How do we know that Earth was not flat in early days ?

Explanation

To determine how we know that Earth was not flat in the early days, we can analyze three key observations made by ancient civilizations that led to the conclusion that the Earth is spherical.

### Step-by-Step Solution:

1. **Observation of Disappearing Ships:**
- When ships sail away from the shore, they gradually disappear from view starting from the hull and then the mast. If the Earth were flat, we would be able to see the entire ship at all times, regardless of how far it traveled. However, the fact that only the hull disappears first suggests that the surface of the Earth is curved. This observation led early thinkers to conclude that the Earth is not flat but round.

2. **Observation of Star Constellations:**
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