You will need timber, nails, tape measure, plus quite a lot of ingenuity. I used a brass thread and wingnut to attach the two pieces of my model, so I also needed a drill.
To make the cross-bar, you hammer small nails into a piece of wood, 35 mm apart. This piece of wood is then mounted on a second piece of wood at least 2 metres long. At a distance of two metres, each nail is just one degree away from its neighbours (you do the sums, using 2πr for the diameter).
The steps to take in making a cross stave. Notice how the nails are in an arc, not a straight line.
Use your cross-stave to make a map of a few of the main stars in the sky.
The moon takes about 30 days to go once around the earth, which means that it appears to shift across the sky by about 12º from one night to the next, which means that it moves about half a degree across the night sky in a single hour. The moon subtends (covers) an angle of half a degree, as we see it from earth, which means that it moves one moon-width across the sky each hour.
Each record needs to have the three sides of the triangle. Why? (Note: this is deliberately not to scale!)
Use your cross-stave to map and measure this movement, looking at the brightest stars within three to five degrees of the moon. Make sure you record your results carefully every hour for several hours. The best way would be to take the measurements as quickly as possible, and enter them onto a sketch that you prepared five minutes before the sighting time.
There is a limit to the accuracy you can get with the cross-stave. Can you improve your results by getting further away from it, and using binoculars? Satisfy yourself first that the binoculars do not change the apparent angle, then use a piece of string 11.46 metres long, and see how well you can map things. At 11.46 metres, one degree will require marks 20 cm apart. Can you estimate angles to the nearest minute?
Note: to get really serious, you may need to use a flagpole with the halyard (rope) attached near the centre of the beam, and you will need to make it more rigid somehow: the magic word is truss. Or maybe girder… (This is how they did astronomy before 1600.)
Try taking photographs of the rising moon, from the same position, over a period of several hours. Once the moon is above the horizon, take shots where the moon is 5º, 10º, 15º, 20º and 25º higher than the original moonrise shot. Does the “inflated moon” illusion work in photographs as well as it does in real life? If you don’t know this illusion, the moon always looks bigger, just after it rises.
(You will notice that I have left you room to add your own “frills” to the experimental design: discuss your plans with a reliable adviser before you go ahead, to make sure you have thought of all the variables.)
Afterthought: can you find a way of making the nail heads glow, maybe with LEDs, so you can tell them apart? I think that red LEDs would be best (you work out why) and I suggest that you think about having two LEDs every fifth marker (again, you work out why).
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