There are four observations that helped Aristotle to understand that Earth is round.
Explanation:
The Ships he observed appeared from to sail first behind the horizon, after then only the hull.
We can see further standing on a higher standpoint. The ships again but also islands in the Aegean sea from a cliff.
The Earth 's shadow caused the eclipse (lunar). He also observed that the shadow always looked circular.
While travelling to south new constellations can be seen above the southern horizon. And as we travel more towards the south we see Polaris lower and lower.
