Self-Centered Globe

Model the patterns of day and night on Earth by holding a globe in just the right position.

Make the pattern of sunlight and shadow on an Earth globe match the pattern of day and night on real Earth.

Subjects:

Astronomy and Space Sciences

Keywords:

earth

sun

shadows

compass

Tools and Materials

Earth globe (use a hard, rigid globe, not an inflatable globe)
Earth-globe base (a bowl, large roll of tape, or other open cylinder that can support the globe)
Magnetic compass
Phone or computer capable of accessing websites from your location
Sunlight
Small paper clip, bent into an L shape (or a toothpick and a small lump of clay)
Tape
Optional: 1-inch (2.5 cm) wide strip cut from a sheet of liquid crystal to about the length of a meridian on your globe; 12-inch ruler and tape measure

Assembly

1. If your globe has a stand, carefully remove it. Then take all your materials to a sunlit outdoor space with level ground.

2. Place the globe on top of its base. Find you location on the globe, and rotate the globe until your location is centered at the top.

3. You need to align your globe so that it's Geographic North Pole points true north. A compass will show you magnetic north, and then you’ll need to adjust for your magnetic declination, which is the difference between magnetic north and true north. First, use a compass to find magnetic north and note the direction. From there, here's how to find true north—the planet’s Geographic North Pole:

a. First, check here to find your location’s magnetic declination. On the right side of the page, type in your zip code, then click “Get & Add Lat/Lon,” which will fill in the latitude and longitude for your location on the left side of the page.

b. Click “Calculate.” Note the information under the word “Declination” in the map box that appears. For the location of the Exploratorium, for instance, it reads: 13° 30' E ± 0° 20', which means magnetic north is 13 degrees and 30 minutes (13.5 degrees) east of true north, and that this measurement is accurate to within plus or minus 20 minutes.

c. Once you have your magnetic declination, you can find true north (the Geographic North Pole) by adjusting your direction by the number of degrees indicated:

If the declination reading indicates your compass needle points too far to the east (as the Exploratorium’s does), turn your compass dial that many degrees back to the west (counterclockwise).
If the declination reading indicates your compass needle points too far to the west, turn your compass dial that many degrees back to the east (clockwise).

4. Keeping your location at the top center, rotate the globe until its Geographic North Pole points true north, toward the planet’s Geographic North Pole. When you’re done, the position of your globe will be aligned with the position of planet, relative to the sun.

To Do and Notice

When you have your globe correctly aligned, look closely at the areas in sunlight. Those same places on Earth are experiencing day right now. Notice the areas of the globe that are in shadow. Those same places on Earth are experiencing night. As the day progresses, the pattern of light and shadow on the globe will change, as will those on real Earth.

By looking at the pattern of sunlight and shadow on your globe, you can determine the approximate time of day at your location. Use the terminator (the dividing line between the light and shadow) to help you. Where on Earth is the sun rising? Where on Earth is the sun setting?

After about 10 minutes, feel the temperature of your globe. Where does it feel the warmest? Where is it the coolest? The temperatures on your globe will indicate relative warm and cool areas at those real locations on Earth, as well.

Use the tape to attach the small L-shaped paper clip to the globe so the upright part of the paper clip is perpendicular to a line tangent to the surface of the globe, as shown in the image below. (You can also use a toothpick supported by a lump of clay.) Notice the direction of this vertical object’s shadow. It will match the direction of a vertical object’s shadow on real Earth.

What's Going On?

When your Earth globe is properly aligned, the areas of temperature, light, and shadow it shows will mimic conditions of temperature, light, and shadow on real Earth.

The shadows on your globe are the same as the shadows on Earth because you have precisely lined up the position of the globe (relative to the sun) with the position of the planet. As a result, your globe’s axis of rotation is parallel to the planet’s axis of rotation. Because these two spheres are similarly aligned in space with respect to the sun, their patterns of light and shadow are also similar.

You can also find approximate time of day with your model. The location of the terminator that lies west of your location on your globe is where the sun is rising on real Earth. You can check this by looking up the sunrise time for a city that’s along the terminator. The location of the terminator that lies east of your location on the globe is where the sun is setting on real Earth. You can check this by using the same website to find the sunset time for a city that’s along the terminator.

After your globe has been in the sun for a while, you’ll find that the night side (which is in shadow), will be cooler than the day side (which is in sun). The warmest places on your globe will correspond to parts of real Earth that are receiving the most solar energy. The point on the globe that is directly in line between the sun and the center of the globe, called the subsolar point, will be hottest. As you move away from the subsolar point, the globe’s surface temperature will decrease. The same is true on real Earth. The subsolar point is always within the tropics—the area bounded by the Tropic of Cancer (23.5 degrees north of the equator), and the Tropic of Capricorn (23.5 degrees south of the equator). The polar regions feel cooler than the tropics, even when they’re in the sun, because sunlight hits the surface near the poles at a glancing angle.

Going Further

Try this activity on special astronomical days, such as solstices or equinoxes. On the December Solstice, the subsolar point is the Tropic of Capricorn, the most southern subsolar point. On the June Solstice, the subsolar point is the Tropic of Cancer, the most northern subsolar point. On the equinoxes, the terminator goes along the meridian, and all parts of the globe receive an equal 12 hours of day and 12 hours of night. The subsolar point on either equinox is the equator.

Let the earth globe cool in the shade for a few minutes. When the globe has cooled, tape a strip cut from a sheet of liquid crystal material so that it runs along the meridian that goes through your location. Re-align the globe as before, and notice how and where the color changes.

Take another look at the paper-clip shadow marker on your globe. Find the ratio of the length of the vertical portion of the paper clip to its shadow. Now hold a 12-inch ruler vertically on the ground nearby, so it casts a shadow on real Earth. Use the tape measure to measure this shadow. Then find the ratio of the length of the ruler (12 inches) to the length of its shadow. What do you notice about these ratios? Are they proportional?