Celestial navigation in a Christmas ball

pallina-di-nataleMany of you, once become acquainted in celestial navigation, have found some difficulties in explaining to others the mechanics of celestial motions and the relationships between terrestrial coordinates and celestial coordinates. But what has to do this with Christmas?

It’s right in this period of the year that, rushing around for shopping in some DIY markets, I’ve seen some Christmas balls that you can decorate youself with various materials, that made me thinking, useless to say, a method to build a model to help explaining the coordinates systems that get involved with celestial navigation.

Thus I’ve purchased the material you can see in picture 1, a plastic transparent ball of a certain size, a smaller polystyrene ball and a spit that, to be honest, was already in my kitchen.

The purpose is to build a 3D model of the earth, surrounded by the celestial globe. At this stage the most challenging task is to run through our polystyrene ball with the spit to simulate the rotation axis of the earth as shown in picture 2 remaining at the center of both hemispheres. At the beginning going straight it’s not that easy, but after a few attempts you’ll get a reasonable result helping yourself with some small notches left by the mould. The same signs that I’ve used, with the help of some pins and some sewing thread to draw up with a thin felt-tip marker, the Greenwich meridian , the date line and the equator as you can see in pictures 3, 4 and 5.

Then we can prepare the transparent plastic ball which will simulate the celestial sphere making two holes to accommodate the spit that simulates the axis of the earth as shown in picture 6. To avoid the earth ball falling onto the bottom of the bigger sphere when inserted inside it, I’ve put two wooden beads at the right distance on the spit, securing them with some glue, and I’ve made a small paper boat pierced with a pin that you can put wherever you want at convenience on the earth surface to simulate positions at various latitude and longitude couples as you can see in picture 7.

Now it’s a great time to put the smaller ball inside the bigger one and complete the job marking the Aries meridian and celestial equator. Our earth model can rotate inside the celestial globe simulating the various combinations of celestial and ground coordinates we might encounter to calculate LHA, Zn and height, knowing the GHA, SHA, declination and, obviously you DR position. In picture 8 you can see the finished model. Couldn’t it cut a fine figure on the Christmas tree of a celestial navigator? Enjoy yourself and best wishes!


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