太阳什么时候离我们最近和最远（地球公转轨道及影响解析）

In December, Earth is more than a million miles closer to the Sun than usual. In early January, we’re the closest we ever get. That’s why the weather is so hot right now.

What’s that you say? The weather’s not hot? It’s freezing cold? With snow on the way? Hmm . . . There must be more to winter than our distance from the Sun.

Indeed there is. As Earth spins, it’s not straight up and down. Instead, it’s tilted. As Earth goes around the Sun, the North Pole always points the same way in space, toward a distant star called the North Star. This tilt of Earth causes the seasons.

During the half of the year we’re in now, the northern half of Earth points away from the Sun. That makes the weather cold for two reasons. First, the time from sunrise to sunset is shorter. So the northern half has fewer hours to soak up heat from the Sun. Second, the Sun doesn’t rise as high in the sky as it does in summer. As a result, winter sunlight is weaker, spread over more area. So even though Earth is closer to the Sun in winter than it is in summer, the planet’s tilt reduces sunlight enough to bring cold weather.

December 21 is the first day of winter. That’s when the northern half of Earth leans the most away from the Sun. But Earth’s northern half started leaning away from the Sun months ago. So wintry weather often arrives long before the official start of winter.

Don’t like the cold? Head to the southern half of the planet. Down there, summer starts on December 21 because that’s when the southern half of Earth leans the most toward the Sun.