Introduction

The universe of marine navigators and surveyors is basically a geocentric one. All calculations necessary for reducing celestial observations to obtain directional or positional information can be carried out within the pre- Copernican two sphere hypothesis. Some mature students on the degree courses have practical experience of navigation at sea but are not used to more abstract ways of thinking. However, most courses in navigation require students to understand the many corrections that have to be applied in astro-navigation. The planetarium can be used to illustrate the basic concepts of the two sphere hypothesis, although other methods are needed to understand the nautical almanac and the principles used in its calculation.

Coordinate Systems and the Planetarium

Obviously a planetarium is very useful to teach star identification, which all practical navigators should be able to do. Projected vertical circles, meridians, prime verticals, celestial equator and ecliptic, hour circles and projected protractors at zenith and pole are very helpful in teaching co-ordinate systems. These circles can also be used as an empirical introduction to the basic concepts of spherical trigonometry. Planetariurns can also be used to demonstrate the effect of precession on right ascension and declination. However, since the planetarium emphasises the geocentric view point it cannot readily be used to explain the physics of precession.

Gyroscopes and Orreries

Many astronomy textbooks take the trouble to explain the dynamics of precession, using the spinning top analogy, but few explain the gyroscopic properties of a spinning body. Even physics students find rotary motion difficult at first, and most good physics textbooks go to great pains to explain and illustrate the concepts.