Direction, Latitude and Longitude — Page 14, Lesson 21

I want to walk you through the concept of latitude, starting with how we define it on a diagram. On Figure 1.8, imagine you draw a line from the Equator into the centre of the Earth. For a Northerly latitude of, say, 40°N, you construct a line from the centre of the Earth with an elevation angle from the Equator of 40°. Where that line touches the Earth's surface is the 40°N parallel of latitude. For Southerly latitudes, you use a depression angle instead of an elevation angle. The range of latitude values runs from the Equator at 0°N/S up to the geographic poles. The North Pole is 90°N, and the South Pole is 90°S. Now, the definition I just gave is based on the centre of the Earth. It's the smaller angle between the line joining the point to the centre of the Earth and the plane of the Equator. That's called Geocentric Latitude. But here's the key point: the Earth is not a perfect sphere. It's actually an oblate spheroid — it's slightly flattened at the poles and bulging at the Equator. Because of that shape, we use a different definition of latitude for practical navigation, called Geodetic Latitude, also known as Geographic Latitude. Geodetic Latitude is the smaller angle between the normal to the meridian at the point on the spheroid and the plane of the Equator. The normal means a line that is at 90° to the surface at that point. This normal line does not necessarily pass through the centre of the spheroid. The diagram exaggerates the shape for illustration, but in reality the spheroid is much closer to a sphere than shown. Here's the important takeaway for you as a pilot: the latitudes plotted on navigation charts are Geodetic Latitudes, not Geocentric. The maximum difference between Geocentric and Geodetic Latitude occurs at approximately 45°N/S, and that difference is about 11.6 minutes of arc. That's a small but real discrepancy. Let me also introduce you to the special parallels of latitude, other than the Equator. These relate to the seasons and periods of day and night throughout the year, and we'll cover them in more detail in the chapter on Time. For now, here are the names and values: - Arctic Circle: the parallel of 66½°N. That 66½° value comes from the Earth's tilt. - Antarctic Circle: the parallel of 66½°S. - Tropic of Cancer: the parallel of 23½°N. The Sun is overhead the Tropic of Cancer on mid-summer's day in the Northern hemisphere. - Tropic of Capricorn: the parallel of 23½°S. The Sun is overhead the Tropic of Capricorn on mid-winter's day in the Northern hemisphere. So to summarise: latitude is measured from the Equator to the poles, but because the Earth is an oblate spheroid, we use Geodetic Latitude on our charts, which is based on the normal to the surface rather than a line to the Earth's centre. And we have four special parallels that mark seasonal boundaries.