Question

Q : Why the horizon is curved ?
Ans : Since the Earth is rounded that's why the Horizon is curved​

Answer 1

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the line or circle that forms the apparent boundary between earth and sky. ... the great circle of the celestial sphere whose plane passes through the center of the earth and is parallel to the sensible horizon of a given position, or the plane of such a circle (celestial horizon ).The Earth does not have just one radius since it isn't spherical. Instead, the Earth is more like an oblate spheroid. It's wider at the equator than at the poles.