Which factor is most responsible for the seasons in the Northern Hemisphere?

• A. Distance from the Sun
• B. Variation in Earth's orbit shape
• C. The Sun's brightness changes seasonally
• D. The tilt of the Earth's axis relative to its orbit around the Sun

Answer: (D)

The key idea is that how sunlight hits Earth and how long a place stays in daylight changes as Earth orbits the Sun because the planet is tilted on its axis. When the Northern Hemisphere tilts toward the Sun, the Sun climbs higher in the sky, days are longer, and sunlight hits more directly, delivering more energy per area and warming temperatures. That’s why summer brings heat and longer daylight. When the tilt moves the opposite way, the Sun stays lower in the sky, days are shorter, and there’s less energy per area, leading to cooler temperatures and winter. The transitions in spring and fall occur as the tilt places the Sun at intermediate heights and daylight lengths.

Distance from the Sun varies only a little over the year, and Earth’s orbit isn’t extremely elongated, so the changing distance has a small effect compared with the tilt. The Sun’s overall brightness doesn’t swing with the seasons in a way that would drive the pattern; solar output is relatively stable over a year. Because of these factors, the axial tilt is the primary reason the Northern Hemisphere experiences seasonal changes.