Light on the World
[from the planetarium show Northern Lights]
1. Earth Globes
Class set of Earth globes between 2"–4" in diameter, each mounted on an axis (dowel or pencil works fine) for easy rotation by students. The equator should be clearly shown and you will need to make clearly visible marker dots at your city (or approximately 45° N.) and Tromsö, Norway (about 70° N.). On some globes, markers can be small wood or metal screws, with the head of the screw being the marker.
Thirty globes will accommodate one globe per student for classes up to 30 students and one for every 2 students for shows with two classes (60 students). You may use either real Earth globes or Styrofoam balls with the following markings on each:
• equator line in black
• dot at approximately 45° N. in red
• dot at approximately 70° N. in blue.
The precise color scheme is not crucial, as long as it is consistent for all the globes.
In order to see why the midnight Sun shines in Tromsø and not in [YOUR CITY], we will each need to look at a globe. The globe is a model of the Earth and this light can serve as the Sun.
Pass out the globes (students may help). Turn up bright light in the center of the room, and darken all other lights.
The pole sticking through the globe represents the axis about which the Earth turns. To make our model accurate the pole should not point straight up and down. It should be tilted.
The Earth’s axis tilts 23° with respect to the plane of the Earth’s orbit. Exaggerating the tilt to about 45° makes this activity more effective.
In the summer the north part of the Earth’s axis is tilted toward the Sun. It is daytime on the Earth where the Sun’s light is shining. On the side of the Earth that is dark it is night.
[Your city] is marked on your globe.
What happens to [your city] as you spin your globe about its axis? [It goes into the light sometimes and sometimes it is in the dark.]
Tromsø is also marked on your globe.
What do you observe happening to Tromsø as the world turns? [As night approaches, Tromsø approaches the dark part of the Earth, but it stays in the light.]
The “top of the world,” the region within the arctic circle is always lit by the Sun.
Half a year later it is winter and the Earth has gone half way around the Sun. Please stay right where you are and simply tilt the north end of the globes away from the Sun.
What happens now? [As the Earth turns [YOUR CITY] still goes in and out of the light, but Tromsø never reaches the light even in the day time.]
In winter in Tromsø, the Sun can not be seen for two months.
Explain that at the equinoxes, the terminator (dividing line between night and day) is perpendicular to the equator giving days and nights of equal length at all places on Earth.