Show the students the video filmed from the ISS as it orbits the Earth every 90 minutes, looking down on the planet’s surface from a height of 370 km.
Ask the students if they recognise Earth’s atmosphere. Emphasise how thin and vulnerable this actually is, in comparison to the size of the Earth. If the Earth were an apple, the atmosphere would be thinner than its skin. Ask them what else they see.
The students have now had a first overview of Earth, although they didn’t see it as just a sphere floating in space (for this, show them 'Earth from Space' image) Explain how the border between day and night shifts from east to west (right to left) across the surface of the Earth. The Earth rotates around its axis in the eastern direction—counter clockwise, if you look from space down on the North Pole—with the Sun as a fixed background light. If you look from space down on the South Pole, the Earth rotates clockwise (still in eastern direction).
Now, we travel even further outwards, to the Moon. Show the students 'Earthrise' image (a photograph taken by the astronauts from the Apollo 8 mission in 1968.) These astronauts were the first people to ever orbit a celestial body other than the Earth, and when they looked back at their home planet, they experienced the so-called overview effect: everything they had ever known and loved was on that tiny blue marble, hanging peacefully in space.
At this point, you can make the shift from photos to model objects. Take the earth ball and hand a Styrofoam sphere (10 cm in diameter) to one volunteer. If you don’t have a ball of that exact size, then use a sphere that approximately fits on Australia on the earth ball, for example an orange. If you use a globe instead of the earth ball, adjust the sizes of the objects accordingly. For example, of you use a globe that is 20 cm in diameter, use a 5 cm moon and also divide the next sizes and distances in this activity in half.
Ask the volunteer to hold this model of the Moon at a distance from the earth ball that he/she thinks is correct, according to this scale.
Ask the other students if they agree, and if not, let them stand at a distance they think is right. The correct answer is a distance of 30 earth balls (or whatever globe you use) in a row. For the earth ball this is 12 meters, meaning all the way to the back of the classroom, or even outside. Let the students look at the earth ball from there and tell them that this is the size of the Earth as it would appear if they were to stand on the Moon.
We proceed on our virtual journey, now, to the other planets. Ask the students to remain at the back of the classroom. Now hold up a sphere of about 0.25 cm in diameter, for example a peppercorn or popcorn seed. The students will be looking at the Earth as viewed from Mars at its closest distance to Earth!
Show the students 'Pale Blue Dot' image, which is a photograph taken by Voyager 1, a spacecraft that was sent out into space in 1977 and has now long since passed the orbit of Neptune—the outermost planet of our Solar System. Of course, Voyager 1 is unmanned. In fact, no human has ever travelled farther than the Moon. In the picture, you can see a teeny tiny ‘blue pale dot’. This is how small the Earth looks from 6 billion kilometres away, which is about the average distance to Pluto. Almost half a million Earths in a row fit in this distance. It takes an airplane more than 600 years to fly there! The stripes in the picture are just ‘noise’.
Ask the students if their perspective of Earth has changed. Do they think the Earth is big enough to provide is with inexhaustible resources? Explain that the Earth is a sphere with a finite atmosphere and finite resources. If we polute our planet, there is no-one in space that can help us. We have nowhere to go. The Earth is the only home we have.
Note: For students aged 9–10 years, you can extend this activity by getting into the subject of searching for life on exoplanets, which are planets outside our Solar System. So far, close to two thousand exoplanets have been discovered. For the current count, check out <http: planetquest.jpl.nasa.gov="">. From this activity, the students have learned that the Earth looks very small from outside the Solar System. This demonstrates that from Earth’s perspective, exoplanets must seem very small indeed and are very difficult to see. Therefore, it’s hard to determine whether life has developed on them. Even with very strong telescopes, astronomers can rarely see the planet, never mind zoom in far enough to look for living organisms!
However, methods are available to examine exoplanets.
Ask the students to think of ways to find out if a planet is hospitable to life, or even to check for actual life. The most important requirement for life is the presence of liquid water. The planet should be far enough from its host star so that water, if present, won’t evaporate. But it shouldn’t be too far, otherwise the water would freeze. Also, an atmosphere is probably necessary to protect life from harmful radiation and large temperature variations. In the future, astronomers might have developed such high-quality telescopes that they can see an exoplanet’s colour, from which they could deduce whether it has vegetation.
So far, however, we haven’t found a planet that is just like Earth. If we do, it will probably be very far away, meaning it will be difficult to study with our telescopes. Emphasise that lots of work still needs to be done in this area: if the students grow up to be astronomers, they might make a breakthrough discovery—they might even find life!