Glossary

Altitude

Angular elevation of an object above the horizon.

AP; Assumed Position

This is the position used for the purpose of computing sight reductions. When using a calculator or computer for sight reductions, this is typically the dead reckoning position. When using tables, the assumed latitude is rounded to the nearest whole degree, and the assumed longitude is taken to be a whole number of degrees from the celestial body's geographic position

Azimuth

Direction of an observed object. Typically (but not always) measured in degrees clockwise of North.

Bearing

The reading of a compass, handbearing/ships, taken to an object.

Body

A celestial object.

CC

Compass Course/Course-to-Steer, the TC corrected for current, variation, and deviation.

CMG

Course Made Good.

D

Declination

The latitude of a celestial body.

Dist

distance in nautical miles.

DR; Dead Reckoning

The dead reckoning position is computed from a previous known fix and the vessel's heading and speed. Calculated by a Sailings method as: Distance = Time X Speed.

Drift

the speed of the current. a)Early were lessons in miles per day(24Hrs) b)Current custom is in Kts per hour.

ENT

Evening Nautical Twilight

EP

Estimated Position, the DR plus the effects of current.

ETA

estimated time of arrival.

First Point of Aries

The position of the Sun at the moment of vernal equinox. This is used as the base coordinate for describing the positions of celestial bodies.

FIX

A position fix, by celestial or land bearings to a known point.

GP; Geographic Position

The spot on the Earth most directly under a celestial object. Determined from the almanac.

GHA; Greenwich Hour Angle

The longitude of a celestial object's geographic position, measured in degrees west of Greenwich. GHA is never measured in east longitude, only west. Thus, GHA may vary from 0 to 360°.

HE

Height of the eye or an object above sea level. Used to calculate distance to the horizon or an object.

HP; Horizontal Parallax

Error caused by moving horizontally relative to some centerline. For example, pretend you're on deck, looking past a buoy at something on shore. Now step a few feet to the side. The buoy and the object on shore will no longer line up. That's horizontal parallax. The amount of error depends on how far you stepped and how far away the objects you were sighting are.

So how does this apply to celestial navigation? Suppose the object you're looking at is the Moon, the Sun, or a planet. And instead of stepping a few feet to the side, you moved 4000 miles (the radius of the Earth). This will change your measurements. For very far-away objects such as stars, the difference is infinitesimal. For the moon, the error can be more than a degree. The tables in the Nautical Almanac will give correction factors for the Sun, Moon and planets.

Hs

Sextant altitude, altitude of a body directly measured with a sextant. No corrections are applied.

Kts

nautical miles per hour (speed).

LHA; Local Hour Angle

The difference in longitude from the observer's position to a celestial body. Computed by subtracting the observer's GHA from the body's GHA, modulo 360°.

Log

a) The book that a navigator records his calculations and observations. b) An instrument with a trailing spinner used to calculate the distance traveled through the water.

LOP

A line of position, by celestial or a line derived from a bearing to a known point, the ship's true position must be somewhere on, or close to, the line.

MNT

Morning Nautical Twilight

mtrs

NA

Nautical Almanac.

Nmi/Nm

nautical miles.

Plotting Sheet

Any paper used by a navigator to graph his LOP's and ships course.

RFIX

a Running FIX, is a previous line of position brought forward to a current line of position.

S

Speed in Kts, usually calculated or from a Knotmeter.

Sailings

DR techniques such as Plane Sailing, Mid-Latitude Sailing, Mercator Sailing, Great Circle Sailing, etc.

Set

The direction *toward* which a current is moving.

SHA; Sidereal Hour Angle

The longitudinal position of a celestial body, measured in degrees west of the first point of Aries.

SMG

Speed Made Good over the ground.

SR

SS

T

Time, the time of an event that is recorded, usually in a log.

TC

True Course, plotted or calculated for a mercator chart from point A to point B.

UT

Universal Coordinated Time, the time from NIST broadcast by WWV. Same as GMT, Zulu, UTC.

ZT

Zone Time, the time on the meridian of a vessel's either DR or FIX, the longitude DR/FIX divided by 15.