Preliminary Measurement: Distance Law

Before we can measure planets, we work out the basic law of photometry: as the distance from the lamp increases, the detector receives less and less light.

Set up

Tools:
Photometer
Lamp
Tape Measure

Procedure:
Measure the irradiance every 10 cm at distances from the lamp: ten readings between 10 and 110 cm

photograph of the set up, evaulation (by ninth graders)

Sketch to explain the quadratic law:

The initial energy is distributed over growing spherical shells of area 4 pi r².

(The circles need to be imagined as spheres.)

Transit Detection

Finally, we let the orbit pass around the lamp and measure the light intensity as a function of time. Whenever the orbit passes in front of the lamp, the brightness goes down a little, but does not become zero: a very good analogy to the planetary transit.

Prototype 2018
Experimental set up 2019
(in the meantime, the plate is laked black and the lamp is better)

More than one Planet?

Test it!

Around christmas time, some gadget stores sell tiny christmas-trains. With one of them orbitting extremely closely, and with all of its wagons making the “planet” train really big, I simulated a hot Jupiter (or even brown dwarf) in narrow orbit.

Note that battery-powered trains typically do not allow to regulate their speed, except for slowing them down with more load (more waggons/ increased mass – e.g. modelling clay) attached. 

Determine Planet Density

Mass Density is a measure of its composition: rock planets have a higher density than gas planets (roughly spoken). We convince ourselves that this principle works by “weighing” some “test planets” with a transparent surface where we can see the inner composition.

With highschool students, we can even compute the density.

With primary school students, we just weigh them and compare the masses of ball with same size.