A:

Actual Depth - the true depth of water as opposed to a predicted depth. Tidal height from the table added to the charted depth.

Almanac - a nautical almanac is a publication describing the positions of a selection of celestial bodies, to enable navigators to use celestial navigation to determine the position of their vessel while at sea.

Altitude - the angular distance of a celestial body above the viewers' horizon.

Anabatic Wind - caused by warm air rising up a slope to be replaced by cooler air, as opposed to katabatic, descending winds.

Apparent Wind - the wind as felt on board, this will be the actual wind modified by the wind created by the vessel's movement..

Aries - the constellation of the Ram in the Northern Celestial Hemisphere.

Astrolabe - an instrument used to determine the altitude of celestial objects before the introduction of the sextant. The first recorded use was 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.

B:

Barometer - an instrument for measuring atmospheric pressure

Barometric Gradient - the amount of atmospheric pressure change between two points, as shown by the distance apart of the Isobars on a synoptic chart.

Bar - any large mass of sand or earth formed and raised above the water surface by the surge of the sea. Bars are mostly found at the entrances of great rivers or havens, and often render navigation extremely dangerous but confer tranquility in the inshore waters by acting as a barrier against strong waves.

Beacon - a lighted or unlighted fixed aid to navigation attached directly to the Earth's surface, such as lighthouses and day beacons.

Bearing - the compass reading taken of an object in relation to the observer.

Beaufort Scale - a numerical scale used to describe wind speed/strength. Devised by Admiral Sir Francis Beaufort in 1808, in which winds are graded by the effects of their force on the surface of the sea or on a vessel.

C:

Cape - a piece of land that projects out into a large body of water.

Cardinal Marks - buoyage marks indicating the direction of safe water with reference to the cardinal points.

Cardinal Points - the four main points of the compass, North, East, South and West.

CC or Compass Course - the course to steer which has been corrected for current, variation, and deviation.

Celestial Body - a natural object which is located outside of Earth's atmosphere, such as the Moon, the Sun, an asteroid, planet, or star.

Celestial Navigation - a method of navigating by referring to the stars or other objects in the sky.

Celestial Sphere - an imaginary sphere with the earth at its centre, on whose surface the stars, planets, and other heavenly bodies appear to be situated.

Characteristics - the distinctive pattern of flashes used to identify a light.

Chart - a nautical map used for navigation.

Chart Datum - the reference level on charts to which tidal heights are referred.

Charted Depth - the depth of water shown on the chart for a particular point on the sea bed.

Chronometer - a very accurate clock or watch that is used for determining longitude at sea.

Circumnavigate - to travel around the entire Earth.

Civil Twilight - occurs when the sun is less than 6 degrees below the horizon. In the morning, civil twilight begins when the sun is 6 degrees below the horizon and ends at sunrise. In the evening, it begins at sunset and ends when the sun reaches 6 degrees below the horizon. These approximate times are given in the nautical almanac for listed latitudes, providing the best time to take morning or evening sights, when both the horizon and the heavenly bodies are visible.

Cocked Hat - a triangle formed by a series of three lines of position (LOP) which do not meet at an exact point.

COLREGs - Convention on the International Regulations for Preventing Collisions at Sea, 1972 - the Rules to follow to prevent collisions at sea.

Compass - a device used for navigation to determine the direction

Compass Rose - is a design on a chart that shows direction. It points which way is north, south, east, west, and some intermediate directions.

Constellations - Star groups.

Conversion Tables - used to convert units in one system to units of another.

Course Made Good (CMG) - the direction in which the vessel is actually traveling as opposed to the direction in which it is pointing. It is measured as the angle from true north.

Course Over Ground (COG) - is the actual direction of progress of a vessel, between two points, with respect to the surface of the earth. The vessel's heading may differ from the COG due to the effects of wind, tide, and currents.

Course to Steer (CTS) - the estimated course that a boat should steer in order to arrive at a waypoint, bearing in mind the effects that wind and tide will have on the vessel.

Corrections - alterations made to charts to update navigational information.

Cross Bearing - using lines of position from several navigational points, aids or marks to obtain a position fix.

Crux - The Southern Cross, a constellation seen in the Southern Hemisphere.

D:

Dead Reckoning (DR) - determining a position by plotting courses and speeds from a known position.

