Why Do We Have Seasons?為什麼會有四季？

🔊Ask a room full of people why summer is hot, and many will say it is because Earth is closer to the Sun in summer. It sounds reasonable, but it cannot be right. Earth is actually closest to the Sun in early January, in the middle of winter in Taiwan. And when it is summer in Taiwan, it is winter in Australia, even though both countries are the same distance from the Sun. The real reason for the seasons is not distance. It is a tilt.

🔊Earth spins around an imaginary line through the North and South Poles, called its axis. The axis is not straight up and down compared with Earth's path around the Sun. It is tilted by about 23.5 degrees. Just as important, the axis keeps pointing in the same direction in space all year, toward the North Star, Polaris. So as Earth travels around the Sun, the Northern Hemisphere leans toward the Sun for half of the year and away from it for the other half.

🔊Leaning toward the Sun does two things. First, the Sun climbs higher in the sky. When sunlight comes from high overhead, it strikes the ground straight on, and its energy is concentrated on a small area. When the Sun is low, the same sunlight is spread across a much larger area, like a flashlight beam shining on a table at a slant. Second, the days get longer. In Changhua, the Sun is up for more than 13 and a half hours in late June but only about 10 and a half hours in late December. More hours of stronger sunlight add up to summer.

🔊Four dates mark the turning points. Around June 21, the summer solstice, the Northern Hemisphere leans most toward the Sun, and at noon the Sun stands straight overhead at the Tropic of Cancer, about 23.5 degrees north of the equator. The Tropic of Cancer runs right through Taiwan, through Chiayi and Hualien, so on that day a pole standing there at noon casts almost no shadow. Around December 21, the winter solstice, the Sun is overhead at the Tropic of Capricorn, far to the south. Halfway between, around March 20 and September 23, come the equinoxes, when day and night are almost equal everywhere on Earth.

🔊Long before anyone knew why, Chinese farmers tracked the seasons with great care. The traditional calendar divides the Sun's yearly path into 24 solar terms, each about 15 days long, with names like Grain Rain, White Dew, and Frost's Descent. The solstices and equinoxes are four of them. Because the solar terms follow the Sun rather than the Moon, they fall on nearly the same dates every year, which made them a reliable guide for planting and harvest. In 2016, UNESCO added them to its list of the Intangible Cultural Heritage of Humanity.

🔊One last puzzle: if the Sun is highest in June, why are July and August the hottest months in Taiwan? Oceans and land take weeks to warm up, the way a pot of water takes a while to boil after you turn on the stove, so the warmest weather lags behind the strongest sunlight. The same delay makes January and February colder than December. And in the Southern Hemisphere, everything is reversed: in Australia, many families spend Christmas at the beach.