Glossary

Terms used across NavMuseum, each with a short definition and a link to where it is explained with a working figure.

Additional secondary phase factor (ASF)

A correction for the extra delay of a ground wave traveling over land rather than sea water. Loran-C time differences were predicted for propagation over sea water; ASF corrections in charts, tables and receivers accounted for slower paths over land. See it in The invisible lighthouse →

Altitude (of a body)

The angle of a star, planet, Sun or Moon above the horizon, 0° at the horizon to 90° at the zenith. See it in Latitude from the sky →

Assumed position (AP)

A convenient position near the DR from which a sight is reduced; chosen to make table entries whole degrees. See it in From a sight to a fix →

Backstaff (Davis quadrant)

An instrument described by John Davis in 1594 for measuring the Sun’s altitude with your back to it, by its shadow. See it in How a sextant works →

Baseline extension

The continuation of the line between two stations beyond either station, where hyperbolic fixes are very poor. On the baseline extension the time difference barely changes as the receiver moves, so small timing errors imply large position errors. Loran charts marked these zones. See it in The invisible lighthouse →

Celestial pole

The point in the sky toward which Earth’s axis points; the sky appears to turn around it. See it in Latitude from the sky →

Circle of equal altitude

All the places that see a body at the same altitude: a circle centered on its GP with radius 60 × (90° − Ho) nautical miles. See it in From a sight to a fix →

Clearing the distance

Reducing a measured lunar distance (limbs, from the deck, through the air) to the geocentric center-to-center distance the almanac lists. Adds semidiameters, removes refraction and the Moon’s parallax along each body’s vertical circle, and re-solves the zenith–Moon–body triangle. See it in Longitude is a clock problem →

Cocked hat

The small triangle formed when three lines of position fail to meet in a point. With independent, symmetric errors the true position lies inside it only about a quarter of the time on average; a small hat does not prove a good fix. See it in From a sight to a fix →

Cone of silence

The zone directly above a radio range station where the signal fades out. On a four-course low-frequency radio range, the loop antennas radiate little straight up, so a pilot passing over the station heard the signal die away; this marked station passage. The zone widens with height. See it in The invisible lighthouse →

Dead reckoning (DR)

Estimating position by advancing a known fix along the course steered and the distance run. A DR ignores current, leeway and steering error; correcting it for an estimated current gives an estimated position (EP). Its uncertainty grows with time since the last fix. See it in From a sight to a fix →

Declination

A body’s angular distance north or south of the celestial equator; the sky’s version of latitude. See it in Latitude from the sky →

Departure

East–west distance corresponding to a longitude difference at a given latitude: 60 NM × Δλ(°) × cos φ. See it in Longitude is a clock problem →

Dilution of precision (GDOP, HDOP)

A geometry factor that converts measurement error into position error. HDOP = √trace((GᵀG)⁻¹), where each row of G is the sensitivity of one measurement to position. Position error (drms) is roughly HDOP times the measurement error expressed as a distance. Large values mean shallow crossing angles or weak sensitivity. See it in The invisible lighthouse →

Dip (of the horizon)

How far the visible sea horizon lies below the true horizontal for a raised eye: about 1.76′ × √(height in meters). See it in How a sextant works →

Doppler shift

The change in received frequency caused by motion along the line of sight: higher while a source approaches, lower as it recedes. To first order f_r = f₀(1 − v_r/c), with v_r the range rate. Transit receivers located themselves from the Doppler history of one satellite pass. See it in You have four satellites →

Double reflection

A ray reflected by two plane mirrors at angle θ to each other is turned through 2θ, whatever its angle of incidence; so a sextant’s arc is engraved at twice the index arm’s rotation. See it in How a sextant works →

eLoran

Enhanced LORAN: a modernised low-frequency terrestrial radio-navigation system studied and trialled as a backup to satellite navigation. See it in You have four satellites →

Emission delay

In Loran-C, the time from the master's transmission to a secondary's: baseline travel time plus a coding delay. The delays ensure the master's pulses always arrive first throughout the coverage area. Published time differences include the emission delay. See it in The invisible lighthouse →