Declination - the angular distance to a point on the celestial sphere measured north and south from the celestial equator along the hour circle.

Deviation - the compass errors caused by metallic or magnetic objects on board the vessel.

Dip - a correction to be applied to sextant altitudes to compensate for the height of eye above sea level.

Doldrums - the Intertropical Convergence Zone (ITCZ) a belt of very still air between 5 degrees north, and 5 degrees south, of the equator where the northeast and the southeast trade winds converge. It encircles Earth near the thermal equator though its specific position varies seasonally.

Drift - the speed in knots of the effect of current or tide on a vessel's progress.

Drying Height - the drying height is the vertical distance of the seabed that is exposed by the tide, above the sea water level at the lowest astronomical tide.

E:

Ebb - to flow back or away, normally associated with the outflowing or falling tide.

Electronic Logs or E-logbooks - a method of standardising the keeping of logs digitally on commercial vessels.

Electronic charts - there are two different types see Vector charts and Raster marine charts.

Evening Nautical Twilight (ENT) - begins when the geometric centre of the sun is 6 degrees below the horizon in the evening and ends (nautical dusk) when the geometric centre of the sun is 12 degrees below the horizon in the evening.

Ephemeris - a published collection of tables giving coordinates for astronomical bodies for specific times.

Estimated Position (EP) - a DR position which has been adjusted to allow for current set and drift.

Equator - an imaginary line drawn around the earth equidistant from the poles, dividing the earth into northern and southern hemispheres. It constitutes the parallel of latitude 0°.

F:

Fathom - a unit of length equal to six feet (1.8 meters), chiefly used in reference to the depth of water.

Flood - to fill with water, used in reference to the incoming or rising tide.

Fix - a position verified by reference to bearings taken off a known point such as a conspicuous landmark.

Flotsam - the floating debris from a shipwreck or its cargo. More broadly it is pieces of broken wood and other waste materials found on the beach or floating on the sea.

G:

Geographical Position (GP) - in celestial navigation it is the point on the earth’s surface directly beneath the Zenith.

Great Circle - the shortest route between two points on the globe. To follow the great circle route (also called orthodromic line), a ship must constantly adjust its course.

Greenwich Hour Angle (GHA) - the angular measure of the celestial body from the Greenwich Meridian (also known as the Prime Meridian) along the celestial equator.

Greenwich Mean Time (GMT) - the international time standard that was replaced in 1972 with Universal Time Coordinated (UTC), which is based on atomic measurements. GMT is clock time at the Royal Observatory in Greenwich, London. It is not affected by Summer Time (Daylight Saving Time) clock changes. When the sun is at its highest point exactly above the Prime Meridian, it is 1200 noon at Greenwich.

Global Positioning System (GPS) - a system which uses information broadcast from orbiting satellites to calculate the longitude and latitude of a receiving device.

Gyrocompass - the navigational instrument which makes use of a continuously driven gyroscope to accurately seek the direction of true (geographic) north. It operates by seeking an equilibrium direction under the combined effects of the force of gravity and the daily rotation of Earth

H:

Halyard - a rope used for raising and lowering a sail, yard, or flag on a sailing vessel.

Handheld Compass - a small portable compass.

Heading - the direction in which a vessel is pointing, as opposed to the course made good

Height of Eye (HE) - the height of a sextant user’s eye above sea level, needed to add as a correction to a sight reduction.

Height of Tide - a figure taken from the tide tables indicating the depth of water above the chart datum at a fixed time.

Horizon - the apparent line, for an observer, where the earth’s surface and the sky meet.

Horse Latitudes - a belt of calm air and sea occurring in both the northern and southern hemispheres between the trade winds and the westerlies. The horse latitudes are regions located at about 30-35 degrees north and south of the equator.

Hull - the main body of a vessel, including the bottom, sides, and deck but not the masts, superstructure, rigging, engines, and other fittings.

I:

IALA Systems - internationally recognised systems of uniform buoyage with the world divided into two regions – Region A and Region B.

International Date Line - the imaginary line in the Pacific Ocean at which the date changes. This line varies from the 180⁰ Meridian to avoid populated land areas.

International Marine Signal Flags - a system of flag shapes and designs used to communicate messages at sea.

