Glossary

GMT, UTC

(Greenwich Mean Time  -  Universal Co-ordinated Time

For all intents and purposes, GMT is the same time as UTC.
GMT is an imperial measurement as measured each noon when the Sun crosses 0° longitude.
UTC is the equivalent time, as measured by various atomic clocks around the World. 

The world is broken into a number of time zones, each time zone covers 15° of longitude or 1 hour (adjusted empirically to align with National boundaries).

Greenwich Mean Time is known as ZULU time passes through the Greenwich Observatory outside London and lies on the Prime Meridian (zero degrees longitude).

Selective Availability

Update: SA was "turned-off" in 2001.

The satellite system used by off-the-shelf GPS receivers is owned by the US Pentagon and whose main purpose is to assist United States and allied military forces. 
Users are able to tap into the system free via a constellation of 24 satellites orbiting the earth known as the United States Global Positioning System.

Modern low-priced GPS units are accurate to less than 10 metres (33 feet), however to decrease their usefulness to the USA's enemies, the signal is intentionally degraded with random errors inserted in the signal. The American military GPS units decode these errors and are accurate to within centremetres! (and very useful for targeting a missile) This is known as Selective Availability and generally referred to as SA. The US has announced that SA is due to be removed imminently, essentially then ALL GPS units will be accurate to centremetres, however due to other natural signal degradations true accuracy would be measured to within a few metres.
review a New York Times article about future GPS accuracy

Before SA is shut down, a highly accurate adjunct to GPS is Differential GPS or DGPS. Here the GPS satellite signal is supplemented by a land-based radio transmission which effectively corrects the GPS error. Obviously once out of range of the transmitter, they revert back to a standard GPS receiver.

Vector versus Raster Charts

Raster charts are simply scanned copies of the paper variety. Vector charts are redrawn (often "traced") as lines, curves and objects.

The raster charts have no knowledge of themselves, they are simply a collection of coloured dots which our brain turns into a picture of land and sea and lighthouses etc. The electronic raster charts do, however, know exactly where each dot lies on the Earth's surface.

Vector charts are digital databases of the foreshores and oceans. Lines, curves and "things" are stored as objects (ie: a beach is a land object at these co-ordinates surrounded by a water object). When a chart is to be displayed on screen, the GPS location triggers the chart to draw itself at the required scale factor. 

Vector charts can be zoomed indefinitely without loosing accuracy.  Raster charts can only be zoomed to the smallest scale to that that the chart was scanned.

Raster Chart Example (unsafe for navigation when over-zoomed)	Vector Chart Example (we could continue zooming down to an individual jetty!)

NMEA

National Marine Electronics Association

The International Standards body which manages electronic protocols utilised by marine instruments.

INMARSAT/CAPSAT

SATELLITE TELEPHONE COMMUNICATION 

The Capsat® Telephone is a mobile Inmarsat-phone terminal which provides access to international telephone, facsimile and data networks. 

Operating the Capsat® Telephone is much the same as making direct international telephone calls from an ordinary telephone. The difference is that the Capsat® Telephone communicates directly with a satellite and therefore does not rely on a local telephone operator to route the call to the desired destination. Instead, the calls to/from the satellite are routed by designated Land Earth Stations (LESs) which also communicates with the same satellite as the telephone terminal.

The necessary requirements to operate a satellite telephone is that

• a) the terminal is registered by a service provider (i.e. payment arrangements).

• b) the location where the telephone is located is covered by a satellite, and

• c) there is a free line of sight from the antenna of the terminals to the satellite.

The satellite system used by the Capsat® Telephone is operated by the international organisation, Inmarsat. The system consists of four satellites which combined guarantee world-wide coverage (view Pacific coverage map).

