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Black Holes to Blackboards: Getting a Global Sense

LuAnn Dahlman

Despite assertions by some of a flat Earth, we really do live on a ball that’s spinning through space. Here’s a an exercise in visualizing our rotating planet.

The twilight zone is not just an old television series. It is a real place: the belt around Earth separating day from night. This globe-encircling band between light and dark is always present, creating morning or evening for whichever part of Earth is sliding under it.

Most places on Earth pass through the twilight zone twice a day; the result is an enchanting hour at dawn and dusk of low sunlight and long shadows. That special quality of morning and evening light seen in the zone is produced by sunbeams that were headed toward Earth but just missed. Captured by our atmosphere and scattered by gases and particulates, these light rays are stripped of shorter wavelengths. High clouds catch the light, revealing its residual reddish hue and creating a continuous circle of majestic sunsets and promising sunrises around the globe.

Earth and Moon

Steppin' into the twilight zone. While heading out for its rendezvous with Jupiter, a camera on NASA's Galileo spacecraft snapped this 1992 image of Earth and its Moon. The terminator, or line between night and day, is clearly visible on the two worlds. Image courtesy of NASA.

Before assigning your students to watch a sunrise or sunset, have them try the following activities to develop their global sense. The activities prepare the students to make connections between what they observe in real life and the plethora of two-dimensional Solar System diagrams they encounter.

Use a globe

In a dark room, point an overhead or slide projector at the side of a globe. Orient the globe to any season you like and give your model Earth a spin. Confirm for yourself that the twilight zone exists by examining the continuous circle and identifying where morning and evening occur. Notice that the twilight zone stays in the same place: Your part of the Earth has to come to it for day to turn to night and back to day again.

Use your imagination to make yourself small. Turn the globe slowly and consider the changes in temperature and light your part of the Earth experiences in one full rotation. Wander up to the polar regions and think about the strange days and nights you see from there, where you are never very far from the twilight zone.

Use your head

Sit on a rotating lab stool in the light of a model Sun, close your eyes, and imagine your own head as a model Earth. If you’re in the northern hemisphere, imagine North America on the right side of your forehead and South America sitting on your left cheek, dripping off your chin like a kind of continental goatee. Africa is hung over your left ear, and Eurasia forms a skullcap across the back of your head. The right side of your head is almost all Pacific Ocean except for Australia, located “down under” and behind your right ear.

Start rotating in the appropriate direction, bringing locations on your model Earth around to experience sunrise, noon, sunset, and midnight. Where is the Sun rising while your part of the Earth sees it setting? Which part of the world is sleeping while you eat your lunch?

The strong light of your model Sun may be uncomfortably bright even with your eyes closed. Use this sensation to help you envision how our own bright Sun warms just one side of the globe at a time, like a planetary rotisserie. Turn slowly enough to become aware of the changes that occur as an area turns from dark night toward full daylight. Notice how sunlight becomes more direct at noon, then wanes as your part of the Earth turns away from the Sun and into the dark again.

Following the your-head-is-a-model-Earth activity, a few of my students discovered for themselves that the ecliptic plane could be seen as a line across the day or night sky. I also found students were more likely to use gestures and pointing in subsequent discussions and activities on celestial motions. They willingly used the model again to solve problems such as the timing of Moonrise and Moonset, variations between seasons, and even to imagine night and day on extra-solar planets around distant stars. By invoking strong visual imagery, you provide your students with a portable and ever-lasting model they can use again and again.

Watching the real thing

Armed with a global sense of our spinning planet, go out to watch the sky as your part of the Earth moves through the twilight zone. Choose sunset for convenience or sunrise for its own unique rewards, but plan to stay out until the transition between light and dark is complete. Think on a large scale: convince yourself that the curved Earth drops away all around you. Give yourself a sense of the rotation that will soon bring you through the twilight zone.

If you choose sunrise, get up and out while it’s still very dark. Watch for a nameless color in the eastern sky; as your part of the Earth turns into it, the glow overpowers the starlight. When you finally cross the terminator, full sunlight hits every upright thing, and shadows are cast straight out to space for just an instant. Diffuse morning light soon gives way to more direct rays, and another part of the Earth moves into the twilight zone from the dark.

To watch a sunset, go outside when the Sun’s daytime glare has softened and shadows stretched. See how long your shadow gets before it dissolves into other shadows. If conditions are right, watch the sunny orb disappear from bottom to top as the roundness of Earth blocks it from your view. Admire the golden glow that remains as you turn into the darkness beyond twilight.

You’ll be sure your students see the bigger picture when their language begins to reflect it. I knew my own daughter was acquiring a global sense when she recently called to me, “Mom, come out and look at the sky. We’re going through the twilight zone.”

LUANN DAHLMAN was for seven years a high-school earth science and astronomy teacher, after which she spent several years writing science curricula and conducting workshops on digital image processing. Now a river guide in Arizona, she is also trying to see how best to distribute her 2D Earth-Moon-Sun Instructional Model and associated lesson plans. Her email address is