It is possible to tell that the earth is round without using technology. You just have to look at the shadow of the earth on the moon during a lunar eclipse.
During the partial phases of a lunar eclipse, the shadow of Earth can be seen on the surface of the Moon. It clearly casts a circular shape. This proves that the earth must be round.
The reason the shadow on the moon is round could be because the earth is a flat disc.
(P1) If the earth is round then there would be a round shadow on the moon during a lunar eclipse (P2) There is a round shadow on the moon during a lunar eclipse (P3) Therefore the earth is round
Reject (P3) Therefore the earth is round
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