When working with the Inmarsat-phone mini-M system, the following abbreviations are often used:

• MES Mobile Earth Station.
  This is the name that Inmarsat uses for terminals (seaphones).
  E.g. the Capsat® Telephone is a MES. Mobile refers to the fact that the terminals are re-locatable. Earth Station is a satellite communication term.
• LES Land Earth Station.
  The LES is located at the "other" end of the MES's satellite link. The LES connects to the local telephone networks and manages calls to and from the MES.
• NCS Network Co-ordination Station.
  The NCS is responsible for assigning communication channels to the MESs. In case of NCS failure the MESs will switch to "Stand-Alone" mode. In Stand-Alone mode designated LESs in each ocean region will act as NCS.

Obsolete

As a complete charting solution including GPS, Auto-pilot, Charting Software and Laptop should not suffer the typical "obsolete the day you buy it" syndrome. GPS & Autopilot characteristics remain essentially constant, that is: no great advancements in the intelligence of these machines has occurred in years, after all, the GPS supplies Lats & Longs and the autopilot steers left & right as necessary! 

The upshot of this is that once installed and working satisfactorily, there should be no need to upgrade the system hardware!
Important considerations: buy quality hardware in the first place. Target well known and well supported and well distributed equipment such as Raytheon and I.B.M. Ten to twenty years from now, you would fairly confidently expect these companies to still exist, and still supply manuals, service, parts and upgrades - all over the world!

The company supplying the Charting Software will include in their pricing policy access to updates to the chart specs from time to time or as necessary.

Peripherals

For harsher usage conditions a new range of flexible and waterproof keyboards are ideal

TV tuner and capture cards turn your computer into a telly, or the main or secondary screen for a VCR. With these video capture cards TV or videos can be watched full screen or in a window while using other programs!  Capture cards can also place a video image on the screen for any attached video camera.  Check the picture for the size of a modern video cam, these could be placed anywhere on board with the image displayed on the computer monitor. These cameras also see in the dark!

This full colour video camera  is the size of a box of matches.

Network hubs for connecting a laptop (back-up, download/upload waypoints, routes, log entries etc etc)

protocol

Electronic devices that can send or receive data to other devices do so using a particular language. For the communication to be successful, they must "speak the same language". 

The easiest way to grasp the relevance and importance of protocols is to think back to the start of video tapes - BETA and VHS, two non-compatible standards competing for the same market space, during the 70's Sony's Beta actually had the early lead, now they're a forgotten memory!

For data devices these are called "protocols",  examples in the marine environment are 'Seatalk' which is a proprietary standard created and used by a single manufacturer and NMEA 0183 which is an official internationally recognised language which all current navigation equipment speaks and understands.

SOB will all accept NMEA 0183 protocol and can often interface with proprietary and competing protocols as well.

New standards are emerging all the time both officially recognised and proprietary ...  

bps, baud rate

bps - baud: depending on who you talk to mean the same thing. A tech-head will say that baud's are the go, a salesman can pronounce bee-pee-ess (when actually the techs pronounce it "bips").

bps = bits per second, ie: the rate that data is transmitted. There are eight bits for each byte. 
One byte is necessary for each letter or number sent, 1 typed page is about a thousand bytes, we know these as kilobytes (Kb see note). Example - a 10 Gigabyte hard disk holds 10,000 Megabytes, or 10 million kilobytes or 10 MILLION PAGES of TYPING.

Communications? Your home internet connection is 33 or 56,000 bps. Your fax talks to other faxes at 9600 or 14400 bps.
see table below ...

EXAMPLE 1:  A long email: 1,000 bytes using 2400bps Seaphone satellite data connection

• 8 bits per byte
• 1,000 bytes = 8 x 1,000 = 8,000 bits 
  
• At 2400 bits per second (bps), 8000 bits will take 8000 divided by 2400 = about 3½ seconds

EXAMPLE 2: A low resolution image attachment over a Mobile phone GSM connection

JPG Image from a cheap digital camera = about 70 kilobytes

• 70,000 bytes = 560,000 bits
  
• 560,000 bits at 9600 bps = about a minute

EXAMPLE 3: A typical sized Microsoft Word document attachment over a Mobile phone CDMA connection

