Dead Reckoning

Navigate Without Stars

Before GPS. Before radio. Before anything. A navigator with a compass, a clock, and a log line. You know your speed. You know your heading. You know how long you've been sailing. From these three facts, you calculate where you must be. And then you go there.

Take a Noon Sight → Open the Log

The Navigation Chronology

1484

The Altitude-Latitude Problem Solved

João II of Portugal commissions a commission. The declination of the sun is measured daily. A table is computed. For the first time, a navigator can fix latitude from the altitude of the sun at noon. The ocean becomes gridded.

1714

The Longitude Act

The British Parliament offers £20,000 to whoever solves the longitude problem. Ships are lost constantly — latitude is known, longitude is not. John Harrison spends his life answering. His H4 chronometer, small enough to pocket, accurate enough to navigate by, wins it.

1759

Harrison's H4

A pocket watch that loses 5 seconds over 81 days at sea. This level of precision means longitude to within half a degree. An entire civilization of lost ships — solved by a clockmaker's obsession with accuracy.

1978

GPS First Satellite

The first NAVSTAR satellite launches. Dead reckoning becomes historical curiosity. But every GPS receiver still does it — between satellite fixes, it estimates position from speed and heading. Harrison's ghost is in every phone.

The Navigator's Tools

The Sextant

Measuring Heaven

The sextant measures the angle between a celestial body and the horizon. From this angle and the time, you can calculate your position on the surface of the earth with an accuracy of one nautical mile.

A brass instrument. Two mirrors, an arc of sixty degrees, a vernier scale. In trained hands, it reads the sky like a sentence.

Accuracy: ±0.2 arc minutes

Dead Reckoning

From Known to Unknown

You know where you were. You know your speed. You know your heading. You know how long you've sailed. Therefore, you know where you are. Simple arithmetic. Except the current and the wind do not cooperate.

Every dead reckoning estimate contains error. The art is knowing how the error accumulates, and correcting it at every star sight.

Formula: Position = Start + (Speed × Time × Direction)