Seasons are caused by distance from the Sun
Earth's seasons are caused by the planet's 23.5-degree axial tilt, not by changes in its distance from the Sun. In fact, Earth is slightly closer to the Sun in January (Northern Hemisphere winter) than in July (Northern Hemisphere summer).
What we know
The intuitive idea that summer is caused by Earth being closer to the Sun and winter by it being farther away seems logical but is incorrect. Earth's orbit is nearly circular, and the difference in distance between its closest point (perihelion, reached in early January) and farthest point (aphelion, in early July) is only about 3.3 percent. This variation affects total solar energy by roughly 7 percent, which is not enough to drive seasonal temperature changes.
The real cause of seasons is Earth's axial tilt of approximately 23.5 degrees. As Earth orbits the Sun, one hemisphere is tilted toward the Sun and the other away from it. The hemisphere tilted toward the Sun receives more direct solar energy because sunlight strikes the surface at a higher angle, concentrating energy over a smaller area. That hemisphere also experiences longer days, extending the period of heating.
NASA and NOAA both explain that in June, the Northern Hemisphere is tilted toward the Sun and experiences summer, while the Southern Hemisphere is tilted away and experiences winter. This reverses in December. The Smithsonian Science Education Center notes a telling inconsistency that disproves the distance hypothesis: Australia experiences warm weather in December when the Northern Hemisphere is cold, even though both hemispheres are at the same distance from the Sun.
The axial tilt also explains why the tropics are warmer year-round (they receive relatively direct sunlight throughout the year) while the poles experience extreme seasonal variation.
Common claims
- Summer is when Earth is closest to the SunFalse - Earth is closest in January (Northern winter)
- Seasons are the same worldwide at the same timeFalse - Southern Hemisphere has opposite seasons
- Earth's tilt causes seasonsTrue - this is the correct explanation