5-10 page Word document = say 245 kilobytes

• 245,000 bytes = 1,960,000 bits
  
• 1,960,000 bits at 19,200 bps = under TWO minutes

NOTE:  In fact a kilobyte is not 1000 bytes, but 1,024 bytes (derived from exponentials of 2). 
In conversation we assume a thousand bytes, when doing computer calculations, though, use 1024. For example your 64 megabyte home computer counts to 65,536 during its boot-up memory check. Yes, 1,024 x 64 = 65,536, while 64 times 1000 is 64 megabytes! Lucky this knowledge is totally irrelevant to your computer use, but be wary of the salesman... 

Data Breakout Box

Often an electrical interface needs to be custom built to connect various electronics with the computer. Sometimes this can be as simple as soldering a computer serial plug to the correct wires, however some custom digital electronics solutions can require ingenious interfacing devices. A breakout-box is a handy device for testing and checking Serial communications.

Navigator's Glossary

A

Aberration. An optical effect due to the earth's orbital motion relative to light from a stellar source.
Altitude. Angular distance of celestial body above the observer's true horizon. Also the arc distance on the observer's vertical circle between the horizon and the observed body.
Aphelion. Point in a planetary orbit where the planet is farthest from the sun, the source of gravitational attraction.
Apparent magnitude. Brightness of a celestial body as established by psychological parameters as opposed to physical scaling.
Apparent sun. The true sun as opposed to the fictitious mean sun.
Arc distance. Distance between two points on a great circle, where on the earth's surface one minute of arc converts to one nautical mile.
Arc measure. One minute of arc on the earth's surface is equal to one nautical mile (by convention, 6,076.1 feet); an angle of one minute at the earth's center subtends an arc of one minute on its surface.
Aries, first point of. Point at which the sun passes from south to north declination; the vernal equinox; also that point on the equinoctial which is the reference for locating the hour circles of stellar objects (i.e., SHA and RA).
Artificial horizon. Any device for giving a horizon reference identical except for dip to that of an observer at sea.
Assumed latitude (aL). Latitude of the assumed position (AP) as adopted for setting up the navigational triangle.
Assumed longitude (aLO ). Longitude of the assumed position (AP) as adopted for setting up the navigational triangle.
Assumed position (AP). Geographic position that is assumed for setting up the navigational triangle in computing a celestial line of position. Usually the DR position but a whole degree of Lat and a Long that gives a whole degree of LHA is used in many sight reduction tables.
Astrolabe. Early instrument for measuring the altitude of a celestial object.
Astronomical twilight. The period when the sun is from 0 degrees to 18 degrees below the horizon.
Augmentation. Apparent increase in simidiameter of sun or moon as the body approaches culmination due to the fact that the body is nearer to the observer at culmination than at rising or setting.
Autumnal equinox. Point on the Celestial equator where the sun moves from north to south declination.
Average deviation. The mean of all deviations of individual observations from the mean, the magnitude of all deviations being treated as positive.
Azimuth angle (Z). Angle within the nautical triangle between the observer's meridian and the vertical circle through the body. Gives the bearing of the body from the observer.

B

Backstaff. An early device for measuring altitude whose observational principles fall between those of the cross-staff and the modern sextant.
Bubble sextant. Sextant that levels upon the horizon by centering a bubble in its chamber rather than by sighting on the visible horizon.

