The nautical mile and astronomical navigation

The nautical mile and astronomical navigation: In navigation and aviation, especially at sea, the nautical mile is used as a unit of measurement. It is equivalent to one minute of angle along a meridian on Earth.

The nautical mile and astronomical navigation, image of nautical mile definition
Giant ocean wave. Author image: Unknown

Use in Navigation and Charts

A nautical mile is defined as one minute of arc measured along any meridian on Earth.

The nautical mile and astronomical navigation, image one nautical mile on latitude scale

In terms of distance, a nautical mile is equal to 1.852 kilometers.

People commonly use it to measure distances at sea or in the air.

The use of NM, the internationally agreed-upon measure of one nautical mile, which is exactly 1.852 meters, remains in place today


The nautical mile and astronomical navigation, image flying fish
Flying Fish. Author image: Mike Prince from Bangalore, India

The nautical mile and astronomical navigation

Understanding the Earth as a sphere helps explain the nautical mile, which is based on Earth’s circumference of approximately 40,000 kilometers. One nautical mile corresponds to one minute of arc along a meridian.

After dividing the half-circle into equal portions of 360°, one can further divide these degrees into 60 minutes.

Decorative image of a ship at anchor and a beach with a black cat.
Classic explanation
The nautical mile and astronomical navigation, image definition NM

Within navigation, one of these minutes (also referred to as minutes of arc) along a great circle on Earth represents a single NM.

decorative image: beach, sea, bench, boat, rum barrel

overall, nautical charts employ one of three map projections: the gnomic, polyconic, and Mercator.

As a matter of fact, among these three, the Mercator projection takes precedence as lines of latitude and longitude intersect at right angles, creating a rectangular grid.

Cylindrical Mercator projection
cylindrical Mercator projection

In fact, this grid enables users to plot straight lines.

This projection is used for nautical charts because it preserves the shape of continents and maintains angles, which is convenient for navigation with a compass.

Mercator maps are not usable at high latitudes due to extreme distortion of distances and areas, making the geographic representation inaccurate and impractical for navigation.

decorative image of the great bear

Also, on this website, we explore creating a simplified plotting sheet inspired by the Mercator concept

Indeed, incorporating the NM, which represents one minute of latitude, greatly simplifies navigation in open water.

Consequently, it assumes a vital role in exploration, shipping, and geography, contributing significantly to these fields.


The nautical mile and astronomical navigation, image Artic sea
Arctic Sea fog at Garths Voe. Author image: John bateston

Difference between a nautical mile and a knot. NOAA

decorative image of a rigging chair