Equal altitudes

Finding the moment of local noon as the midpoint between two times at which the Sun has the same altitude, morning and afternoon. See it in Longitude is a clock problem →

Equation of time

Apparent solar time minus mean solar time: how far a sundial runs ahead of or behind a clock, between about −14 and +16½ minutes over a year. Caused by the eccentricity of Earth’s orbit and the tilt of its axis. Ignoring it in a noon longitude costs 1° for every 4 minutes. See it in Longitude is a clock problem →

Error ellipse

The region, centered on a fix, within which the true position lies with a stated probability (for example 1σ or 95 %). Computed from the covariance of a least-squares fix. Lines that cross at a shallow angle give a long, thin ellipse along the lines. See it in From a sight to a fix →

Estimated position (EP)

A DR position corrected for an estimate of current and wind (set and drift). See it in From a sight to a fix →

Etak

A Carolinian (Micronesian) way of tracking progress: the canoe is pictured as still while a reference island off to the side moves back beneath successive star points. See it in Reading the world →

Geographic position (GP)

The point on Earth directly beneath a celestial body: latitude = declination, longitude = −GHA. See it in From a sight to a fix →

GNSS

Global navigation satellite system: GPS, GLONASS, Galileo, BeiDou and similar systems that position receivers from satellite pseudoranges. See it in You have four satellites →

Group repetition interval (GRI)

The interval between successive pulse groups from a Loran-C master; it identifies the chain. Given in microseconds; the designator is the GRI divided by ten (for example 99,600 µs, chain 9960). All Loran-C chains shared 100 kHz and were distinguished by GRI. See it in The invisible lighthouse →

Horizon mirror

The fixed, half-silvered glass through which you see the horizon directly and, in its silvered half, the body reflected from the index mirror. See it in How a sextant works →

Horizontal parallax (HP)

The angle Earth’s equatorial radius subtends at a body; for the Moon 54′–61½′. Parallax in altitude is HP × cos h. See it in Longitude is a clock problem →

Hyperbolic navigation

Finding position from differences in arrival time (or phase) of signals from synchronized stations. A constant difference of distances to two stations defines a hyperbola with the stations at its foci; two such differences give two hyperbolas whose crossing is the fix. Gee, Loran and Decca were hyperbolic systems. In three dimensions the locus is a hyperboloid. See it in The invisible lighthouse →

Index arm

The sextant’s pivoted arm carrying the index mirror; its position on the arc gives the reading. See it in How a sextant works →

Index error

The reading a sextant gives when the two images of one distant object coincide; it is applied with reversed sign to every sight. See it in How a sextant works →

Intercept (altitude intercept)

a = Ho − Hc in arcminutes, which equals nautical miles: how far the LOP lies toward (+) or away from (−) the body, measured from the AP. The basis of the Marcq Saint-Hilaire method (1875). Hc is the altitude computed for the AP; Ho is the observed altitude. See it in From a sight to a fix →

Ionospheric delay

The extra apparent range of a satellite signal crossing the ionosphere's free electrons; largest at low elevation and removable with two frequencies. See it in You have four satellites →

Iṣbaʿ

“Finger”: the unit of star altitude used by Arab and Indian Ocean pilots, roughly 1.5° to 1.75°. See it in Reading the world →

Kamal

An Indian Ocean altitude instrument: a small board on a knotted cord held so it spans horizon to star; its origin and chronology are contested. See it in How a sextant works →

Lane (Decca)

The strip between two successive in-phase hyperbolas of a phase-comparison system; λ/2 wide on the baseline. A phase measurement fixes the position within a lane but not which lane; lanes were counted as a ship moved and identified by a coarser measurement. A lane slip is choosing the wrong one. See it in The invisible lighthouse →

Latitude

Angular distance north or south of the equator, 0° to 90°. Navigators measure it as the angle between the local vertical (a plumb line) and the plane of the equator. See it in Latitude from the sky →

