Short Answer 
The Earth is spherical due to gravity, which pulls uniformly toward its center, leading to a round shape. Observations of horizon curvature and the disappearing ship masts as they sail away, alongside global time zones and photographic evidence from space, further support this conclusion.
1. Understanding Gravity
Gravity is a fundamental force that pulls objects toward the center of mass. This force acts uniformly in all directions, which naturally results in celestial bodies, including Earth, forming a spherical shape. The consistency of gravity’s pull across the planet reinforces the concept that Earth is round.
2. Observing Curvature and Disappearance
When observing objects at a distance, two phenomena provide insight into the Earth’s shape. First, the curvature of the horizon is evident from elevated viewpoints where distant objects appear lower than expected. Additionally, the disappearance of ship masts as they sail away demonstrates how Earth’s curvature limits our line of sight, supporting the assertion of a spherical shape.
3. Global Time Zones and Visual Evidence
The existence of time zones around the world is directly related to the Earth’s rotation and confirms its round shape. If Earth were flat, we wouldn’t experience varying sunlight hours across different regions. Furthermore, photos from space captured by satellites and astronauts provide clear visual proof of Earth’s spherical shape, showcasing its rounded silhouette against the cosmos.