How did ancient philosophers deduce the size and shape of the Earth?

  • 4 Replies
  • 1407 Views

0 Members and 1 Guest are viewing this topic.

*

Offline chris

  • Neilep Level Member
  • ******
  • 5392
  • The Naked Scientist
    • View Profile
    • The Naked Scientists
Ancient thinkers knew the Earth was round and they also had a concept of how large it is. What methods did they employ to achieve these findings?
I never forget a face, but in your case I'll make an exception - Groucho Marx

*

Offline Colin2B

  • Global Moderator
  • Neilep Level Member
  • *****
  • 2031
    • View Profile
I read somewhere that they used height of masts on water, but before that when building wells they noticed that different stars could be seen from widely spaced wells. That told them they were not looking up parallel but at diverging angle, rest is geometry.
and the misguided shall lead the gullible,
the feebleminded have inherited the earth.

*

Offline PmbPhy

  • Neilep Level Member
  • ******
  • 2794
    • View Profile
Ancient thinkers knew the Earth was round and they also had a concept of how large it is. What methods did they employ to achieve these findings?
It was assumed to be spherical due to the shadow the Earth cast on the moon when the Earth eclipsed the moon. To determine it's radius, one experiment utilized poles. Each pole was constructed very straight and were erected precisely vertical. Then at the same time of the day they took measurements of the shadows the poles cast and from those measurements they could get a rough estimate of the radius of the Earth.
« Last Edit: 14/06/2015 18:22:19 by PmbPhy »

*

Offline David Cooper

  • Neilep Level Member
  • ******
  • 1505
    • View Profile
There was one particular well in which the sun could be seen being reflected back up at midday in the middle of summer. Further north such a thing never occurred. Someone, I forget who but it's discussed in the first edition of Karl Sagan's series Cosmos, went several hundred miles further north to measure the angle of the sun at midday at the same time of year and came up with a very accurate calculation of the size of the Earth.

*

Offline chiralSPO

  • Global Moderator
  • Neilep Level Member
  • *****
  • 1916
    • View Profile
https://en.wikipedia.org/wiki/Eratosthenes

Erastosthenes determined the radius of the Earth by the method described by PmbPhy: by measuring the difference in declension of the sun at two distant points on the Earth, and using some simple geometric relations, he was able to calculate the radius of the Earth with reasonable precision (error of 1-20%, depending on how we convert the ancient distance metrics t our own...)