Latitude sailing

Reaching a destination by first sailing to its latitude, then running east or west along that parallel. See it in Latitude from the sky →

Line of position (LOP)

A line on which an observation says you lie. A celestial LOP is the straight tangent to a circle of equal altitude near your position. First used at sea by Captain Thomas Sumner in December 1837. Radio bearings, ranges and time differences give other kinds of LOPs. See it in From a sight to a fix →

Local apparent noon

The moment the Sun crosses your meridian and stands highest in your sky. See it in Latitude from the sky →

Local apparent time

Time kept by the real Sun at your meridian: 12:00 when the Sun crosses it. See it in Longitude is a clock problem →

Local mean time

Time kept by a fictitious mean Sun moving uniformly; differs from apparent time by the equation of time. See it in Longitude is a clock problem →

Longitude

Angular distance east or west of a reference meridian (today Greenwich), 0° to 180°. Because Earth turns 15° per hour relative to the Sun, a longitude difference equals a difference in local time: 4 minutes of time per degree. See it in Longitude is a clock problem →

Lunar distance

The angle between the Moon and the Sun, a planet or a bright star near its path; it changes by about 30′ per hour and so served as a clock. Compared with almanac predictions it gives Greenwich time: 1′ of distance ≈ 2 minutes of time ≈ 30′ of longitude. See it in Longitude is a clock problem →

Marine chronometer

A portable timekeeper built to keep a steady, predictable rate at sea, set to the time of a reference meridian. What matters is predictability, not zero error: the navigator applies the measured daily rate, and only the unpredicted part becomes longitude error (1 second of time = 15″ of longitude). See it in Longitude is a clock problem →

Meridian

The north–south great circle through your zenith and the celestial poles. A body is highest when it crosses it. See it in Latitude from the sky →

Micrometer drum

A graduated drum on a sextant’s tangent screw: one turn moves the arm one degree, and the rim reads minutes of arc. See it in How a sextant works →

Monsoon

A seasonal reversal of the prevailing wind; in the Arabian Sea, broadly from the north-east in the northern winter and from the south-west in summer. See it in Reading the world →

Multipath

Reception of a signal by a reflected path as well as (or instead of) the direct one, which lengthens the measured pseudorange. See it in You have four satellites →

Nautical mile (NM)

1852 m exactly; originally one arcminute of latitude, so 60 NM ≈ 1° of a great circle. See it in Latitude from the sky →

Observed altitude (Ho)

A sextant altitude corrected for index error, dip, refraction, semidiameter and parallax: the altitude of the body’s center above the true horizon, as from Earth’s center. See it in How a sextant works →

Octant (Hadley’s quadrant)

The first practical reflecting instrument for use at sea (presented 1731): a 45° arc measuring up to 90°. See it in How a sextant works →

Parallax (in altitude)

The difference between a body’s altitude seen from Earth’s surface and from its center; for the Moon up to about 1°, for the Sun under 0.15′. See it in How a sextant works →

Periplus

An ancient sailing guide listing coasts in order, with distances, harbours and hazards (plural periploi). See it in Reading the world →

Precession

The slow circling of Earth’s axis, about once every 26,000 years, which moves the celestial poles among the stars. See it in Latitude from the sky →

Pseudorange

The distance a satellite signal appears to have travelled, measured with the receiver's imperfect clock: true range plus c times the receiver clock error. ρ = c(t_received − t_sent) = |s − r| + c·b. Because the same clock error b affects every pseudorange equally, a receiver solves for it as a fourth unknown alongside x, y and z. See it in You have four satellites →

Radial (VOR)

A magnetic bearing from a VOR station outward; a line of position, not a heading. An aircraft is on a given radial whatever its heading. The VOR receiver derives the radial by comparing the phase of two modulations in the station's signal. See it in The invisible lighthouse →

Radiogoniometer

A small rotating coil between two fixed crossed antennas that finds a radio null without turning the antennas. Introduced with the Bellini–Tosi direction finder (patent filed 1907). The fixed crossed loops reproduce the arriving field inside the instrument, and the operator turns the small search coil to find the null and read the bearing. See it in The invisible lighthouse →

