Satellite Photography and Direct Observation
Since the 1960s, thousands of satellites have orbited Earth and taken photographs showing its spherical shape. These images from multiple countries, space agencies, and private companies all show the same round Earth. Modern satellites continuously send back images that confirm Earth's shape from various angles and distances.
The Horizon and Ships Disappearing
When you watch a ship sail away on the ocean, it doesn't just get smaller and smaller until it disappears. Instead, the bottom disappears first, then gradually the rest of the ship. This only happens when you're standing on a curved surface. On a flat surface, the entire ship would shrink equally in all directions until it was too small to see.
Earth's Shadow on the Moon
During a lunar eclipse, Earth passes between the Sun and Moon, casting a shadow on the Moon. This shadow is always circular, no matter when the eclipse occurs or where Earth is in its orbit. A flat object can cast different shaped shadows depending on its angle, but a sphere always casts a circular shadow. This observation has been recorded for thousands of years.
Gravity and How Objects Fall
Gravity pulls all objects toward Earth's center with equal force. This means objects fall straight down relative to the ground, no matter where you are on Earth. If Earth were flat, gravity would need to work differently in different places, but it doesn't. The way gravity behaves everywhere matches what we'd expect on a sphere.
Different Stars in Different Locations
People at different locations on Earth see different stars in the night sky. Travelers heading south see new stars appear on the horizon while others disappear. This pattern matches what would happen on a sphere where the sky appears different depending on your position. On a flat Earth, everyone would see the same stars.
Time Zones and Sun Position
The Sun is directly overhead at different times in different locations. In one location it's noon while in another it's midnight. This only makes sense on a sphere where different places face the Sun at different times. A flat Earth would have the Sun directly overhead everywhere or nowhere simultaneously. 
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