C

Cartesian coordinates. A two coordinate geometry in which a point is located on an x, y grid whose axes are mutually perpendicular (akin to latitude and longitude on a Mercator chart).
Celestial equator. The equinoctial, that equatorial band made by projecting the earth's equator into the celestial sphere.
Celestial poles. Projection of the terrestrial poles into the celestial sphere.
Celestial triangle. Triangle projected onto the celestial sphere with the celestial pole, the observer's zenith point, and the celestial position of body as the vertices, and codeclination, coaltitude of the body, and colatitude of the observer's zenith as its sides. (Used interchangeably with the navigational triangle.)
Chronometer. A clock designed to keep accurate time.
Circle of equal altitude. The locus of points on the earth's surface from which identical altitude observations of a given body will be obtained.
Circle of uncertainty. A circle within which the navigator will locate his position granting an allowance for error.
Circumpolar body. A celestial object visibly rotating about the pole, thereby never dipping below the horizon.
Civil twilight. The period when the sun is from 0 degrees to 6 degrees below the horizon.
Clearing the distance. Correcting the apparent angular distance between the moon and another celestial object to its true angular distance as measured at the center of the earth.
Coaltitude. The complement of altitude (i.e., 90 degrees - altitude).
Coast Pilot. A book containing detailed information on coastal waters, harbors, currents, weather, etc., for the guidance of the mariner.
Codeclination. The complement of declination (i.e., 90 degrees - declination).
Colatitude. The complement of latitude (i.e., 90 degrees - latitude).
Compass error. The difference between true heading or direction and compass heading.
Computed altitude (Hc). Altitude as computed by methods of sight reduction, for comparison with observed altitude in determining a celestial LOP.
Conjunction. The alignment of bodies such that they are located on the same celestial meridian.
Course line LOP. An LOP parallel to a vessel's course, hence the true azimuth (Zn) cuts the course at a right angle.
Cross-staff. Early instrument for measuring the altitude of a celestial object based on the principle of a shadow astrolabe, with a sliding crosspiece indicating altitude along a horizontal staff.
Culmination. The point at which a celestial body reaches its maximum altitude for the given observer. Also transit, meridian passage.

D

d correction (d). An interpolated correction due to the inconstancy of hourly change in declination.
Dead reckoning (DR). Determining position from a knowledge of heading, speed, and drift.
Declination (Dec. or d). Angle at the center of the celestial sphere between the equinoctial and parallel of latitude through a celestial body. Corresponds to the latitude of a celestial bodies sub-point.
Departure (p). A term used in plane sailing to indicate difference of longitude along a given parallel of latitude, the difference being expressed as distance in nautical miles.
Dip (D). The angular difference between the geometrically true and the apparent or visual horizon.
Distance line. An LOP that cuts the course at a right angle, hence the true azimuth Zn parallels the course. (Speed Line)
Diurnal motion. The apparent daily motion of a celestial object; its track in a twenty four hour period.
Double altitude. Method of determining latitude from the altitude observations at different but proximal times.

E

Eclipse. The obscuring of one body by the interposition of another vis-à-vis the observer.
Ecliptic. Apparent path of the sun through the stellar sphere as generated by the earth's axial orientation and orbit of revolution about the sun.
Elongation. Arc-angle between two celestial objects as measured from the earth.
Ephemeris. Astronomer's almanac of celestial information.
Equation of time. The difference in arc, translated into time, between the true sun and the fictitious mean sun.
Equatorial plane. Plane perpendicular to polar axis, midway between the North and South poles; inscribes the great circle of the equator on the earth's surface.
Equinoctial (Q). Celestial equator as projected by the earth's equatorial plane.
Equinox. Nodal point at which the ecliptic crosses the equinoctial (the spring and Autumnal equinoxes).
Euclidean space. Our conventional three dimensional space, making use of Euclid's geometry.
Ex-meridian. Methods involving the computation of rates of change in altitude as a function of differences of time between meridian passage and the time of an actual observation.

