Visit Your Local PBS Station PBS Home PBS Home Programs A-Z TV Schedules Watch Video Donate Shop PBS Search PBS
NATURE Home Current Season Episode Index NATURE Shop Contact Us For Teachers
Video DatabasePuzzles & FunEpisode PreviewsAnimal Guides
Featured Program
Forces of the Wild
Flowers Sunset and moon A wave

The Equinox

No matter how expected and predictable they are, other forces in our world, from bird migration to the changing of the leaves, can be hard to understand. Many seemingly mysterious events are products of the shifting seasons -- which are in turn caused by the simple fact that the Earth is tilted on its side just a bit.

Our daily lives are ruled by the sun.
The sun, of course, is the center of our solar system. The Earth sits some 93 million miles away from it, absorbing its energy every day. This is not a static system, however. The Earth turns on its own axis each day, and whichever side of the Earth faces the sun experiences daytime, while the other side is in night. At dawn each morning, the sun doesn't actually "rise"; rather, each portion of the Earth slowly turns to face the sun.

So far, so good. The Earth makes a daily rotation. But if the Earth is turning at more or less the same speed all the time, why aren't our days and nights of equal length? The complicating factor is that the Earth does not stand up straight. Looking at an imaginary line that stretches from the North Pole to the South Pole, the Earth is actually tilted on its side -- 23.5 degrees to be exact. And that makes all the difference to our days, nights, and seasons.

The angled Earth, while still spinning daily on its own axis, makes one grand sweep around the sun each year. The tilt of the Earth never changes, however. So at one end of the journey, the North Pole, along with the northern hemisphere, is tilted toward the sun. At the other end of the tour, six months later, the northern half of the globe is pointed away from the sun. At that time, it gets less sunlight -- and shorter days -- than the southern hemisphere does. As the Earth rotates, the northern hemisphere just doesn't face the sun for very long each day. Up top, the North Pole does not receive any sun at all; even during "daytime," it never faces the sun.

Six months later, the planet is at the other end of its yearly sweep and the northern hemisphere is pointing towards the sun. Exposed to the sun's rays for a longer period as it goes through its daily rotation, the northern hemisphere enjoys longer days and shorter nights. At that time, the sun never sets in the northernmost regions of the planet -- the famous "midnight sun" of the polar circle.

The two extremes of day length are marked by the Winter Solstice, the shortest day of the year, and the Summer Solstice, the longest day of the year. Midway between these two extremes lie the equinoxes, when the Earth, on its annual trek around the sun, is exactly parallel to the sun. The Earth's axis is not pointing away from or toward the sun; instead, both northern and southern hemispheres receive equal amounts of daylight. The autumnal equinox usually occurs around September 21 and the vernal, or spring, around March 21. On the equinoxes, days and nights are the same length.

What does all this have to do with life on Earth? Just about everything. Life on Earth depends on the energy from the sun, and how much energy -- or daylight -- an area receives has a great influence. As you see in FORCES OF THE WILD, when days are longer, plants flower, birds migrate, snakes and bears emerge from winter dens, and the energy of the sun powers circulatory forces in the atmosphere, causing cyclones, hurricanes, and other weather phenomena.

Recipe for Life
Why did life evolve on Earth?
Volcanic Action
The secrets of erupting volcanoes.
The Equinox
Why aren't our days and nights of equal length?
Understanding El Niño
What is El Niño?
Avoiding Destruction
Meet an earthquake hunter.
Volcanic eruption
Significant events in geological history.
For Teachers
View FORCES OF THE WILD lesson plans.
A bird
Web links and books related to the program.

episode homepage