Direction, Latitude and Longitude — Page 14, Lesson 21

Let’s start with the idea of latitude itself, because everything in navigation hangs off this. I want you to picture the Earth as a sphere for a moment, with the Equator drawn around its middle. Now, come in from the Equator to the centre of the Earth. From that centre point, construct a line that rises up at an elevation angle of, say, 40° from the Equator. For a Northerly latitude, that’s an elevation angle of 40°. For a Southerly latitude, it’s a depression angle of 40° — same idea, just going down instead of up. Where that line touches the surface of the Earth is the 40°N parallel of latitude. So a parallel of latitude is literally a circle on the Earth’s surface defined by that angle from the centre. The range of latitude values runs from the Equator, which is 0°N/S, all the way to the geographic poles. The North Pole is 90°N, and the South Pole is 90°S. So latitude is always expressed with a hemisphere — N or S — and it can never exceed 90°. Now, here’s a subtlety that matters a lot in real navigation. The definition I just gave you 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 is called Geocentric Latitude. But the Earth is not a perfect sphere — it’s an oblate spheroid, meaning it’s slightly flattened at the poles and bulging at the Equator. Because of that, we use a different definition: Geodetic Latitude, also called 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 is the line at 90° to the surface at that point. And here’s the key point: that normal line does not necessarily pass through the centre of the spheroid. On a perfect sphere, the normal would always point to the centre, but on the oblate spheroid, it doesn’t. The diagrams in the book exaggerate the shape for illustration, but the real spheroid is much closer to a sphere than shown. Now, which one do we actually use? The latitudes plotted on navigation charts are Geodetic Latitudes. That’s the one you’ll work with in practice. The difference between Geocentric and Geodetic Latitude is small, but it’s not zero. The maximum difference occurs at approximately 45°N/S, and it’s about 11.6 minutes of arc. That’s a real, measurable difference — about a fifth of a degree — and it matters when you’re doing precise navigation. Now, there are a few special parallels of latitude, other than the Equator, that you’ll need to know. These relate to the seasons and the periods of day and night throughout the year, and they’ll be explained fully in the chapter on Time. But let me introduce them now. The Arctic Circle is the parallel of 66½°N. Note that 66½° is the value of the Earth’s tilt — that’s why it’s special. The Antarctic Circle is the parallel of 66½°S. Then we have the Tropic of Cancer, which is the parallel of 23½°N. The Sun is overhead the Tropic of Cancer on mid-summer’s day in the Northern hemisphere. And the Tropic of Capricorn is the parallel of 23½°S. The Sun is overhead the Tropic of Capricorn on mid-winter’s day in the Northern hemisphere. So you have four special parallels: two at 66½° — Arctic and Antarctic Circles — and two at 23½° — Tropic of Cancer and Tropic of Capricorn. The Equator, at 0°, is the fifth special parallel, but it’s the baseline for everything else. So, to tie it together: latitude is the angular distance from the Equator, measured from the centre of the Earth for Geocentric, or from the normal to the spheroid for Geodetic. We use Geodetic on charts. The range is 0° to 90°N/S. And the special parallels — 23½° and 66½° — mark the boundaries of the tropics and the polar circles, tied to the Earth’s tilt and the Sun’s overhead position through the seasons. That’s the foundation you need before we move on to longitude.