NASA - National Aeronautics and Space Administration

It is autumn, the season of change! In the north the hottest days of summer are past and each day is shorter than the last. Trees will soon turn brilliant 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, and the weather is warmer. Soon wild flowers will be blooming, bringing 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 and part of the time 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 and 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. And short winter days have long, cold nights.

The longest day is in the middle of summer, June 21st or 22nd north of the equator. That's called the summer solstice. The shortest day is in the middle of winter, around December 21st or 22nd, north of the equator. That's called the winter solstice.

But right in between summer and winter, this year on September 22nd, 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, and the first day of spring in the southern half of the world.

In between winter and summer there is another equinox, called the vernal equinox. Just like the autumnal equinox, day and night are the same length. Only this change is the first day of spring north of the equator, and the start of fall to the south.