Luke A Vavra Rare Maps And Books

The above sketch depicts a great circle and one possible rhumb line connecting two distant locations. Although I am a graduate Civil Engineer and should remember all of the following narrative, I admit I referred to Wikipedia several times.

Most of us who look at antique maps have seen lines radiating in many directions from a point on a map. Those are called “rhumb” lines. You may sound the “b” or ignore it, as you please. By definition they are lines on a map or globe which intersect “meridians” at a constant angle. (Meridians are the lines of longitude, or north-south lines, on a map, sea chart or globe.) They may be straight or curved. On a chart constructed using the Mercator projection, rhumb lines are straight lines. On other projections such as a hemisphere being shown within a circle, they must curve to keep a constant angle at all intersections with meridians. I have just used several terms that are not part of our daily conversations so I will try to explain them.

MERCATOR PROJECTION: The Mercator projection is a cylindrical map-projection introduced by Flemish geographer and cartographer Gerardus Mercator in 1569. It became the standard map projection for navigation due to its ability to represent north as ‘up’ and south as ‘down’ everywhere while preserving local directions and shapes. However, as a result, the Mercator projection inflates the size of objects the further they are from the equator. In a Mercator projection, landmasses such as Greenland and Antarctica appear far larger than they actually are relative to landmasses near the equator. Despite these drawbacks, the Mercator projection is well-suited to marine navigation and internet web maps and continues to be widely used today.

MERIDIANS: Locations on a globe or map are designated by degrees “longitude and “latitude”. Meridians are circles of constant longitude passing through a given place on the earth’s surface and the terrestrial poles. A meridian is an imaginary line that runs from the North Pole to the South Pole on the surface of the Earth. A meridian asses through a given point on the Earth’s surface and connects the two poles. Meridians are used to define longitude. On maps or globes, meridians are represented as circles or half-circles numbered for longitude. They help describe the position of a place.

RHUMB LINES: Rhumb lines usually radiate from a compass rose, but not necessarily so. Compass roses are placed at intersections of lines of longitude and latitude. The word “rhumb” comes from the name of angle measurement representing the “point” on the old fashioned compass cards. There are 32 “rhumbs” in 360 degrees, so a rhumb is 11 1/4 degrees. Rhumb lines can make a sea chart interesting to see, but their usefulness may be questioned. How did early navigators know they were on a particular rhumb line shown on the chart, or did they even want to be on one shown? No, it was the idea conveyed by the rhumb line – that if you know where you are and know where you want to go, plot that information on the sea chart, and follow the straight line between them at the bearing (or azimuth) measured on the chart. It’s not the shortest route, but is the easiest to navigate. You will recall that a “great circle” is the shortest distance between two points on a sphere, but a great circle is tedious to navigate because you must change direction periodically to stay on the great circle which actually is curved when laid on a flat sea chart. Typically, rhumb lines on antique charts originate at intersections of lines of latitude and longitude.

All parallels, including the equator, are rhumb lines, since they cross all meridians at 90º. Additionally, all meridians are rhumb lines, in addition to being great circles. A rhumb line always spirals toward one of the poles, unless its azimuth is true east, west, north, or south, in which case the rhumb line closes on itself to form a parallel of latitude (small circle) or a pair of antipodal meridians. What does “antipodal” mean?

ANTIPODES: In geography, the antipode of any spot on Earth is the point on Earth’s surface diametrically opposite to it. A pair of points antipodal to each other are situated such that a straight line connecting the two would pass through Earth’s center. Antipodal points are as far away from each other as possible. The North and South Poles are antipodes of each other. In the Northern Hemisphere, “the Antipodes” sometimes refers to Australia and New Zealand because they are on opposite sides of the Earth from England.

LONGITUDE: Longitude is the angle east or west from the Prime Meriam as if measured at the center of the Earth. For example, the Prime Meridian (which passes through Greenwich, England) is the starting point for measuring longitude. Greenwich is at “0” degrees longitude. Not all early maps conformed to the Greenwich convention and often used the nation’s capital for the location of the Prime Meridian.

LATITUDE: In geography, la Latitude and longitude are used together as a coordinate pair to specify a location on the surface of the Earth. Latitude is a coordinate that specifies the north–south location of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from −90° at the South Pole to 90° at the North Pole, with 0° at the Equator. Lines of constant latitude, or parallels, run east–west as circles parallel to the equator. Latitude has been understood and measured for more than a thousand years. It is simply, but not exactly, when measured at sea level in the Northern Hemisphere, the vertical angle to Polaris, our North Star. That will not be true in a few thousand years, but that is another story.