Why Are Nautical Miles Used To Measure Distance In Space?

If you use a fitness band or smartwatch, you probably receive daily messages of how many miles (or kilometers, depending on the setting) you've walked. But that's only on land. If you travel by water or air, your distance is measured in something known as "nautical miles," and the same is true for venturing out in space because they all rely on the same coordinate system.

In normal land-based travel, the distance between where you need to go and how far you need to travel is usually relatively small compared to the size of Earth. During these scenarios, regular miles (and kilometers) are sufficient. However, since oceans take up so much of Earth's surface with nary a landmass as a reference point, people must navigate using map points based on latitude (the angular distance from the equator). Nautical miles measure the distance between these points. Air and space travel work on roughly the same principle — landmasses are too far away to rely on while flying — so cartographers just carried nautical miles over from one to the other. If it ain't broke, don't fix it.

Despite using the word "mile" in the name, a nautical mile is slightly longer than a land-based mile. In order to travel a nautical mile, you need to venture the equivalent of 1.1508 standard miles. And before you ask, there's no such thing as a "nautical kilometer." 

When you plot a path on your navigation apps, the distance you need to travel is often further than the distance between the start and end points due to topography. You need to make turns that add to total distance. Even "straight" highways have curves that increase the total mileage. But nautical miles don't need to worry about any of that, so they are measured using actual straight lines (that take the Earth's curvature into account) and how they translate to distance on a map.

In order to determine the number of nautical miles between two points, you use geographic coordinates. One nautical mile is equal to one minute of latitude, which there are 60 of in a degree of latitude. Longitude, meanwhile, doesn't come into play when measuring nautical miles because unlike latitude, the degrees of longitude aren't consistent — nautical miles rely on the consistency of latitudinal degrees to work.

Let's say you need to measure nautical distance by hand (a useful skill inside a submarine since GPS signals don't work underwater). On a nautical chart, draw a straight line between your starting position and your destination. Then take a specialized tool known as a divider (similar to the compass you used in grade school to draw circles) and line it up with the width of a nautical mile as provided on the side of a Mercator-projection map. Starting at one end, walk the divider along your line until you get to the other end. The number of times you need to rotate the divider on a point to get from one end to the other is the distance in nautical miles. If the line isn't long enough to reach the divider width on the final rotation, readjust the tool and measure it against the chart's nautical mile ruler. Add that to the total number of full nautical miles, and you have your distance.