In November, each day gets a little bit shorter. Sunset is a few minutes earlier. Outdoor playtime ends sooner. And you may have to switch on your desk lamp to do your homework, even before supper.

What's going on? We're moving through autumn toward winter. As the last season of the year approaches, less and less sunlight strikes our part of the planet, that is, the northern hemisphere for people in the United States.

Seasons change because of the way the earth journeys around the sun. This can be a little hard to picture, but give it a try. The earth is always spinning. It rotates around its axis, an imaginary line that runs from the North Pole to the South Pole. It takes the planet 24 hours to make one complete spin, or rotation. That 24 hours equals one day and one night.

It's daytime in the Northern Hemisphere when our part of the earth faces the sun, and nighttime when we face away from the sun. Pretty simple, right?

Here comes the more complicated part. As we spin on our axis, we also revolve, or travel along a giant path around the sun. It takes us just a touch over 365 days to make one trip, or revolution. That's one year.

Okay, you say, it takes us 24 hours to rotate, and 365 days to revolve. What's so complex about that? Well, earth spins at an angle, and it's always tilted in the same direction. During part of our journey around the sun, the North Pole slants toward the sun. Then it's summer. During another part of the trip, the North Pole slants away from the sun. That's wintertime.

The length of time that sunlight strikes the planet each day varies, too -- unless you live at the equator, the line that encircles earth's middle like a big belt, where the sun's rays strike straight on. In the northern hemisphere, above the equator, summer days are longer than nights. In winter, nights are longer than days. In spring and autumn, days and nights are about the same length. At the equator itself, it's summer all year long.

It can be hard to picture this in your mind, so here's a simple demonstration you can do that might help. You'll need a lamp and a round piece of fruit (an orange works well) and a pencil. Stick a pencil through the orange to represent the axis -- and to give you something to hold on to. In this show, the lamp plays the sun, the orange plays the earth, and the pencil the earth's axis. You'll have to put the lamp in the middle of the room on a table that you can walk around as you "revolve."

Turn on the lamp, and darken the rest of the room. Tilt the orange so that the "sun" shines toward the stem end of the fruit. The stem end represents the North Pole. You're creating summer on your "planet."

Everywhere north of the equator, days are longer than nights. The longest day, which arrives around June 21 or 22, is called the summer solstice. Rotate the orange on its axis. Your North Pole is in sunlight all the time, but your South Pole's in the dark. Now move one quarter turn around the lamp but keep your orange

tilted so that the pencil is still pointing in the same direction. Rotate the orange again. The sunlight now reaches all parts of the orange.

It's time for the equinox, the period in our journey around the sun when days and nights are the same length everywhere on earth. This happens twice a year, around March 21st in summer and September 23 in autumn. The equinox marks the transition between seasons. In your orange's northern hemisphere, it's autumn. In the southern hemisphere, spring has arrived. After the autumn equinox, days get shorter. After the spring equinox, days get longer.

Okay, time for another quarter turn. Now, the earth's axis -- the pencil through the orange -- is pointing away from sun. You're now halfway through your revolution. By now, the sun is shining on your planet's South Pole. It's wintertime. When earth's tilt takes us farthest from the sun, we experience the year's longest night and shortest day -- around December 22. (That's the time what we're heading for right now.) That day is called the winter solstice.

One more quarter turn will bring spring to your orange earth as another equinox arrives. Repeat the demonstration. Imagine that you're sitting on the orange looking up at the sun as it changes position in the sky.

During real winter, you may wish that you could change the seasons as fast as you can by doing this demonstration. But remember, after the winter solstice arrives next month, each day will actually start getting a teeny bit longer. That's something you can count on.Tips for Parents

To while away the long nights of autumn and winter, you and your kids might want to take up stargazing. Bundle up well, get a seasonal star chart or star wheel, and identify the changing positions of the constellations. You can get the charts at the library or at map or hobby stores. A good one is "Seasonal Star Charts" published by Hubbard Scientific, which includes a rotating luminous dial. A good source for kids is "Look at the Night Sky: An Introduction to Star Watching" by Seymour Simon (Viking; $6.95).

Catherine O'Neill is a children's writer in Minnesota.