Q.If the Earth has no tilt, what happens to the seasons of the Earth?

The common misconception. Many people assume seasons happen because the Earth's distance from the Sun changes over the year -- closer to the Sun means summer, farther away means winter. This is wrong: Earth's orbit is nearly circular, and its distance from the Sun barely changes over the year; more importantly, this explanation cannot even account for the Northern and Southern Hemispheres having opposite seasons at the same time, since both hemispheres are at the same distance from the Sun at any given moment.

The real cause: axial tilt. The Earth's rotation axis is tilted by about $23.5^\circ$ relative to the plane of its orbit around the Sun, and crucially, this tilt stays pointed in a fixed direction in space as Earth goes around the Sun. As a result, for roughly half the year the Northern Hemisphere leans toward the Sun (receiving more direct sunlight -- summer there) while the Southern Hemisphere simultaneously leans away (receiving more slanted sunlight -- winter there); six months later, the geometry reverses. …