[thedoc (OP)]

Greigan Cunningham Douglas asked the Naked Scientists:
   
My half brother watched a video on 200 facts on how the earth is flat and it's impossible to argue with him even if you give him facts but what proof is there that the earth is round and what makes it a fact that the earth is round?

What do you think?

[alancalverd]

1. Go anywhere on the planet and drop something. Since gravity always acts downwards on the surface of the earth, and towards the centre of any mass in the laboratory, the earth cannot be any other shape than spherical. Use brain.

2. If the earth is flat, it must have an edge beyod which you cannot go.  Go to the edge and tell us what you see when you get there. Use telephone.

[RD]

... what proof is there that the earth is round ...

The face of the moon appears rotated 180^o when viewed from different hemispheres,
 see ... http://www.primaryhomeworkhelp.co.uk/moon/hemispheres.html
That would not be the case if all the countries were on the same face of a flat earth.

At sunrise & sunset an observable shadow is created by the curvature of the Earth...

http://www.atoptics.co.uk/atoptics/earshad.htm

A distant ship's masts being visible before the hull appears shows the Earth is curved.
[ If the flat-earthers claim that's due to waves, point out that a wave as high as a ship's mast would be a tsunami ].

[Colin2B]

If you measure the evelvation of the pole star and then move north or south a set distance, measure it again, repeat this a few times. If you then work out the triangles created you will find that only a curved surface will create them. Work out what that curved surface would be and it is a sphere.

[chiralSPO]

On a clear night one can take a time lapse photograph, and clearly see that the stars all appear to rotate about a single point in the sky. This point changes based on the latitude of the photographer.

I have attached such images that I found online, one taken in Scotland, where the center of the rotation is pretty much in the middle of the sky, one from Joshua Tree Park in the Southern US, where the center is closer to the horizon, and one from Ecuador, which is essentially at the equator, where there appear to be two centers (one on the Northern horizon, and one on the Southern horizon). This is very easily explained qualitatively by a spherical Earth rotating on an axis (and can even be explained quantitatively by comparing the relationship between the latitude and the angle between the horizon and the center of rotation).

[Colin2B]

You can take what ChiralSPO is saying a stage further. If you have friends around the world you can Skype them and discuss/photo what they are seeing in the sky and compare at say 4 hr intervals.

[RD]

On a clear night one can take a time lapse photograph, and clearly see that the stars all appear to rotate about a single point in the sky. This point changes based on the latitude of the photographer ...

I'd never seen an equatorial star-trail photo before.

[RD]

... If you have friends around the world you can Skype them and discuss/photo what they are seeing in the sky ...

Or ask them what is the local time ...

[Bored chemist]

Commercial air flight timetables only make sense if you have a round Earth.

[alancalverd]

"Actual flight times may vary". Do they know something we don't?

And when I'm President of Earth, the time everywhere will indeed be the same. None of this nonsense about arriving in London before you leave Paris.