## Monster Comet Promises Big Show

Classroom Activity: A Viewgraph Comet

Julieta Fierro

Using a very simple viewgraph simulator, teachers can explain why comets take different shapes.

Materials:

1. Two transparencies
2. Colored pens and drawing materials
3. A thick needle
4. A snap or round-headed fastener
Construction:

1. Draw a 10-centimeter (4-inch) diameter circle on one of the transparencies. This will simulate the Earth's orbit around the Sun. Somewhere on the orbit, draw a colored circle about 1 centimeter (half-inch) in diameter; this will represent Earth. Draw the Sun at the center of the orbit. Cut the transparency about a centimeter outside the Earth's orbit. You should now have a circular piece of transparency, which you will later spin to represent the Earth's yearly motion.
2. On the second transparency, draw a section of the comet's orbit. This should look like an arc of an elongated ellipse (see photo). Mark the Sun's position. Draw several comets along the path with their tails pointing away from the Sun. The tails should be smaller when the comet is farther out. In preparing the viewgraph, you may want to draw first on ordinary paper and later trace it onto the transparency.
3. Heat the point of the needle about 5 seconds on a stove. Poke a hole through both transparencies at the center of the Sun.
4. Place the Earth's orbit transparency on top of the comet one. Fasten them with the snap. You should be able to spin the Earth and change its position relative to the comet.

You can create other viewgraphs that have a smaller scale and show several comet orbits or constellations. Pupils can construct their own comet simulator. They can use cardboard instead of a transparency for the comet's path.

Use:

1. Show how the Earth's orbit is almost circular compared to the comet's. Explain that the Sun is at the focus of the orbits of the planets, asteroids, and comets. Comet orbits are highly elongated; this takes them far from the Sun. In fact, the elongated orbit is why astronomers believe comets originally came from the outer reaches of the solar system.
2. Explain that the comet's tail always points away from the Sun and grows as it approaches it. The tail on the viewgraphs is to scale: Comet tails really do stretch tens or even hundreds of millions of kilometers. Actually, comets have two tails. One, made of hot gas, point directly away from the Sun -- pushed back by the force of the particles that stream outward from the Sun. The other tail, made of dust left behind in the cometary orbit, is short and curved.
3. Turn Earth to different positions and investigate how comets look from several vantage points. We can see a comet head-on (its tail will look short) or on its side (the tail will look long).
4. Explain that sometimes we cannot see the comet, because it is behind the Sun. In January and February, Comet Hale-Bopp will be very close to the Sun from our vantage point.
5. Explain that the tail changes size and that Earth can go through the tail.
6. Be sure to explain that the orbit on the viewgraph is a projection. All bodies orbit in a plane, but a comet does not necessarily orbit in same plane as Earth does.
JULIETA FIERRO is in charge of astronomy popularization efforts at the Instituto de Astronomía of the Universidad Nacional Autónoma de México in Mexico City. One of her many projects has been to work with homeless children in Mexico City -- the subject of an upcoming article in Mercury magazine. Her email address is julieta@astroscu.unam.mx.

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