Eng: Making a clinometer

In the time before telescopes, all astronomers made their own observatories, and some of the best were in China, India and the Middle East: look up <observatories history> on the web, and you will find that there were seven major observatories older than this one below.

A Renaissance observatory at Uraniborg in Denmark.

Instruments were mainly designed to measure the distances between bodies in the sky, with all measurements written down as angles, and the old astronomers had to make do with that. You can measure some angles with a clinometer made from a protractor, a drinking straw and a weight on a string. You will also need sticky tape (and the string should be thinner than shown in the next few pictures).

Starting the clinometer.

The first thing to do is to drill a hole for the string, and then cut a small notch, because you want the string to hang from the very centre of the protractor. The next three pictures show you what to do.

Note: never look at the sun, as I appear to be doing! (I wasn’t.)

This actually came out of my design for a theodolite, which can measure angles between two objects like buildings, mountains or trees, any two things that you can see. A real theodolite uses accurate optics and a very fine scale, but we are going for a simple version, where we sight two landmarks through straws, and measure the angle between the straws.

The error is likely to be several degrees, but the principle is the same. (A slim drinking straw shows a field of view just twice the diameter of the moon, so it is about 1° across. A fat straw shows a field about 2° across.)

Tape one of the straws to the protractor but the other one has to be able to move. After a few failures, I sticky-taped a bent paper-clip beneath the protractor so the poke-out bit was at the dead centre of the protractor. Then I made two holes in the turning straw with a kitchen knife, and threaded it on and looked through: I could see past the wire.

The steps in making a theodolite.

Hint: When you are using this, it may help to have the protractor and bottom straw attached to a stand. Otherwise, set it down on something at eye height, so you can line up the fixed straw first, and then line up the moving straw. For serious work, a cross stave was what the old astronomers used when they wanted to measure angles.

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