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In the night sky: Orion
In the night sky: Orion

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4.3.4 Transit experiment

Try for yourself to see how a transit works.

Activity 4.4

You will need a light source, for example in a table lamp with no lampshade, and a set of spoons of different sizes (e.g. teaspoon, dessert spoon, tablespoon). The lamp represents the star, and the spoons are planets.

Point the lamp at a flat surface, so that it illuminates part of the surface. Take the largest spoon and slowly move it in front of the lamp, watching the pool of light on the surface. You should see that it darkens where the spoon is in front of the lamp, i.e. less light is falling on the surface when the spoon is in position. This is an analogy for a planet passing between an observer and a star as it orbits the star – i.e. as it transits in front of the star.

If you repeat the experiment with a smaller spoon, you should see that the shadow again darkens, but not by quite as much. So you can also say something about the relative sizes of transiting planets.

Did the smaller spoons have a noticeable effect? Note down your findings.

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Next, you will find out about planets found using the radial velocity method.