Earth's tilt, called obliquity, is why Earth has seasons. Over the last million years it has varied between 22.1 and 24.5 degrees, and as of November 2026 it is about 23.4 degrees, roughly halfway between its extremes. The tilt is very slowly decreasing in a cycle that spans about 41,000 years. The greater the tilt angle, the more extreme the seasons are, and larger tilt angles favor periods of deglaciation, the melting and retreat of glaciers and ice sheets.
Tilt is only one of three slow orbital cycles that help set Earth's long-term climate. The other two are eccentricity and precession. Eccentricity measures how much the shape of Earth's orbit departs from a perfect circle, and it is the reason our seasons are slightly different lengths. Axial precession makes seasonal contrasts more extreme in one hemisphere and less extreme in the other, and it affects seasonal timing relative to Earth's closest and farthest points around the Sun.
Today's orbit has its own timing. As of November 2026, Earth is closest to the Sun in early January, in the middle of northern winter, and farthest in July. In 2023 Earth's center was 152,093,250 kilometers from the Sun's center at aphelion, and NASA gives the average distance as about 150 million kilometers. Perihelion increases the intensity of sunlight reaching Earth by about seven percent compared to aphelion, yet January is still northern winter, because Earth's axial tilt is the dominant driver of seasons. Because Earth moves a little faster in northern winter, northern winter is about three days shorter than northern summer.
Water and wind turn the same sunlight into very different seasons. Water has a heat capacity that is three times that of land, so it takes about three times as much energy to warm water as a similarly sized volume of land. That is why places near large bodies of water have smaller seasonal temperature changes. The hottest days of summer come after the summer solstice, because the land and oceans simply need extra months to warm up. By August, the ocean water on the same beach will be much warmer. In the tropics the seasons are about rain. The Intertropical Convergence Zone is a region near the equator where intense sun and warm water heat the air, raising its humidity and making it buoyant. Aided by converging trade winds, the air rises, expands, cools and releases its moisture in an almost perpetual series of thunderstorms. Over land this rain belt is distorted by monsoon circulations. During the Asian summer monsoon, land surfaces on the continent of Asia get hotter than the surrounding oceans as days lengthen, and warm, moist air from the Indian Ocean flows onshore, bringing rain.
Living things keep a record of all these clocks. Phenology is the study of the timing and cyclical patterns of events in the natural world, such as the budding of leaves, the arrival of migratory birds and the flowering of plants. Cues like day length, temperature and rainfall tell plants and animals when to act. Phenology records the timing of leaf-out, flowering, migration and spawning, and it helps assess the vulnerability of species, populations and ecological communities to ongoing climate change. It also tracks how global warming is altering the timing of key biological events.
Words to know
- obliquity: Earth's tilt
- precession: slow axis wobble
- eccentricity: orbit's stretch
- deglaciation: ice melting back
- phenology: timing of nature
- monsoon: seasonal rain wind
