Earth takes about a year, or about 365 days, to go around the Sun. Earth is tilted, so it does not stand up straight, and this tilt gives us the seasons. The tilt is about 23 and a half degrees. As Earth goes around the Sun, its tilt always points the same way in space. Earth does not wobble back and forth each year.

In Northern Hemisphere summer the Sun's path is higher in the sky, rising in the east and setting in the west. In Southern Hemisphere winter the Sun is more northerly in the sky. A more direct sun angle in the northern hemisphere causes the warmth of northern summer. A lower sun angle in the southern hemisphere brings less solar heating power and the chill of southern winter. The hottest days of summer come after the summer solstice, not on it, because land and oceans need extra weeks to warm up.

The farther north you go, the bigger the differences in the seasons. Helsinki, Finland, sees 18.5 hours of daylight in the middle of June. In mid-December it is light for less than six hours. On the Arctic Circle the Sun does not set at all on the June solstice. At the South Pole the Sun sets once for the winter. It rises again around the September equinox.

Two lines on the map, the Tropic of Cancer and the Tropic of Capricorn, mark where the Sun can be straight overhead. On about June 21 the Sun is directly over the Tropic of Cancer, and around December 21 it is directly over the Tropic of Capricorn. Between the two tropics, which include the equator, the Sun is directly overhead twice per year. Outside those two lines the Sun is never straight overhead.

Near the equator regions typically have alternating rainy and dry seasons. Places there have about the same amount of daylight and darkness throughout the year, so they remain warm year-round. The four-season year is typical only in the mid-latitudes. Those are the places that are neither near the poles nor near the equator.

Seasons can be counted in two ways. Astronomical seasons are based on the position of Earth in relation to the Sun. Earth's elliptical orbit makes them unequal, from 89 to 93 days. Meteorologists often use seasons that follow the calendar. In the north, meteorological winter includes December, January and February. They use whole months because it becomes much easier to calculate seasonal statistics from monthly statistics. These seasons are more even, lasting about 90 to 92 days.

Other planets have seasons too. Mars is tilted about 25 degrees, close to Earth's 23.4 degrees, and a year on Mars lasts 687 Earth days. Uranus is tilted 97.77 degrees, so it spins almost on its side. It has the most extreme seasons in the solar system. Each pole gets about a quarter of its 84-Earth-year orbit in direct sun.

Living things keep time by the seasons. Phenology is the study of the timing and cyclical patterns of events in the natural world. Examples are the budding of leaves, the arrival of migratory birds and the flowering of plants. Many animals deal with the cold and food scarcity of winter by going dormant. A true hibernator reduces metabolism, slows heart rate, and lowers body temperature. Some animals migrate to places with a better winter. Some travel thousands of miles by land, air, or sea.

Many people guess that summer comes when Earth is nearest the Sun. In northern summer, Earth is not closer. As of November 2026, Earth is closest to the Sun in early January, in the middle of northern winter. That point is called perihelion. The farthest point, in July, is called aphelion. In 2023 Earth's center was 152,093,250 kilometers from the Sun's center at aphelion. NASA gives the average distance as about 150 million kilometers. So the distance varies only a little over the year.

Perihelion does matter in one way, because it changes how long the seasons last. Earth moves faster in early January, so the seasons that occur then pass more quickly. Earth travels about 2,000 miles per hour faster at perihelion than at aphelion. Kepler's second law explains why Earth speeds up near the Sun. The smaller the distance, the faster the motion. Because Earth moves a little faster in northern winter, northern winter is about three days shorter than northern summer.

Perihelion also brings a little more sunlight. It increases the intensity of sunlight reaching Earth by about seven percent compared to aphelion, yet January is still northern winter. Earth's axial tilt is the main driver of seasons, not Earth's distance from the Sun.

Words to know

• perihelion: closest to Sun
• aphelion: farthest from Sun
• meteorologists: weather scientists
• phenology: timing of nature
• hibernator: winter sleeper