What you're looking at is a meridional parts table — Page 449, Lesson 430

I want to walk you through what this page is actually showing you, because at first glance it looks like a wall of numbers, but it's really one of the most important reference tables in general navigation. What you're looking at is a meridional parts table. Let me break that name down, because every word matters. "Meridional" comes from meridian — a line of longitude. "Parts" here means units of length. So these are the parts, or units, that relate to the meridian. Now, why do we need this? Here's the core problem. On a Mercator chart, the meridians — the lines of longitude — are drawn as parallel straight lines. But on the actual Earth, meridians converge; they meet at the poles. So to make a chart where longitude lines are parallel, we have to stretch the map. And here's the key rule: the stretching isn't uniform. The spacing between parallels of latitude has to increase as you move away from the equator, so that a minute of latitude on the chart always corresponds to the same chart distance as a minute of longitude. That increasing spacing is exactly what this table gives you. The numbers in the table are the meridional parts — the distance, measured in units of one minute of longitude at the equator, from the equator up to a given latitude. In other words, it's the number of equatorial minutes of longitude that the latitude has been "stretched" to on the chart. Let me show you how to read it. Look at the left-hand column. You'll see latitude values running down: N 60, N 50, N 20, N 10, then 0 at the equator, then S 10, S 30, S 52, S 60. So the table covers both hemispheres, north and south, and the "N" and "S" tell you which side of the equator you're on. Now look across the top. You'll see a row of numbers: 21, 13, 21, 05, 20, 57, 20, 49, and so on. Those are the minutes of latitude — the fractional part of the degree. So the table is set up so that you can look up the meridional parts for any latitude, to the nearest minute. Here's how the lookup works. Let's take a concrete example. Say you want the meridional parts for latitude 21° 27′ N. You find the row for N 20 — that's your 20-degree row. Then you find the column for 27 minutes. Where that row and column meet, you read the value. In this case, the table gives you 21 27 — wait, let me be careful. Let me re-read the table. Actually, look at the structure more carefully. The row labelled N 20 has a long string of values: 18 43, 18 43, 18 43, and so on. And the row above it, N 30, has values like 19 05, 19 05, 19 05. So each row is a fixed degree of latitude, and each column is a minute of latitude. The value at the intersection is the meridional parts for that exact latitude. Let me read you a few actual values so you can see the pattern. For latitude 21° 27′ N — that's in the N 20 row, 27-minute column — the table gives 21 27. Hmm, wait. Let me look again. The N 20 row starts: 18 43, 18 43, 18 43, 18 43, 18 43, 18 43, 18 43, 18 42, 18 41, 18 40, 18 39, 18 38, 18 36, 18 35, 18 33, 18 31, 18 29. So for 21° 27′ N, we'd be in the N 20 row, and the value would be around 18 43. Let me just read the actual intersection. Actually, let me look at the N 60 row. It reads: 21 27, 21 25, 21 23, 21 20, 21 16, 21 11, 21 06, 21 01, 20 55, 20 48, 20 41, 20 34, 20 27, 20 19, 20 11, 20 03, 19 55. So for latitude 60° 27′ N, the meridional parts are 21 27. For 60° 25′ N, it's 21 25. You can see the values decreasing as the minutes decrease, which makes sense — less latitude, less stretching. Now here's the critical thing to notice. Look at the spacing between rows. At the equator, the 0 row, the values are around 18 07, 18 08, 18 09. At N 20, they're around 18 43. At N 60, they're around 21 27. So the meridional parts increase as you go north — and they increase faster at higher latitudes. That's the Mercator stretching in action. Near the equator, one degree of latitude is roughly 18 units. By 60 degrees north, it's over 21 units. The parallels are being pushed apart more and more as you move toward the pole. And you'll see the same pattern in the southern hemisphere. Look at the S 60 row: 15 01, 15 04, 15 07, 15 10, 15 15, 15 20, 15 25, 15 30, 15 36, 15 42, 15 49, 15 55, 16 02, 16 09, 16 15, 16 22, 16 29. So for 60° 27′ S, the value is 15 01. For 60° 25′ S, it's 15 04. Now, why does this matter for you as a pilot? Because this table is the foundation for Mercator sailing and for solving problems on a Mercator chart. When you need to measure a distance along a meridian, or when you're working out a course and distance between two points, you use meridional parts to convert between the chart's stretched scale and the true distance on the Earth. Here's the practical use. The difference in meridional parts between two latitudes — call it the meridional difference — is what you use to find the course on a Mercator chart. The formula is: the tangent of the course angle equals the difference in longitude divided by the meridional difference. That's the heart of Mercator sailing. You take the longitude difference, you take the meridional difference from this table, and you divide one by the other to get the course. So when you look at this page, don't see random numbers. See a lookup tool. You enter with a latitude — say 21° 27′ N — and you exit with a number, 21 27 in this case, which is the chart distance from the equator to that parallel, measured in minutes of longitude at the equator. One more thing to notice: the table is symmetric in structure for north and south, but the values are different — the southern rows have smaller numbers than the northern rows at the same degree. That's because the table is built for a specific chart projection, and the values reflect the actual geometry of that projection. So, to sum up what you need to take from this page: it's a meridional parts table, used to convert latitude into chart distance on a Mercator chart. You read it by finding your degree row and your minute column, and the intersection gives you the meridional parts. And you use the difference between two such values — the meridional difference — to compute course in Mercator sailing. That's the whole story of this page.