# Sun, Moon and Earth

20 the earth geometry, metrology and ancient geodesy The Earth is a giant flywheel in space, a stable rotating platform from which to observe the heavens. Ancient geocentric earth centred observations for calendars, navigation and surveying appear to have been greatly aided by the use of traditional units of length which harmonised with the principal fixed dimensions of the Earth see opposite. We have already seen how the diameter of the Earth may be accurately expressed as 8 x 9 x 10 x 11 miles see page 3. The Earths equatorial circumference of 24,902.86 miles turns out to be 360,000 x 365.242 English feet, enabling space, angle and time to be calibrated in either feet, degrees or days. Because the Earth spins she is slightly fatter at the equator than at the poles, the precise shape being an ellipsoid known as the Geoid. The polar radius of the Earth 3949 miles multiplied by an approx. for 2p, neatly accounts for the Earths equatorial bulge, giving a value of 24,883.2 miles for the meridian circumference through the poles. The ellipsoidal shape causes surface degrees of latitude to measure longer at the poles than they do at the equator. One degree of latitude averages at 69.12 miles, a length quoted by Ptolemy as 300,000 Roman remens and which, astonishingly, remains the UK Admiralty figure today for navigational purposes. The global system of ancient measures related to each other through simple fractions, making calculations easier in a world without electronic calculators. Their elegant system is now lost, almost totally obscured by the adoption of the metric system. 5 7 63 10 21