I hope this brief glossary of navigation terms will be of assistance to those wishing to improve their navigational skills.
the true depth of water
as opposed to a
predicted depth. Tidal height from table added to the charted depth.
an annual publication
containing Tide Tables
astronomical ephemerals etc.
the angular distance of
a celestial body
above the viewers horizon.
by warm air
rising up a slope to be replaced by cooler air, as opposed to kabatic,
the wind as felt on
board, this will be the
actual wind modified by the wind created by the vessels movement.
the constellation of the
Ram in the Northern
Celestial Hemisphere, see also, First Point of Aries.
an instrument used to
determine the altitude
of celestial objects before the introduction of the sextant. The first
use by Greek astronomers in around 200 B.C.
Azimuth; the horizontal angular distance from a reference position to a celestial body, usually measured clockwise in degrees from the direction of the nearest pole.
an instrument for
the rate at
which atmospheric pressure rises of falls.
an unlit navigation mark.
the compass reading
taken of a object in
relation to the observer.
numerical method of
describing wind strength.
a celestial object.
piece of land that that
projects out into a large body
the direction of safe water with reference to the cardinal points.
four main points of
the compass, North, East, South and West.
the study and the
course to steer which
has been corrected for current, variation, and deviation.
navigation; a method of
navigating by referring to the stars or other objects in the sky.
imaginary sphere with
the earth at its center, on whose surface the stars, planets and other
bodies appear to be situated.
the distinctive pattern
of flashes used to
identify a light.
reference level on
charts to which tidal heights are referred.
depth shown on the
chart for a particular point on the sea bed.
a very accurate clock or
watch that is used
for determining one's longitude at sea
travel around the
Civil twilight; times given in the almanac, for listed latitudes for approximately the best time to take a morning or evening sight, when both the horizon and the heavenly bodies are visible.
CMG; the Course Made Good is
the course over the ground.
hat; a triangle formed by a
series of three LOPs
which do not meet at an exact point.
international rules for
prevent of collisions at sea.
a device that always
points towards magnetic
north, used for navigation.
a design on a chart
that shows direction. It points which way is north, south, east, west,
Conversion tables; used
units of one system to units in another.
astronomer credited with disproving the Ptolemaic System.
direction in which a
vessel is heading or is intended to be steered, the direction through
Course made good; the
which the vessel is actually traveling as opposed to the direction in
is pointing. Corrections;
alterations made to
charts to update
using LOPs from several
navigational aids or
marks to obtain a position fix.
Crux; the Southern Cross a constellation seen in the southern hemisphere.
reckoning; determining a
position by plotting courses and speeds from a known position.
angular distance to
a point on the celestial sphere measured north and south from the
equator along the hour circle.
the compass errors
caused by metallic o
magnetic objects on board the vessel.
a correction to be
applied to sextant
attitudes to compensate for height of eye above sea level.
Convergence Zone (or ITCZ) a belt of very still air between 5 degrees
5 degrees south of the equator.
the speed in knots of
the effect of current or tide on a
height above chart
datum, on a chart, of any areas which are uncovered at low water.
Duration of tide; the time between high an low water, normally just over 6 hours.
Electronic logs or E log
method of standardising the keeping of logs digitally on commercial
are two different
types see Vector charts and Raster marine charts.
, Evening Nautical
Estimated Position, the
DR plus the effects of
a published collection
of tables giving
coordinates for astronomical bodies for specific times.
a DR position which has
been adjusted to
allow for Set and Drift.
estimated time of
Equator; an imaginary circle around the earth, halfway between the north and south poles.
an old fashioned unit of
measurement to measure depth,
approximately equal to 6 feet or 1.8 meters.
point on the celestial
sphere which is used as a reference point, it is in fact where the
‘Ecliptic’ and the Celestial Equator cross.
a position verified by
reference to bearings
take off a known point such as a conspicuous land mark.
the floating debris
resulting from a
Forecasting Terms; used to convey specific, concise, information on weather conditions.
celestial navigation it
is the point on the earths surface directly beneath the Zenith, and the
the surface you are standing on is your body’s Geographical Position or
measurement of a
position, in degrees, westward from the Greenwich meridian. Read more
referred to as UTC,
Coordinated Universal Time, or UT, it is the mean of the times of noon
system which uses
information broadcast from orbiting satellites to calculate the
latitude of a receiving device.
Prime Meridian it
is the 0°longitude line, it is referred to as the Greenwich Meridian
because it passes through the Royal Observatory at Greenwich in the UK.
Gyrocompass; a non magnetic type of compass.
the direction in which a
vessel is pointing,
as opposed to the course made good.
Eye; the height of a sextant user’s eye above
needed to add as a correction to a site reduction.
Height of tide;
a figure taken from the
tide tables indicating the depth
of water above the chart datum.
the apparent line, for
an observer where the
earth’s surface and the sky meet. Also a circle formed on the celestial
sphere by a plane tangent to a point on the Earth's surface.
Horizon Mirror half
clear glass half mirror, on a sextant it allows the observer to view
horizon at the same time as a reflection of whatever is showing in the
Mirror. Read more:
Horse Latitudes; See Doldrums and ITCZ.
recognised systems of buoyage
mirror at the apex
of a sextant frame which pivots with the index arm.
two or more lines
such as LOPs cross.
imaginary line in the
Pacific Ocean at which the date changes. This line varies from the 180⁰
meridian to avoid populated land areas.
marine signal flags; a system of
flag shapes and designes used to communicate messages at sea.
almanac tables to
estimate value between the
Convergence Zone; the
of normally still air lying approximately between 5⁰ north and south of
Isolated Danger Mark; buoyage marks indicating a hazard, a such as a wreck directly below the mark,
Jetsam; are objects that have been thrown overboard from a vessel, originally stuff jettisoned by a ship which was in trouble.
