During an eclipse, the Sun or the Moon seems to disappear. What is happening? Why not explore this fascinating phenomenon in the classroom, with an easy to build model?
Solar and lunar eclipses are astronomical phenomena that have been shrouded in myth and legend throughout history. The ancient Chinese, for example, believed that solar eclipses occurred when a celestial dragon devoured the Sun. The Chinese word for an eclipse, ‘chih’, even means ‘to eat’.
Because eclipses are frequent and can be observed without a telescope, they are an excellent topic for introducing astronomy at school. This article describes a simple activity to help students aged 6-14 to understand eclipses, our Solar System and the motion of Earth and the Moon within it. The students begin by building and using their own model of the Sun, Moon and Earth. To consolidate and expand on what they have learned, the UNAWE projectw1 – an astronomy programme that fosters universe awareness – then enables the students to work with other schoolchildren around the globe. What time of day is it for them? What season? What can they see in the sky at a particular moment?
Although the Moon appears brightly in the night sky, it does not in fact shine but simply reflects the light from the Sun. The shape of the Moon that you see depends on both where you are on Earth and where the Moon is in its orbit around Earth. Every month the Moon completes a cycle, starting as a complete circle (at full moon), shrinking to become almost invisible (at new moon), then swelling to a full circle again; these stages are called the phases of the Moon (figure 1). To learn more, see Mitchell et al., 2008.
Usually, when the Moon is on the far side of Earth from the Sun (figure 1B), the three bodies do not lie in a straight line; instead, the Moon is slightly above Earth and is still illuminated, so we see a full moon. Occasionally, however, Earth passes directly between the Sun and Moon, and the Moon is completely in the Earth’s shadow – we call this a lunar eclipse. You can see it with the naked eye, as the Moon enters the Earth’s shadow (the umbra) and travels across it, taking on a beautiful dim red gloww2. As the Moon crosses in or out of the umbra, you can even see the curve of Earth partially shadowing the Moon. This partial shadow is called the penumbra.
Unlike lunar eclipses, solar eclipses are very rare. Most people will never see one because solar eclipses are only visible from relatively small areas of Earth at any one time. Solar and lunar eclipses have somewhat different causes: during a solar eclipse, we are in shadow. When the Moon passes exactly in front of the Sun, it casts a shadow on Earth. Because the apparent size of the Moon is so much less than that of the Sun, this shadow will only be seen in a small area of Earth.
To understand this, hold your thumb in front of the Sun so that it is completely covered. Your eyes are in the shadow of your thumb, but it does not affect the person standing next to you, as they are not in the same shadow. They see the Sun as normal. Once the Moon moves directly between Earth and the Sunw3, the sky takes on a spooky twilight, which causes animals to become confused. The air cools and, during a total eclipse, the Moon appears as a black disk in the sky surrounded by a crown of fire. That fire is actually the Sun behind the Moon; as the Moon passes, the full burning light of the Sun returns.
Eclipses in the classroom
Building the Sun-Earth-Moon system described below will allow your class to discover how and why eclipses happen. They will be able to understand exactly what they are seeing if ever they see a real eclipse. Building the model, which is not to scale, takes about 45 minutes.
For each model, you will need:
Two cardboard tubes (e.g. empty toilet rolls)
Scissors (suitable for cutting cardboard)
Sturdy but bendable wire (35-50 cm long)
Styrofoam ball the size of a large orange
Ping pong ball (or a Styrofoam ball of a similar size)
Large strip of cardboard (about 60 cm in length and no less than 20 cm in width)
Stack of books or magazines
Divide the class into groups of three or four. Give each group their own materials to make the model.
Take one cardboard tube and make a series of small (2 cm) even, vertical cuts around the circumference of each end.
At each end, bend the cut pieces out, then stand the tube upright. At the top, the cut edges should fan out like a flower (see image).
Using adhesive tape, fasten one end of the cardboard tube to the strip of cardboard; this is the base of the model. The tube should be at least 30 cm from one end of the cardboard strip.
Using tape or glue, attach the larger ball to the open flower of the tube. This ball is the Earth.
Cover the smaller ball with aluminium foil, shiny side out. This is the Moon.
Insert one end of the wire into the top of Earth, so that the wire is vertical.
Measure a finger’s length along the wire. Bend the wire at a right angle to give a horizontal arm.
Insert the other end of the wire into the Moon.
About halfway between Earth and the far end of the cardboard strip, measure a finger’s length along the wire and bend it downwards at a right angle, toward the cardboard base. The Moon’s equator should be at the same height as Earth’s equator.
Balance the torch on a stack of books or magazines at the other end of the cardboard strip from Earth. Make sure the height is correct: the middle of the torch beam should hit Earth’s equator. If the beam is too diffuse, attach the second cardboard tube to the end of the torch to direct the light horizontally. Ensure the beam hits the nearest half of Earth and the Moon directly. If the beam is not bright enough, move the stack of books closer.
Using the model
Duration: 30-45 minutes.
Creating a solar eclipse.
Click on image to enlarge.
Image courtesy of Marissa
Ask your students if they have ever seen an eclipse. Was it a solar or a lunar eclipsew4? Explain that solar eclipses are much rarer but today they will be lucky enough to see both.
Create a solar eclipse. Stand facing the torch and swing the wire around until the Moon casts a shadow on Earth; if necessary, dim the lights. The Moon is now between Earth and the Sun and is blocking the sunshine for some people on Earth. Point out that only people directly in the shadow see a complete eclipse of the Sun. You can show how the shadow moves by slowly rotating the wire.
Now create a lunar eclipse. Stand facing the torch and swing the wire so that the Moon is behind Earth. No light should be hitting the Moon: Earth is between the Sun and the Moon, casting a shadow over the entire Moon. Explain that unlike during the solar eclipse, the entire ‘night side’ of Earth can see the lunar eclipse.
Creating a lunar eclipse.
Click on image to enlarge.
Image courtesy of Marissa
Collaborating with other schools worldwide can help your students to understand the subject even better.
Marissa Rosenberg graduated in astrophysics from the University of California, Los Angeles (UCLA), USA, then completed a master’s degree at the International Space University in Strasbourg, France. She is currently working on her PhD in astrophysics at Leiden Observatory in the Netherlands.
Marissa is involved with EU Universe Awareness, writing astronomy articles for children. She has also completed three years of science teacher training through the California Teach programme at UCLA.