seasons

Earth's 23.5-degree axial tilt, not its elliptical orbit, is why we have seasons. The tilt means each hemisphere receives more direct sunlight and longer daylight hours for part of the year (summer), then less direct light and shorter days for another part (winter). The two hemispheres always experience opposite seasons simultaneously. Solstices and equinoxes mark the extremes and midpoints of this tilt cycle.

Pluto has an atmosphere. In 2015, New Horizons measured its surface pressure at about 1 Pascal — roughly 1/100,000th of Earth's at sea level. That atmosphere is not stable. Over Pluto's 248-year orbit, it inflates and then freezes entirely out onto the surface, cycling between a thin gaseous envelope and nothing at all. We arrived just in time to see it while it still existed.

Earth's 23.5° tilt creates seasons and stabilizes climate. What would happen if it vanished — or flipped to 90°?. Learn the science, key examples, and...