Let me walk you through what you’re actually looking at here — Page 457, Lesson 454

This is a classic grid navigation chart — the kind you’ll use in polar regions where magnetic compasses are useless. Let me walk you through what you’re actually looking at here. First, the big picture. This is a grid chart — a map where we’ve overlaid a grid of parallel lines that point to Grid North, not True North. The whole reason we do this is that near the poles, meridians converge so sharply that True North changes direction from place to place. So we invent a fixed reference direction — the grid — and navigate relative to that. Now, look at the numbers. You’ll see latitude labels like N60, N50, N20, N10, 0, S10, S30, and so on. Those are your parallels of latitude. The 0 is the Equator. Above it you have North latitudes, below it South latitudes. So this chart spans from about N60 down to S60 — a huge swath of the globe. But here’s the key feature: the longitude values printed along each parallel. Look at the row for N60 — you see values like 16 04, 16 13, 16 09, 16 04, 16 01, 15 57, 15 55, 15 53, 15 51, 15 50, 15 50, 15 51, 15 52, 15 54, 15 57, 16 00, 16 03. These are longitudes — degrees East or West of the Greenwich Meridian. The numbers are in degrees and minutes — so 16 04 means 16 degrees 04 minutes of longitude. Now, here’s the crucial part. Look at how those longitude values change as you move across the chart at N60. They start at 16 13 on the left, drop down to 15 50 in the middle, then climb back up to 16 03 on the right. That’s a U-shape — the longitudes decrease to a minimum, then increase again. Why? Because the meridians converge toward the pole. At high latitude, the lines of longitude bunch together. So when you plot a straight line on this chart — a grid line — it doesn’t follow a constant longitude. It cuts across the meridians. The longitude values along that line first decrease, then increase, because the line is curving relative to the converging meridians. Now compare that to the row at N10. Look at the values: 17 57, 17 57, 17 58, 17 58, 17 59, 18 00, 18 01, 18 02, 18 03, 18 05, 18 06, 18 08, 18 09, 18 11, 18 12, 18 14. Here the longitudes steadily increase — no U-shape. They just climb from 17 57 to 18 14. That’s because at lower latitude, the meridians are more parallel, so a straight grid line crosses them more uniformly. And at the Equator — the 0 row — you see 18 11, 18 12, 18 13, 18 14, 18 15 — again a steady increase, no reversal. So the pattern is this: the higher the latitude, the more the longitude values along a grid line curve — decreasing then increasing. At N60 you get that pronounced U. At N10 and the Equator, it’s a straight climb. That’s the signature of meridian convergence — the closer you get to the pole, the more the meridians pinch together, and the more a straight grid line has to bend relative to them. Now look at the S latitudes — S10, S30, S40, S50, S60. At S10 you see 26, 27, 29, 30, 32, 33, 35, 37, 38 — those are the longitude values, and they’re increasing. But look at S60 — the bottom row: 21 32, 21 43, 21 55, 22 06, 22 17, 22 27, 22 39, 22 48, 22 57, 23 04, 23 08, 23 10, 23 09, 23 07, 23 03, 22 56. Here the values increase to a peak at 23 10, then decrease — an inverted U. Same convergence effect, mirrored in the Southern Hemisphere. So the takeaway: on a grid chart, the longitude values along a grid line are not constant. They change because the grid is a fixed rectangular framework overlaid on a converging spherical surface. The amount of change — and whether it’s a U or an inverted U — depends on your latitude and hemisphere. Let me show you the actual grid construction. — this is Figure 27.7, "Creation of grid." It shows how you take the converging meridians and overlay a set of parallel grid lines. The grid lines are all parallel to each other, pointing to Grid North, which is a single fixed direction across the whole chart. That’s what makes navigation possible in polar regions — you have one consistent reference instead of a True North that shifts with every meridian. And here’s the other figure. — Figure 27.1. It shows a point A at longitude 70W, and the line defining True North is the line joining A to the North Pole. That’s the fundamental idea — True North at any point points toward the geographic North Pole. But on a grid chart, we replace that with Grid North, which is the same everywhere. So when you read this chart, every one of those longitude numbers — 16 04, 15 50, 18 14, 23 10 — is telling you where the grid line crosses a particular meridian. And the pattern of those crossings — the U-shapes, the steady climbs — is the visual proof of meridian convergence and how the grid compensates for it. That’s the core of grid navigation: a fixed reference direction, overlaid on a converging sphere, with the longitude values along each grid line showing exactly how the grid relates to the true meridians at every latitude.