Intertropical Convergence Zone (ITCZ) - the varying area of normally still air lying approximately between 5⁰ north and south of the equator.

Isolated Danger Mark - buoyage marks indicating a hazard, such as a wreck directly below the mark

J:

Jetsam - unwanted material or goods that have been thrown overboard from a ship and washed ashore, especially material that has been discarded to lighten the vessel.

K:

Keel - the lengthwise timber or steel structure along the base of a vessel, supporting the framework of the whole. In some vessels, it extends downwards as a ridge to increase stability.

Knot (kn or kts) - a unit of speed equal to one nautical mile per hour, exactly 1.852 km/h (approximately 1.151 mph or 0.514 m/s). A vessel traveling at 1 knot along a meridian travels approximately one minute of Geographic latitude in one hour.

L:

Landfall - the approach to or sighting of land after an ocean passage.

Lateral Marks - buoyage indicating the edge of a safe water channel.

Latitude - the angular distance north or south from the equator on the earth surface.

Leading Lights or Range Lights - a pair of lights deliberately placed to provide a transit along a narrow channel.

Leeway - the sideways movement/drift of a vessel to leeward of the desired course.

Leeward - the side sheltered from the wind (downwind).

Line of Position (LOP) - a line drawn on a chart on which the position of the vessel lies. The line deduced from either a compass bearing of a mark or a sight reduction. A minimum of two or preferably more are necessary to establish a fix.

Local Noon - the time when the sun reaches its highest altitude at your position and is directly north or south of you. it will then be at your Zenith and its GHA will correspond to your longitude.

Local Time - time as reckoned based on meridian or solar time in a particular region or time zone at your position.

Local Hour Angle (LHA) - is calculated from your assumed longitude, adjusted by the GHA. It is the angle between the meridian of your assumed position and the meridian of the geographical position of the celestial body.

Log - an instrument used to calculate speed and distance traveled, through the water.

Logbook - a book which records data about the vessel's voyage.

Longitude - the angular distance east or west from the Greenwich or Prime Meridian (0⁰) on the earth’s surface.

M:

Magnetic Compass - a device used for navigation which aligns with the earth’s magnetic field thus indicating the direction of magnetic north.

Magnetic Bearing - a bearing taken from a magnetic compass.

Magnetic North - the northerly position of the earth’s magnetic field, this varies from the True North.

Making Way - being propelled through the water by sail, machinery, or oar.

Mark - an artificial or natural object of easily recognisable shape or colour, or both, situated in such a position that it may be identified on a chart or related to a known navigational instruction.

Mercator Projection - a method for representing the spherical globe as a flat chart (map) with lines of latitude and longitude intersecting at right angles.

Meridian - a line of longitude, the imaginary great circles on the surface of the earth which converge at the poles and have their centre at the centre of the earth.

N:

Nautical Almanac - see Almanac.

Nautical Mile - a nautical mile is a unit of length used in air, marine, and space navigation. Historically, it was defined as the meridian arc length corresponding to one minute of latitude (one 60th of a Nautical Mile). Today the international nautical mile is defined as exactly 1.852 meters.

Navigation - the process of plotting and directing the course of a vessel.

Navigation Lights - a set of lights shown by a vessel or aircraft at night to indicate its position and orientation.

Neap Tide - refers to a period of moderate tides when the sun and moon are at right angles to each other.

North Magnetic Pole - the northerly point toward which a compass needle always points, the north of the earth’s magnetic field as opposed to the geographic north pole

North Star - designated α Ursae Minoris, it is the common name of Polaris or Pole Star, the brightest star of the constellation Ursa Minor, located almost directly over True North.

Notices to Mariners - advises mariners of important matters affecting navigational safety, including new hydrographic information, changes in channels, and aids to navigation, and other important data.

P:

Parallax - the apparent differences in the positions of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines.

Passage Plan - a written navigational plan of a proposed voyage.

Pilot Book - a guide book giving details of coastlines and harbours, as well as sailing directions.

Polaris - see North Star.

Position Fix - see Fix.

Position Line - see LOP.

Q:

Quartering Sea - seas approaching a vessel from between 105° and 165° to port or starboard.

R:

Range - the greatest distance at which an object or a light source can be seen under conditions of perfect visibility, as limited only by the curvature of the earth, by the refraction of the atmosphere, and by the elevation of the observer and the object or light.

