By Eric Moll | Updated Mar 24, 2022
Although the Sun’s radiance remains essentially constant year‑round, the way its rays strike Earth changes with the planet’s tilt. Imagine a flashlight illuminating a sheet of paper: when the paper is perpendicular to the beam, the light is concentrated; tilt the paper, and the same light spreads over a larger area, becoming less intense. The same principle applies to sunlight hitting the globe.
At the equator, the Sun’s rays strike the surface almost straight on, delivering maximum energy per unit area. Higher latitudes receive those rays at a slant, spreading the energy over a wider area. Even without any tilt, this geometry would make equatorial regions warmer and polar regions colder.
Earth’s axis is inclined by roughly 23.5°. During the Northern Hemisphere’s summer, the tilt directs that half of the planet toward the Sun, increasing the incident angle and warmth. Simultaneously, the Southern Hemisphere leans away, receiving less direct light and experiencing winter. The tilt remains fixed; as Earth orbits the Sun, the opposite hemisphere assumes the Sun‑facing orientation, and the seasons alternate.
At the equinoxes (mid‑March and mid‑September), the axis is neither pointed toward nor away from the Sun, resulting in equal day and night lengths worldwide. After each equinox, one hemisphere’s days lengthen while the other’s shorten. The solstices—June 21/22 and December 21/22—mark the longest and shortest days, respectively, for each hemisphere.
