Which factors cause the seasons on Earth?

• A. Earth's tilt (23.5 degrees) and orbit around the Sun
• B. The Sun's energy output changes dramatically with seasons
• C. The Moon's gravity is the main cause of seasons
• D. The Earth's distance from the Sun remains constant

Answer: (A)

The season pattern comes from the tilt of Earth’s axis as it orbits the Sun. The axis is tilted about 23.5 degrees relative to the plane of Earth’s orbit, so different hemispheres tilt toward or away from the Sun at different times of year. When a hemisphere tilts toward the Sun, the Sun climbs higher in the sky, days are longer, and sunlight hits that part of Earth more directly, warming it (summer). When it tilts away, the Sun sits lower, days are shorter, and sunlight is less direct, cooling it (winter). The opposite seasons occur in the other hemisphere at the same times.

The Sun’s energy output doesn’t change enough over the year to drive the seasons, and the Moon’s gravity isn’t what creates seasonal changes. Earth is also slightly closer to the Sun at certain times, but that distance change is small and can even occur during winter in the Northern Hemisphere, so distance isn’t the main cause. The tilt and the orbit together explain why there are seasons, with the tilt controlling the angle of sunlight and day length.