Why do we have seasons?

Answer: Earth's axis is tilted

The Sun's heat output changes — Wrong. The Sun's energy output is remarkably constant over short timescales like years. Small variations (about 0.1%) follow an 11-year solar cycle, but this doesn't cause our annual seasons.

Earth's speed varies in orbit — Wrong. Earth's orbital speed doesn't cause seasons—Earth moves at nearly constant speed throughout its orbit. The 23.5° axial tilt is responsible for the dramatic seasonal temperature and daylight changes we experience.

Earth's axis is tilted ✓ — Correct! Earth's axis is tilted 23.5° from vertical. As Earth orbits the Sun, this tilt means the Northern Hemisphere points toward the Sun in June (summer there) and away in December (winter). When tilted toward the Sun, that hemisphere gets more direct sunlight and longer days, creating summer. The opposite hemisphere experiences winter. This tilt causes seasons.