The astronomical seasons are set by Earth's tilt and the Sun's alignment over the equator, and the equinoxes mark the times when the Sun passes directly above the equator. The Southern Hemisphere seasons are reversed but begin on the same dates. Because of the tilt, the bulge of our spherical Earth blocks sunlight from the far hemisphere at the solstices, which means different lengths of days and differing amounts of warming sunshine. In the south the June solstice is the winter solstice, the coldest weather comes in July and August, and the Sun is more northerly in the sky.

Where you live on a continent matters. Places near large bodies of water have smaller seasonal temperature changes, because large bodies of water cool much more slowly than land. 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. For the same reason, the hottest days of summer come after the solstice, since land and oceans need extra weeks to warm up.

Near the equator regions typically have alternating rainy and dry seasons, and the tropical rain belt near the equator follows the Sun through the year. A monthly rain-rate animation centered on the Pacific Ocean shows the belt shifting with the season. Over land the belt is distorted by monsoon circulations, in which the rising air near the equator is replaced by air from north and south of the equator. During the Asian summer monsoon, land surfaces on the continent of Asia get hotter than the surrounding oceans as days lengthen and heating increases. Warm, moist air from the Indian Ocean flows onshore, bringing rain.

The far north and south show the extreme. Helsinki, Finland, sees 18.5 hours of daylight in the middle of June, but 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 and rises again around the September equinox. When the South Pole points toward the Sun in summer, the Sun does not set. When it points away in winter, the Sun does not rise.

Living things answer the cold in different ways. Many animals deal with the cold temperatures and food scarcity of winter by going dormant. A true hibernator reduces metabolism, slows heart rate and lowers body temperature during dormancy. Brumation is a special kind of winter dormancy that snakes and other reptiles use. It is triggered by cold weather and fewer hours of daylight, similar to hibernation.

Mars is tilted about 25 degrees, close to Earth's 23.4 degrees, and like Earth, Mars has distinct seasons. On Mars, northern spring, which is southern autumn, is the longest season at 194 sols. Northern winter is 154 sols and northern summer is 178 sols. A year on Mars lasts 669.6 sols, which is the same as 687 Earth days. Uranus is tilted 97.77 degrees, so it spins almost on its side, and it has the most extreme seasons in the solar system. For nearly a quarter of each Uranian year the Sun shines directly over each pole, plunging the other half of the planet into a 21-year-long, dark winter.

Words to know

• hemisphere: half of Earth
• monsoon: seasonal rain wind
• dormancy: winter rest
• brumation: reptile winter rest
• sols: Mars days