Range Markers or Transits - see Leading Lights.

Raster Chart - is essentially an electronic picture of a paper chart, obtained through an accurate, detailed scanning process.

Rhumb Line - a course sailed using a constant compass course. An imaginary line drawn on a chart which crosses all the meridians at a constant angle.

Rigging - the system of ropes or chains employed to support a vessel's masts (standing rigging) and to control or set the yards and sails (running rigging).

Rule of Twelfths - a method for making quick estimations of tidal height. It’s called the 'Rule of Twelfths' because it says In the first hour after low water, the tide rises one-twelfth of its range In the second hour after low water, the tide rises two-twelfths of its range In the third hour after low water, the tide rises three-twelfths of its range. The converse to high tide exists.

Running Fix - a fix based on two separated bearings of the same fixed object and the distance run between the bearings.

Running Rigging - is the rigging of a sailing vessel that is used for raising, lowering, shaping and controlling the sails.

S:

Safe Water Mark - as defined by the IALA, is a sea mark used in maritime pilotage to indicate the end of a channel.

Secondary Port - in the Tide Tables this is a port where the details have to be worked out in relation to a Standard Port.

Sector Lights - is a pilotage and position fixing aid that consists of horizontal angle light beams to guide water-borne traffic through a safe channel at night in reasonable visibility. Sector lights are most often used for safe passage through shallow or dangerous waters. This may be when leaving or entering harbour.

Set - the direction towards which a tide or current is flowing.

Sextant - an instrument used to measure angular distances, like the altitude of the celestial bodies for navigation.

Slack Water - the interval at high and low water where the stream ceases momentarily before changing direction.

Sounding - a method of measuring the depth of water.

Southern Cross - see Crux.

South Magnetic Pole - the southerly point toward which a compass' needle always points, the south of the earth’s magnetic field as opposed to the geographic south pole.

Special Marks - buoyage indicators of special areas or objects mentioned on the charts or in other nautical and publications.

Speed Over Ground (SOG) - the actual speed over from one geographical point on the earth to another, as opposed to the speed made through the water.

Speed Through Water (STW) - is the speed of the vessel relative to the water. Currents, tides and drift can all affect the vessel's progress to the destination.

Spring Tide - a tide just after a new or full moon, when there is the greatest difference between high and low water.

Standard Port - a port for which the Tide Tables give full details and from which Secondary Port details can be extrapolated.

Standing Rigging - comprises the fixed lines, wires, or rods, which support each mast or bowsprit on a sailing vessel and reinforce those spars against wind loads transferred from the sails.

Strait - 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.

T:

Tide - the alternate rising and falling of the sea, usually twice in each lunar day at a particular place, due to the attraction of the moon and sun.

Tidal Diamond - a symbol on a nautical chart that indicates the direction and speed of tidal streams.

Tidal Race - is a natural occurrence whereby a fast-moving tide passes through a constriction, resulting in the formation of waves, eddies and hazardous currents.

Tidal Stream - it refers to the currents associated with the tides, generally near a coastline or harbour.

Top Mark - the distinctive shape on top of a buoy.

Track - the plotted course of a vessel.

Trade Winds - steady easterly winds that blow through the tropics and subtropics.

Transit - the action of passing through or across a place.

Transit Fix - a method of lining up charted objects to obtain an LOP.

True North - geographic north as opposed to magnetic north.

U:

Universal Time Coordinated (UTC) - is the primary time standard by which the world regulates clocks and time. It is within about 1 second of mean solar time at 0° longitude, and is not adjusted for daylight saving time.

Underway - a vessel which is not at anchor, or made fast to the shore, or aground, however, is not “Making Way” (see above).

V:

Variation - the angle between True North and Magnetic North.

Vector Chart (ENC) - is a type of electronic chart or "smart charts" that are coded with additional information not available on paper or raster charts.

W:

Waypoint - is an intermediate point or place on a route or line of travel, a stopping point or point at which course is changed.

Wind Over Tide - when the wind is blowing in the opposite direction to which the tide is setting, creating sharp choppy conditions.

Windward - the side facing the wind (upwind).

Z:

Zenith - the point directly above the observer’s head.

Zenith Distance - the angular distance between a celestial object and the observer’s zenith.