Direction, Latitude and Longitude — Page 8, Lesson 10

Let’s start with the shape of the Earth. The simple view is that the Earth is a sphere, and that’s nearly true. But in reality, the Earth’s shape is commonly described as an oblate spheroid — that is, a sphere which is slightly flattened at its poles. This flattening happened when the Earth formed from a gas cloud; the spin of that cloud caused higher centrifugal forces at the equatorial region than in regions nearer the poles. That centrifugal force pushed material outward at the equator, making the Earth bulge there and flatten at the poles. This flattening is called compression. For the Earth, compression is approximately 0.3%, or 1/300th. To put that in more concrete terms: the Earth’s polar diameter is 23 nautical miles — or 43 kilometres — less than its equatorial diameter. So the Earth is wider around the middle than it is from pole to pole. Now, recent satellite surveys have shown something even more interesting: the Earth is also slightly pear-shaped. Its maximum diameter actually occurs south of the Equator, in the Southern Hemisphere. This Southern hemisphere distortion is considerably smaller than the compression distortion — it’s measured in tens of metres, not kilometres. Why does this matter for navigation? If the Earth were perfectly spherical, its cross-section would be a perfect circle. Mathematicians could handle that easily when producing the equations needed for map projections. If the compression were completely symmetrical, the cross-section would be a perfect ellipse — still not too mathematically complicated. But the Earth is not quite either of those. So the only word to describe it is “Earth-shaped”. That’s exactly what the word geoid means — it comes from Greek origin, and it simply means “Earth-shaped”. Different agencies have measured and modelled the Earth, each producing their own equations to define their geoid. Each agency has tended to optimize its geoid to give the best fit to the actual shape of the Earth over the area in which it is interested. This field of study is called geodesy — the science of measuring and modelling the Earth’s shape. Let me recap the key terms: oblate spheroid — a sphere flattened at the poles; compression — the amount of that flattening, about 0.3% or 1/300th; geoid — the Earth-shaped model that accounts for all these irregularities; and geodesy — the science of measuring and modelling that shape. These are the foundations we need before we can talk about direction, latitude, and longitude.