When do we know people knew the earth was a sphere?

Eratosthenes used geometry to estimate the circumference of the Earth.

Eratosthenes measured the altitude of the noontime sun at Alexandria at its maximum on Jun 21st. On that date, the Sun is directly overhead at noontime at Syene, in southern Egypt (latitude = 23.5 degrees north). The zenith distance is the angle from the zenith to the point where the Sun was at noon; it is also 90 degrees minus the altitude. At Syene, the zenith distance was 0 degrees; at Alexandria it was about 7 degrees. He knew how far it was from Alexandria to Syene, so he used geometry and the difference in zenith angle to estimate the size of the Earth. Eratosthenes also measured the tilt of the Earth axis by 23.5 degrees, which gives us the seasons .

Luke 17:30 -36 "It will be just like this on the day…" "…on that night"

"day", "night" referring to the same hour and event - only possible on a sphere. Note that by this time the idea of a spherical earth was well known in Greek society.

So passages in the New Testament can easily be seen in the light of a spherical earth.

In the Old Testament we have Isaiah 40:22 "He sits enthroned above the circle of the earth."    The original Hebrew for circle is "sphercity or roundness."

In Job 26:7 we have "He spreads out the northern skies, over empty space; he suspends the earth over nothing." Job dates 2000-18000 B.C.

ROUND research note on Psalm 103:12

Required concept: If you go north, you eventually go south. If you go east, you never go west. If you can never reach west going east, then east is an infinite distance from west.

All of the world views in ancient days that I am familiar with postulate a finite earth. This was also the Jewish view. The earth is described as smaller than God’s hand. (Isaiah 40:12, 48:13) So, as a poet, how do you describe something that is at least the same size as God, who is omni-everything?

You look for a phrase that means very large. Something that just means the size of the earth is just not big enough. Our transgression moved from His palm to His knuckle just doesn’t mean much. So "as east is from the west" has to be bigger than God’s hand - which is quite a bit larger than earth.

Therefore a flat earth won’t work as "east from west" would be too small. So earth must be a sphere so that the distance will be very large (infinite in modern terms.)

 If you think you understand the passage, the study question is:

How far did God remove their transgressions?

Notes:

I have found that some people reject the conclusion because they are sure that no one knew that the earth was round, and they reject the idea that there could be anything in the Bible before the world knew it. None of these people answered the question.

So I tried taking a survey. I asked "If you start going east, when do you start going west?" The answer was always "never." Then I asked, "So how far is the east from the west?" 7 people said infinite one said "they are not related" which I will take to mean infinite. One said "oh, you mean as in the Psalm?" So I did not use that one.

Conclusion: This is a straightforward concept in a non-Bible discussion.

See also Inventing the Flat Earth; Columbus & Modern Historians Jeffrey Burton Russell (Praeger, 1991)