Knot, (Kts.); a unit of speed, nautical miles per hour.
the approach to or
sighting of land after an ocean
edge of a safe water channel.
the angular distance
north or south from the
equator on the earth surface.
Range Lights; a pair of lights
deliberately placed to provide a transit along a narrow channel.
sideways movement of a
vessel usually caused by the
when taking a sun or
moon sight it is difficult to guess
where the center is so the observer aligns either the top edge (Upper
more usually the bottom edge (Lower Limb) of the object on to the
sight is then corrected using the appropriate correction tables.
line drawn on a chart on
which the position of the vessel must lie. The line deduced from either
compass bearing of a mark or a sight reduction. A minimum of two or
more are necessary to establish a fix.
the time when the sun
reaches its highest
altitude at your position and is directly north or south of you. it
be at your Zenith and its GHA will correspond to your longitude.
time at your
Angle, LHA; the angular distance
of an observed celestial object to the west of the observers meridian.
phenomena; alterations made to the
prevailing weather by local
an instrument used to
calculate speed and
distance travelled, through the water.
a book in which the
navigator records data
about the vessels progress
the angular distance
east or west from the
Greenwich or Prime Meridian (0⁰) on the earth’s surface.
Lower limb; see Limb. LOP; see Line of Position.
device used for
navigation which aligns with the earths magnetic field thus indicating
direction of magnetic north.
baring taken from a
northerly position of
the earths magnetic field, this is varies from the True North.
used to denote progress,
as in making way or a rising
tide as making.
a fixed buoyage
projection; a method for
representing the spherical globe as a flat map with lines of latitude
longitude intersecting at right angles. Devised by Geradus Mercato in
Meridian; a line of longitude, the imaginary great circles on the surface of the earth which converge at the poles and have their center at the center of the earth.
unit of distance that is
equal to 1/60 of a degree as measured along a line of longitude at the
of the distance to be measured.
the process of plotting
and directing the
course of a vessel.
the lowest tides in the
occurring when the sun and moon are in quadrature.
northerly point toward
which a compass' needle always points, the north of the earth’s
field as opposed to the geographic north pole
Notices to Mariners; updates and corrections to chart data.
the apparent differences
in the positions of
objects viewed along different lines of sight.
written plan of a
a guide book giving
details of coastlines and
harbours and sailing directions.
navigation plan for a
a paper sheet separate
from the cart where a
navigator can plot his LOPs and the vessels course.
is the pole star of the
it is located almost directly above the true position of due north
Ptolemy; Claudius Ptolemaeus, c. AD 90 to c. 168 a Roman mathematician, astronomer, geographer, possibly of Greek or Egyptian origin.
Quadrant; a precursor to the sextant as an instrument used to measure altitudes up to 90° or one quarter of a circle, hence the name. First proposed by Ptolemy as an improvement on the astrolabe.
distance at which a light or
mark can be seen from
sea level. Also the difference between the height of low water and high
Transits; marks or lights which
when in line indicate a safe channel.
conform to IHO specifications are produced by scanning a paper charts.
course sailed using a
constant compass course. An imaginary line drawn on a chart which
the meridians at a constant angle.
a LOP or
previous line of position forward to a current line of position. A fix
two separated bearings of the same fixed object and the distance run
which, indicates that
there is safe water for navigation is all around the mark
the Tide Tables
this is a port where the details have to be worked out in relation to a
emitted from a
single position which will show up as either red, white or green
the angle of the observer, designed to indicate a safe navigational
direction towards which a
Tide or Current is flowing.
an instrument used to
distances, like the altitude of the sun, moon and stars for navigation.
angular distance of a
star from the First Point of Aries.
the interval at high and
low water where the
stream ceases momentarily before changing direction.
a method for
combining three compass bearings to obtain a position fix, one where
variation and deviation need not to be taken into account.
Solid state compass;
a method of measuring
the depth of
water shallow enough to
allow depth to be
measured by lead line.
the southerly point
toward which a compass'
needle always points, the south of the earth’s magnetic field as
to the geographic south pole.
special areas or objects mentioned on the charts or in other nautical
Speed Made Good, SMG;
speed over the ground as opposed to through the water.
the rise and fall
or range of the tide is greatest, occurring around the time of the full
port for which the
Tide Tables give full details and from which Secondary Port details con
a narrow channel of
water connecting two
larger bodies of water.
Synoptic charts; a diagrammatic representation of weather data, summarised and overlaid on a chart.
method for showing
tidal information on a nautical chart.
where the tidal stream
creates confused and
sometimes dangerous water.
the horizontal flow of
the distinctive shape on
top of a
plotted course of a
steady easterly winds
that blow through the
tropics and subtropics.
two fixed objects in
lined up by an
a method of lining up
charted objects to
obtain an LOP.
True north; geographic north as opposed to magnetic north
Universal Coordinated Time, UTC or UT; the time at the Greenwich or 0⁰ meridian, GMT, Zulu time.
the angle between True
North and Magnetic
Vector charts; a type of electronic chart.
entered in a GPS.
are the prevailing
west-to-east winds of the middle
latitudes of both hemispheres on Earth.
the point directly above
between a celestial object and the observer’s zenith,
clock time within
Zulu time; see Universal Coordinated Time.
I will be adding more Navigation Terms when I think of them. In the mean time, if there is a navigation term you would like to know more about let me know using the comment form below.