Question

how do people come to an understanding that earth is spherical

Answer 1

In ancient times it was believed that earth is round in shape but later some sailors proved this wrong by performing and interesting experiment around world
------------------------------------------------------------------------------------------------------------------------
They started from a point and sailed for about months and after sometime they came to same point.Which proved that earth is spherical.

Answer 2

there are some scientists who have been explained that one scientist he want to know about time so he used sun as to know the time so he put a long stick on the land from starting he observed that the shadow of the stick is reached same place and made a sphere shape.... many sailors also said about that, a scholar started his sea route journey from a place and reached same place after many days... by this we can say that the earth is spherical...