It is autumn, the season of change! In the north, the hottest days of summer are past. Each day is shorter than the last. Trees will begin to turn bright colors. Soon it will be time for hot cocoa and warm coats.
Far to the south, across the equator, spring has arrived. The days are growing longer. The weather is warmer. Soon flowers will be blooming. They bring the promise of summer's heat and new life.
The reason for these changes has to do with the Earth's yearly trip around the sun. For part of the year the Earth's North Pole points away from the sun. Part of the time it points toward it. This is what causes our seasons. When the North Pole points toward the sun, the sun's rays hit the northern half of the world more directly. That means it is warmer and we have summer. But, when the North Pole is pointed toward the sun, the South Pole is pointed away. So the Earth south of the equator gets less warmth from the sun. It is winter there.
Summer is even warmer and winter is colder because of the length of our days and nights. In the summer, daylight lasts longer and nighttime is shorter. In winter, the days are shorter and the nights longer. That means there is more time for the sun to warm us during long summer days. Short winter days have long, cold nights.
The longest day is in the middle of summer. North of the equator, it happens on June 21st or 22nd. It is called the summer solstice. The shortest day is in the middle of winter. This happens around December 21st or 22nd north of the equator. It is called the winter solstice.
There is a special day in between summer and winter. On this date, day and night are each 12 hours long. This is called the autumnal equinox. It is the first day of fall north of the equator. It is the first day of spring in the southern half of the world.
In between winter and summer there is another equinox. It is called the vernal equinox. One day and night are each 12 hours long. This change is the first day of spring north of the equator. It is the start of fall to the south.