Blue Planet

How to construct
an instrument that will teach you
how to recognize the stars

by Cesare Petitti
teacher of technical education
at the lower middle school and instructor for the CEMEA
(Centres of Practice in Methods of Active Education)

Here we will be explaining how to make your own astrolabe. It is a very simple instrument and can be made out of "poor" materials that can all be found in the home. You will need some sheets of white card and a stiff carboard disk (like a cake base), to which we will glue our celestial planisphere.

The centre of the celestial planisphere should be attached to a piece of cardboard of the same width, and slightly more than twice its size in length, which is then folded inwards onto it. A calendar, shaped rather like a goniometer but with the degrees substituted with the days of the year, should then be placed around the planisphere disk. A pin can be used to fix the disk through the Pole Star onto the rigid base that is slightly larger in circumference than the disk itself, and with the 24 hours of the rotation of the earth marked on it.

Once the movable planisphere is completed, the correct position has to be found; this can be done by drawing in the horizon at our hemisphere's average latitude of 45° North. This is an oval window that should be drawn and cut out on the carboard we folded earlier, so that only the stars that are above the horizon at that particular moment can be seen.

The observer's Zenith, placed exactly above his head, is an extremely important reference point. We will add a transparent moveable index to our instrument to help us discover which stars are passing our Zenith at any given moment, also useful for simplifying the hour-day observation sequences.

Our astrolabe is now complete. It should be raised upwards towards the sky, turning it carefully to coincide with the cardinal points (in other words, the North marked on the instrument's horizon should correspond with the real North), and downwards when observing the sky to the North.