G

Geocentric parallax. The difference of altitude of a body as measured at the center of the earth and as measured at a point on the earth's surface.
Geocentric system. A hypothetical celestial system placing the earth at the center of the celestial sphere (as in traditional celestial navigation).
Geodesic. A line in some designated space that represents the shortest distance between two points (on the earth's surface, a great circle).
Geographic position (GP). Position on the earth's surface directly under a celestial body. Point at which the body's altitude is precisely 90 degrees.
Gibbous moon. State of moon when less than full but more than half.
Gnomon. A vertical pin whose shadow, projected on a horizontal base, indicates the altitude of the sun.
Gnomonic chart. A chart constructed by projecting the earth's surface onto a plane tangent at some given point. Sometimes known as a great-circle chart, since a great circle will appear upon it as a straight line.
Great circle. Circle inscribed on a sphere made by passing a plane through the center of the sphere. (On the earth's surface all meridians are great circles, as is the equator).
Great-circle course. The great circle, or shortest distance line between a departure and a destination.
Great-circle sailing. Determining courses and distances along the great-circle track.
Greenwich hour angle (GHA). Hour angle between the prime meridian of Greenwich and an hour circle through a celestial object, the angle measured westerly through 360 degrees.
Greenwich hour angle of Aries (GHA). Hour angle between Greenwich and the first point of Aries measured westerly through 360 degrees.
Greenwich mean time (GMT). Also Greenwich civil time. Time as measured from the lower branch of Greenwich meridian to the hour circle of the mean sun. Also local apparent time at Greenwich, England. Now reported as Coordinated Universal Time [UTC].
Greenwich meridian. Meridian through the Royal Observatory, Greenwich, England; the standard reference for longitude and Greenwich time.

H

Haversine. A trigonometric function, (1 - cos) / 2, whose advantage in early navigational practice was to eliminate angular ambiguities inherent in ordinary trigonometric functions.
Heliocentric system. A hypothetical celestial system placing the sun at the center of the Celestial sphere.
Horizon. The plane at right angles to the zenith-nadir axis that theoretically intersects the center of the earth, that plane subtending a great circle on the celestial sphere.
Horizon, apparent. Horizon as seen at sea unobscured by intervening waves. The difference between the true and the apparent horizon will be due to the factor of dip and in the case of the moon, both dip and parallax.
Horizon glass. The fixed mirror of a sextant, half of which is clear glass permitting direct viewing of the horizon.
Horizontal parallax (HP). The maximum value of parallax when the moon is on the horizon.
Hour angle (HA). Angle between any two hour circles or between a meridian of longitude and an hour circle in the celestial sphere, generally measured westerly through 360 degrees, as in LHA. If hour angle is measured east or west through 180 degrees, it is identical with meridian angle, "t".
Hour circle. Celestial meridian made by projecting meridians of longitude into the celestial sphere.

I

Index arm. The movable arm of a sextant.
Index error. Instrumental error due to imperfect alignment between the horizon and index mirrors. If the error is "on the arc" the instrument reads too high and the correction is subtracted; "off the arc," and vice versa.
Index mirror. The mirror attached to the movable index arm.
Inferior planets. Mercury and Venus, the two planets whose orbits lie inside the earth's orbit; only Venus is used for navigational observations.

Instrument Error. A constant or fixed error due to imperfections in an instrument such as a sextant.
Intercept. Difference in minutes of arc between the observed altitude Ho and the computed altitude Hc, labeled toward (T) or away (A) as Ho is greater or smaller than Hc.
International date line. Lower branch of the meridian passing through Greenwich; requires advancing a clock twenty four hours in crossing from W to E, or retarding it twenty four hours in crossing E to W.

Inverse of a trigonometric function. The angle for which the given trigonometric function holds.
Irradiation. An illusory error generated by the contrast of a bright sky against a dark Horizon; its effect is to depress the apparent horizon and slightly increase the apparent diameter of the sun.
Isomorphic relationship. Congruence of terrestrial and celestial position in navigational astronomy.

K

Kamal. An ancient Arabian device used to measure altitude, whereby the observer sights on the base and top of a rectangle while holding the rectangle at a determined distance from the eye.

