When did people first know the Earth was a sphere?

People first knew the Earth was a sphere around 2,000 years ago, like when your grandparents were kids.

Imagine you're on a boat and you watch another ship come toward you. At first, only the top of the ship is visible, like just the sails. But as it gets closer, more of the ship shows up. That happens because the Earth is curved, like a big ball.

Some smart people looked at the Moon and noticed that when it was partially hidden by Earth, it made a circle, just like how a ball would look if you covered part of it with your hand. That helped them guess the Earth might be round too!

Later on, some travelers even went all the way around the world and came back to where they started, kind of like going around a big ball and ending up where you began!