# Earth Grids

34 35 The size and shape of the Earth has only been known for the last two centuries, but the ancients may have got there first. Measurement systems found at sites such as Stonehenge appear to be derived from an accurate understanding of the Earths size, and builders of the ancient temples seem to have used this system to a high degree of accuracy. In the ancient system, miles, cubits, feet and inches were all perfect subdivisions of the Earths polar or equatorial circumference or its radius. For example, the meridian polar circumference is 24,883.2 miles, equal to 135,000,000 Roman feet, 63,000,000 Sacred cubits or 129,600,000 Greek feet 129,600 is the number of seconds in the 360 degrees of a circle. The values of the various ancient measures were grouped together by whole number ratios that relate to the English geographic foot, considered by some to be the root of metrology. The spheroid shape of the Earth means that degrees of latitude measured at the poles are longer than those at the equator. The average mean degree is 69.12 miles, a length quoted by Ptolemy as 300,000 Roman remens and which is still used officially today. The canonical diameter of the Earth 7,920 miles can be expressed as 8 x 9 x 10 x 11 miles. Likewise, the equatorial circumference 24,902.86 miles is 360,000 x 365.242 English feet also the number of days in a year, enabling space, time and angle to be worked out in either feet, days or degrees. The equatorial circumference relates to the meridian through the fraction 12611260 see other examples in table bottom right. Various people have tried to explain how this could have been worked out in ancient times, but no one really knows measurinG The earTh geodesy and ancient metrology Left Geodetic map of the Earth with Giza as the Prime Meridian 30 degrees north, showing degree values. Meridian circuMference 24,883.2 Miles equatorial circuMference 24,902.948 Miles Polar radius 3,949.7142 Miles equatorial radius 3,963.42857 Miles Mean radius 3,958.6909 Miles Mean diaMeter 7,917,3838 Miles Above Eratosthenes 276195 BC experiment to measure the size of the Earth. He calculated the polar circumference to within 180 miles by noting the angle of a midday midsummer shadow in Alexandria. He knew that at Syene, 500 miles to the south, the midday midsummer Sun cast no shadow. The angle of the shadow at Alexandria was roughly 7 degrees, or onefiftieth of 360, so 50 x 500 miles gave him 25,000 miles as the circumference 24,821 miles is the modern measurement. Above The principal dimensions of the Earth from the study of ancient geodesy courtesy Robin Heath, after John Neal. These numbers pop up frequently in metrology. For example, the Royal cubit identified by Petrie at the Great Pyramid differs as 441 to 440 to a Royal cubit of 127 English feet. The fractional relationships seem to have been known to the builders of ancient sacred sites.