L

Latitude (L). Angle at the center of the earth between the equator and a point on the earth's surface as measured north or south of the equator along the meridian passing through the point.
Latitude by meridian passage. Finding latitude by observing the altitude of a body at transit.
Law of cosines, spherical triangle. The cosine of any side is equal to the product of the cosines of the other two sides plus the product of the sines of those two sides times the cosine of their included angle; e.g., cos x = cos y cos z + sin y sin z cos X.
Law of sines, spherical triangle. sin x / sin X = sin y / sin Y = sin z / sin Z. ( where x, y, and z are the arc sides opposite the angles X, Y, and Z.)
Limb. Upper and lower points of tangency on the circumference of the sun or moon.
Line of position (LOP). Locus of points all of which are consonant with an observer's observation. In celestial observation the LOP is a segment of a circle of equal altitude.
Local apparent noon (LAN). Moment at which the true sun crosses the upper branch of an observer's meridian.
Local hour angle (LHA). Hour angle between an observer's meridian and an hour circle through an observed body, measured westerly through 360 degrees.
Longitude (Lo). Angle at the center of the earth between the prime meridian through Greenwich and meridian through a point on the earth's surface, as measured east or west through 180 degrees.
Lower branch of a meridian. The half of a body's or observer's meridian between the poles which intercepts its antipodal location.
Lower limb. Point on the circumference of the sun or the mood yielding the last altitude from the horizon.
Lower transit. Transit of celestial body at the lower branch of an observer's meridian.
Lunar distance. The angular difference between the moon and another celestial body (usually the sun) as measured at the center of the earth.

M

Magnetic bearing. Bearing relative to magnetic north.
Marcq St. Hilaire method. Determines an LOP by comparing an observed altitude (Ho) and a computed altitude (Hc) and striking the difference (the intercept) along the true azimuth (Zn) of the body.
Mean. The arithmetic average of a set of observations obtained by dividing the sum of all such measures by the total number of observations.
Mean sun. Fictitious sun projected to move along the equinoctial at a constant rate, and completing its circle in one solar year.
Mean time. Time as measured relative to the motion of the mean sun.
Mercator chart. Chart made by the projection of the earth's surface onto a cylinder tangent to the earth usually at its equator.
Mercator sailing. Methods for computing course and distance on a Mercator chart where the measure of distance is a function of latitude.
Meridian. A great circle made by passing a plane through the polar axis. Meridians on the earth's surface constitute lines of longitude.
Meridian angle (t). Hour angle, the angle between the observer's meridian and the hour circle through a celestial object, measured east or west to 180 degrees.
Meridian passage. SEE: Meridian transit.
Meridian transit. The moment when a celestial body intersects the observer's meridian. Also meridian passage, and culmination.
Meridional part (M). One minute of arc on a meridian at the equator as represented on a Mercator chart. Meridional parts will be the number of such units required to measure the expansion of latitude on a Mercator chart.
Mid-latitude sailing. Method of computing the departure (p) of plane sailing by assuming a latitude midway between that of a vessel's geographic departure and its destination.
Moonrise. Local mean time at which the upper limb of the moon is seen to emerge above the true horizon.
Moonset. Local mean time at which the upper limb of the moon is seen to disappear below the true horizon.

N

Nadir. Point on the celestial sphere opposite the observer's zenith.
Nautical Almanac. An ephemeris of celestial and navigational data published jointly by Great Britain and the United States.
Nautical astronomy. That aspect of astronomy which concerns the determination of position on earth from observations of celestial bodies.
Nautical mile. One minute of arc on a great circle inscribed on the earth's surface, visualized as a perfect sphere; by convention a distance of 1,852 meters, or 6,076 feet, or 1.15 statute miles.
Nautical triangle. The navigational triangle as generalized both for celestial sight reduction and great-circle sailing.
Nautical twilight. The period when the sun is from 0 degrees to 12 degrees below the horizon; the period when most observations of stars and planets are made by the navigator.
Navigational planets. Those for which navigational data are provided in the Nautical Almanac, namely, Venus, Mars, Jupiter, and Saturn.
Navigational triangle. Spherical triangle with zenith (Z), pole (P), and geographic position of the body (GP or M) as its vertices, and codeclination, coaltitude, and colatitude as its sides.
Nodal point. Point of intersection between two or more celestial tracks.
Node. The intersection of two or more tracks, such as the ecliptic and the equinoctial or the moon's celestial track and the ecliptic.
Normal. The line of sight or direction at right angles to the plane of a mirror.

