s Razor argue the simplest explanation is usually true…
‘the heck does that mean?
A curved earth is the parsimonious way to explain how you would see a shadow in one place and no shadow in another on the same day.

5. … concave or convex? How do you tell the difference?
Of course, the earth could curve in a convex manner like the picture on the left or it could curve in a concave manner like the picture to the right. Both would result in a shadow in one spot and no shadow in the other.
How do you tell the difference?

6. … concave or convex? How do you tell the difference?
The direction of the shadow is important. If the shadow points towards the other city, a concave earth makes sense, if it points away, a convex.
In the real city of Alexandria, the shadow points away from Syene, indicating a convex earth.

7. Evidence that the Earth is Round
Eratosthenes experiment
Ships and buildings on horizon
Seeing farther from higher
Lunar eclipses
Different constellations
Everyone else is round...
Circumnavigation & exploration

8. Obscured Bottoms…
When we look at distant objects like ships and tall buildings (like these in Chicago across Lake Michigan) and cannot see the bottom due to the curvature of the earth.

9. Evidence that the Earth is Round
Eratosthenes experiment
Ships and buildings on horizon
Seeing farther from higher
Lunar eclipses
Different constellations
Everyone else is round...
Circumnavigation & exploration

10. On a Flat Earth…
Two observers--one on top of a mountain, and the other at the bottom, would experience the sunrise at the same time.
6:08am

11. 6:08am On a Spherical Earth…
The mountaintop observer sees the sun sooner than the observer at the ground. She sees the sun when it becomes tangent to her– well before it becomes tangent to the observer lower down.
6:08am

12. 6:12am On a Spherical Earth…
The mountaintop observer sees the sun sooner than the observer at the ground. She sees the sun when it becomes tangent to her– well before it becomes tangent to the observer lower down.
6:12am

13. Where does the Sun rise first in the US?

14. West Quoddy Head, Maine? Elevation 30’ Easternmost point in contiguous US.

15. Cadillac Mountain Bar Harbor, Maine
Cadillac Mountain Bar Harbor, Maine? Elevation 1529’, slightly west of West Quoddy

16. Where does the Sun rise first in the US?
On a Flat Earth…
There is no debate. It is West Quoddy Head– the easternmost point in the contiguous US.
On a Spherical Earth…
Cadillac Mountain has a chance due to its higher elevation.

17. Two Sunsets? Just Gain Elevation!

18. Ramadan
Ramadan is a Muslim commemoration of the revelation of the Quaran to the prophet Muhammed. For a lunar month from waxing crescent to waxing crescent, observers do not eat or drink during daylight hours.

19. Ramadan Fasters Wait…
On the Burj Khalifa, the world’s tallest building, the sun sets about two minutes later than it does on the ground.
Muslims clerics declared that Ramadan fasters must wait a little longer to end their fast from this building.

20. Evidence that the Earth is Round
Eratosthenes experiment
Ships and buildings on horizon
Seeing farther from higher
Lunar eclipses
Different constellations
Everyone else is round...
Circumnavigation & exploration

21. Lunar Eclipses
Pythagoras was the first person who documented this evidence of a round earth around 570 BC!
LunaL
During a lunar eclipse, the moon turns this amazing blood-red color.
Lunar eclipses occur because the earth passes between the moon and sun blocking nearly all of the the suns light from reaching the moon
As the eclipse transpires, the shadow of the earth passes across the moon, and it is round…

23. Evidence that the Earth is Round
Eratosthenes Experiment
Ships and buildings on horizon
Seeing farther from higher
Lunar eclipses
Different constellations
Everyone else is round...
Circumnavigation

24. North Celestial Pole
South Celestial Pole

25. We see different constellations from different spots on the earth…
As you move around the planet you either see the same constellations from different perspectives, or COMPLETELY DIFFERENT CONSTELLATIONS. This makes sense from a spherical earth and less-so from a flat one.
We also see the constellations spinning differently from different spots. This motion is EASILY explained from a sphere.

26. On a flat earth, one would expect to have the entire world see the same stars.

27. On a spherical earth, it makes sense that we see different stars based on where you are…

28. Different Constellations 4:40

29. Evidence that the Earth is Round
Eratosthenes experiment
Ships and buildings on horizon
Seeing farther from higher
Lunar eclipses
Different constellations
Everyone else is round...
Circumnavigation & exploration

30. We are either part of the club or not…
All the other objects we can see in our solar system appear spherical.
Either we are the great exception, center of the universe with different physical laws and all, or we are like everyone else.
The more parsimonious explanation is we are a sphere too.

31. Spheres and not discs?
But wait, all the objects in our solar system could be flat discs and not spheres right?

32. Spheres and not discs?
For this to be the case, they would have to be oriented at right angles to all observers!

33. Spheres and not discs?
Otherwise, the discs would appear as ovals, not circles.

34. Spheres and not discs?
Spheres look like circles regardless of the observer’s position

35. Spheres or Discs? The Moon

36. Plus… we see phases of the moon and some planets
Plus… we see phases of the moon and some planets. This too makes sense for spheres alone.
Our Moon
Venus

37. Evidence that the Earth is Round
Eratosthenes Experiment
Ships and buildings on horizon
Seeing farther from higher
Lunar eclipses
Different constellations
Everyone else is round...
Circumnavigation & exploration

38. 1519 Magellan Circumnavigates the Globe (Sort-of)

39. Flat Earth Map
Modern “flat earthers” claim the earth is a large disc with a wall of ice keeping the oceans in place.
Among other things, like Australia being over a thousands miles larger than the US on this map even though they are about the same width, this means there is no south pole.

40. No Fram, No Amudsun, No Scott…
In 1911, Norwegian explorer, Roald Amudsen led the first expedition to the south pole aboard the Fram.
Several months later, British explorer, Robert Scott make it to the pole then died on the return.

41. Amudsen-Scott South Pole International Research Station
Today, there is an international research station at the south pole. They have a freaking webcam
Either all this is a huge international conspiracy for a very unclear reason, or, parsimoniously, the earth is a sphere!

42. Evidence that the Earth is Round
Eratosthenes Experiment
Ships and buildings on horizon
Seeing farther from higher
Lunar eclipses
Different constellations
Everyone else is round...
Circumnavigation & exploration