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. 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. Mars has seasons because it is tilted 25 degrees, which is another similarity with Earth's 23.4 degrees. On Mars the seasons vary in length because of its elliptical, egg-shaped orbit around the Sun.
Earth's tilt is 23.5 degrees relative to the Sun and the ecliptic plane. The axis is tilted away from the Sun at the December solstice and toward the Sun at the June solstice. It is not the Sun that moves north or south through the seasons. At the equinoxes the tilt is at a right angle to the Sun and the light is spread evenly. The Sun sits directly above the equator, and daytime and nighttime are equal lengths everywhere on the planet.
Why does the angle of light matter? A beam of light that is concentrated provides more heat than one that is more dispersed, like a flashlight shone straight at a wall versus at a slant. In northern summer the Sun's more concentrated rays warm the land and water more. Earth's distance does not explain it, because Earth is nearest the Sun in early January. The warming is slow, because water has a heat capacity that is three times that of land. It takes about three times as much energy to warm water as a similarly sized volume of land. That is why water is slow to warm or cool, and why the ocean keeps coastal cities milder. It also explains why the hottest days come after the summer solstice. By August, the ocean water on the same beach will be much warmer.
Near the equator the seasons are about rain, not cold. 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. Seasonal shifts in its location drastically affect rainfall in many equatorial nations. They produce the wet and dry seasons of the tropics.
Plants and animals read these cycles. Phenology is the study of the timing 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. Farmers and gardeners can use phenology to choose the best time to plant, fertilize and harvest crops, and it also helps predict events such as allergies and mosquito season.
Autumn color has its own chemistry. Cooling, but not freezing, autumn nights signal leaves to produce red, crimson and purple anthocyanins. Unlike yellow and orange carotenoids, these red pigments do not occur in leaves in spring and summer. Temperature and rainfall can affect the brightness of the fall colors, and in a cool fall red maple leaves appear brighter. Every tree has a different combination of pigments remaining, which gives leaves their unique colors.
Winter can be survived by sleeping. Arctic ground squirrels, who usually have a body temperature of 99 degrees Fahrenheit, can drop as low as 27 degrees Fahrenheit in hibernation. They spend 7 to 8 months in hibernation every year, and the below-freezing body temperature can last for a few weeks. Pregnant bears give birth to their cubs in the den, most likely within the first two months of denning. The family stays in the den for the winter while the mother sleeps and the cubs nurse and grow.
The length of bear denning depends on location, and can vary from a few days or weeks to a few months or more. In fall, bears eat and drink nearly nonstop in a stage called hyperphagia and put on weight. Their bodies use the fat they stored in summer and fall as energy during winter.
Words to know
- obliquity: Earth's tilt
- ecliptic: Earth's orbit plane
- phenology: timing of nature
- anthocyanins: red leaf pigments
- hyperphagia: nonstop eating