O

Observed altitude (HO). The sextant altitude corrected for systematic and personal factors or error.
Occultation. The obscuring of a body by the bright background of a larger body, as for example when Jupiter's moons pass between the observer and Jupiter itself.

P

Parallatic angle (é). One of the angles of the navigational triangle, viz., that between the meridian through the celestial body and the vertical circle from the body to the observer's zenith.
Parallax. The difference in angle between altitude as measured at the center of the earth and as measured at the surface of the earth.
Parallel of latitude. That circle passing through all points of a given latitude and inscribed on the earth's surface by a small circle parallel to the equator.
Pelorus. An instrument for sighting relative bearings.
Perihelion. Point in a planetary orbit where the planet is nearest the sun, the Point of gravitational attraction.
Personal error. Error due to bias or idiosyncrasy in a particular observer's observations.
Pilot Chart. Chart providing information on ocean currents and temperatures, wind, storm tracks, magnetic variation, and other items of interest to the mariner.
Plane sailing. Method of finding course and distance by trigonometric means when a small section of the earth's surface is regarded as a simple plane.
Polar axis. The axis of the earth's rotation between the true North and South poles.
Polar Coordinates. A two-coordinate geometry in which position from an origin is specified by direction and distance, for instance, course and distance on a Mercator chart.
Polar distance. The arc measure between a celestial object and the nearest pole. Same as codeclination of the body.
Polar-equational system. The celestial hemisphere as defined by the polar axis and the equinoctial.
Polyconic projection. A map or chart that is constructed by projecting the earth's surface onto one or more cones placed tangent to the earth's surface.
Precession. Change of direction of the axis of a rotating body due to applied torque.
Prime meridian (G). The meridian passing through Greenwich, England, which services as a standard for measuring longitude and Greenwich time.
Prime vertical circle. Vertical circle intersecting the observer's geographic east and west points.
Prime vertical sight. Determining when a body is due east or west by comparing observed altitude with a precomputed altitude.

R

Radian. Arc subtended at the center of a circle equal to the radius of the circle, there being 2 (PI) radians in a circle.
Random error. Error that is not systematic in nature, i.e., that which is due to unpredictable fluctuations in the accuracy of observation.
Rate of change. Change per unit of time.
Refraction. The deflection of linear light as it passes between media of differing densities. Upon entering the earth's atmosphere the light from a stellar source is refracted toward the earth's surface resulting in an apparent or observed altitude greater than the true altitude.
Relative bearing. Bearing between two objects as seen from an observer's Point of reference; bearing relative to the fore and aft axis of a vessel.
Retrograde motion. Reversal in direction of a planet's apparent motion against the stellar background.
Revolution. The orbital motion of one body about another.
Rhumb line. A line constructed to cut all meridians of longitude at a constant angle; a straight line on a Mercator chart.
Right ascension (RA). Hour angle between first Point of Aries and the hour circle through a body, measured easterly through 360 degrees and converted to time.
Rotation. The circular motion of a body about the polar axis.
Running fix. A positional fix established by advancing or regressing one or more LOP's to the time of a base LOP.

