Low down on the sidelands of Lundy’s east coast, separated by an exact distance of one nautical mile (= 1.151 miles or 1,852 m), are two pairs of white-painted metal poles with diamond shapes on their tops. You should be able to see the southern pair when you arrive on the Oldenburg; it’s just above a small embayment known Ladies Beach. The lower East Side path passes just above it. The northern one is at Tibbets Point, about 3/5^th of the way up the island.

They look completely out of place, against a backdrop of bracken and rugged scenery. So what are they there for? Well, they do indeed have a specific function. They are a simple way of allowing a ship to check if its actual speed through the water is the same speed its instruments on the bridge are indicating. To test this out, a ship will run on a parallel course to the markers a short distance offshore at, say, 10 knots (or 10 nautical miles per hour). When perpendicular to the first marker pair (indicated when the diamond on each pole is in line), a stopwatch is started, being stopped when the ship passes the second marker pair. If there was no tidal current at the time, then the time elapsed would be expected to be 60 mins/10 = 6 mins. This calculation gets a little trickier when there is a tidal current running, which may either allow the ship to move faster (“over the ground”) or slower, depending on the direction and speed of the current.

A nautical mile is defined as “the meridian arc length corresponding to one minute (1/60^th of a degree) of latitude at the equator”, so the Earth’s polar circumference is very near to 21,600 nautical miles (that is,  60 minutes x 360 degrees).