Rate (of a chronometer)

How many seconds a chronometer gains or loses per day, measured in port and applied at sea. See it in Longitude is a clock problem →

Receiver clock bias

The offset b of a receiver's clock from system time; it adds c·b (300 km per millisecond) to every pseudorange. See it in You have four satellites →

Refraction (astronomical)

The bending of light in the atmosphere that makes bodies appear higher than they are: about 1′ at 45°, roughly 34′ at the horizon. See it in How a sextant works →

Running fix

A fix from lines of position taken at different times, with the earlier lines advanced along the DR track to a common time. See it in From a sight to a fix →

Rutter

A book of sailing directions: courses, distances and landmarks from one place to the next. See it in Reading the world →

Schuler tuning

Designing an inertial navigator so its errors oscillate with an 84-minute period (√(R/g)·2π) instead of growing without limit. See it in You have four satellites →

Selective Availability

The deliberate degradation of the civil GPS signal, discontinued at midnight after 1 May 2000. See it in You have four satellites →

Semidiameter

Half the angular diameter of the Sun or Moon (about 16′); added for a lower-limb sight, subtracted for an upper-limb sight. See it in How a sextant works →

Sextant

A reflecting instrument whose 60° arc measures angles up to 120° by bringing the image of a body into coincidence with the horizon. See it in How a sextant works →

Sextant altitude (Hs)

The angle read from the sextant’s arc, before any corrections. See it in How a sextant works →

Sight reduction

Turning a corrected sextant altitude and its time into a line of position: find GHA and declination, choose an AP, compute Hc and Zn, compare with Ho, plot. See it in From a sight to a fix →

Spoofing

Broadcasting counterfeit navigation signals so that a receiver computes a position or time chosen by the attacker. See it in You have four satellites →

Star compass

A mental ring of directions around the horizon defined by where named stars rise and set. Documented in the central Caroline Islands (Micronesia). The Hawaiian star compass of 32 houses is a modern framework developed by Nainoa Thompson from Mau Piailug’s Micronesian compass. See it in Reading the world →

Star path

A sequence of stars that rise (or set) at nearly the same point on the horizon, one after another through the night, so a navigator can steer toward each in turn. Stars of similar declination share a rising bearing at a given latitude: cos A = sin δ / cos φ. See it in Reading the world →

Stick chart

A Marshallese teaching model of sticks and shells representing how swells bend and reflect around islands; studied ashore, not carried at sea. See it in Reading the world →

Sunstone (sólarsteinn)

A crystal mentioned in Icelandic texts; the idea that Norse navigators used polarizing calcite to find the Sun through cloud is plausible but contested. See it in Reading the world →

Swell

Long, regular ocean waves raised by distant wind systems, which keep their direction for days and can be felt through a hull. See it in Reading the world →

Swinging the arc

Rocking the sextant about its line of sight so the body’s image swings; the lowest point of the swing marks the vertical, where the reading is correct. See it in How a sextant works →

Time sight

A single altitude of a body, with latitude and declination, solved for local time and hence longitude. See it in Longitude is a clock problem →

Transit (Navy Navigation Satellite System)

The first operational satellite navigation system (1964–1996), which fixed position from the Doppler shift of polar-orbiting satellites at 150 and 400 MHz. See it in You have four satellites →

Vernier

A short auxiliary scale whose divisions are slightly smaller than the main scale’s, so the line that coincides gives a fraction of a division. See it in How a sextant works →

Zenith

The point directly overhead, straight up along your plumb line. See it in Latitude from the sky →

Zenith distance

90° minus altitude: the angle from the zenith down to a body. On Earth it equals the distance from you to the point beneath the body, in degrees (60 NM per degree). See it in Latitude from the sky →

Zenith star

A star that passes straight overhead at a particular latitude (where latitude equals its declination), used in modern Pacific voyaging teaching to judge north–south position. See it in Reading the world →