S

Sailing Directions. Publications containing information useful to the mariner in coastal pilotage and navigation. For the U.S. coast, the U.S. publications are known as Coast Pilots.
Sailings. Methods for computing courses and distances, latitudes and longitudes, by trigonometric means.
Semidiameter (SD). Half the diameter of a stellar body such as the sun or moon; the magnitude of semidiameter is the correction applied to a sextant observation of the upper or lower limb of these bodies.
Sextant altitude (hs). The altitude actually observed on a sextant prior to introducing corrections for systematic, personal, and instrument errors.
Shadow bearing. The bearing of the sun determined by a shadow pin. For the magnetic compass the shadow bearing will be the compass bearing plus 180 degrees.
Shadow pin. Vertical pin at the center of a compass card designed to cast a linear shadow on the compass rose.
Sidereal day. Duration of one rotation of the earth with a fixed stellar Point as reference, namely, the first Point of Aries.
Sidereal hour angle (SHA). Hour angle measured westerly from first Point of Aries to the hour circle through a particular star.
Sidereal period. A rotational or orbital period of time with a fixed stellar Point taken as reference.
Sidereal year. Time of one orbital revolution of the earth about the sun, with a fixed stellar object as reference.
Sight reduction. Procedure of computing altitude and azimuth angle of a celestial object for an assumed position. More generally, the process of determining a line of position from a celestial observation.
Sight reduction tables. Tables of solutions of the navigational triangle, giving computed altitudes and azimuths on entry information of latitude, declination, and hour angle.
Small circle. Circle inscribed on a sphere by passing a plane through the sphere but not intersecting its center. (On the earth's surface all parallels of latitude except for the equator are small circles.)
Solar time. Time based on the rotation of the earth relative to the sun.
Solstice. Points of maximum declination, north and south, reached by the sun on the path of the ecliptic.
Spherical trigonometry. The application of trigonometric principles and relations to spherical triangles. The mathematical foundations of nautical astronomy.
Standard deviation. A statistical measure of variability; the square root of the mean of squared deviations.
Statute mile. A traditional unit of distance in Great Britain and the United States equal to 5,280 feet, or .869 nautical mile.
Sumner line. An LOP, a segment of a line of equal altitude and then determining corresponding longitudes compatible with the latitudes and the given altitude.
Sunrise. Local mean time at which the upper limb of the sun is seen to emerge above the true horizon.
Sunset. Local mean time at which the upper limb of the sun is seen to disappear below the true horizon.
Superior planets. Those planets whose orbits lie outside the earth's orbit; Jupiter, Mars, and Saturn are the navigational superior planets.
Swinging the arc. Rocking the image in the sextant view piece so as to assure perpendicularity of the sextant.
Synodic period. Period of time required for two successive conjunctions of celestial objects, usually the sun and another planet.

T

Theory of error. The statistical treatment of errors of observation.
Time diagram. Circular diagram plotting the location of objects with respect to the Greenwich meridian using time and angular measures interchangeable.
Time sight. Method of determining longitude by computing the hour angle between an observation at meridian transit and a later observation. 
Traverse sailing. Tracking courses and distances over a set of short tacks by the method of plane sailing.
True azimuth (Zn). The bearing of a body from the observer measured from true north westerly through 360 degrees.
True bearing. Bearing of an object with respect to true north; magnetic bearing corrected for variation.

U

Universal plotting sheets. Approximate segments of Mercator charts constructed on fixed lines of latitude but variable lines of longitude.
Upper branch of meridian. The half segment of a meridian between the poles that intercepts the observer's or the body's position.
Upper limb. Point on the circumference of the sun or moon yielding the greatest altitude.
Upper transit. Transit at the upper branch of the observer's meridian.

V

v correction. An interpolated correction due to the fact that hourly increment for GHA of a body such as Venus or the moon is not constant.
Vernal equinox. The first Point of Aries; the Point at which the sun moves from south to north declination.
Vertex. Point of highest latitude through which the extended great-circle course passes.
Vertical circle. Great circle passing through the observer's zenith and nadir.

Z

Zenith (Z). Point on the celestial sphere directly over the observer's head.
Zenith distance. The complement of latitude (i.e., 90 degrees - latitude); the distance of the zenith from the elevated pole.
Zenith system. The celestial hemisphere as defined by the observer's zenith and horizon.
Zodiac. The region straddling the ecliptic, 16 degrees in width, which includes the orbits of the visible planets.
Zone time. Greenwich mean time adjusted on the basis of one hour of difference for each 15 degrees time zone, the prime Greenwich time zone being from 7.5 degrees E to 7.5 degrees W longitude.