Angular elevation of an object above the horizon. Altitude measured by sextant is Sextant Altitude (Hs). When sextant errors are corrected, it is known as Aparent Altitude (Ha). After all errors are corrected, it is known as Observed Altitude (Ho). A computed altitude is Hc.
Compass Course/Course-to-Steer, the TC corrected for current, variation, and deviation.
The latitude of a celestial body.
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.
The speed of the current.
Evening Nautical Twilight
Estimated Position, the DR plus the effects of current.
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.
A position fix, by celestial or land bearings to a known point.
The spot on the Earth directly under a celestial object. E.g. a line drawn from the object to the center of the earth would pass through the Earth's surface at the GP. Determined from the almanac.
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°.
An object's GHA varies rapidly as the earth turns.
Height of the eye or an object above sea level. Used to calculate distance to the horizon or an object.
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.
Sextant altitude, altitude of a body directly measured with a sextant. No corrections are applied.
Nautical miles per hour (speed).
The difference in longitude from the observer's position west to a celestial body. Computed by subtracting the observer's GHA from the body's GHA, modulo 360°.
The edge of a celestial body that shows a disk.
Calculations are normally based on the center of a celestial body, but this is very hard to identify on an object, such as Sun or Moon, that shows a disk. Instead, measurments are made from the edge ("limb") of the object and then corrected for the radius ("semidiameter") of the object.
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.
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.
Nautical miles.
Any paper used by a navigator to graph his LOP's and ships course.
A Running FIX, is a previous line of position brought forward to a current line of position.
Speed in Kts, usually calculated or from a Knotmeter.
DR techniques such as Plane Sailing, Mid-Latitude Sailing, Mercator Sailing, Great Circle Sailing, etc.
The direction *toward* which a current is moving.
The longitudinal position of a celestial body, measured in degrees west of the first point of Aries.
An object's SHA value varies very slowly as the object moves through the sky. The SHA values of stars change the slowest of all.
Speed Made Good over the ground.
Sunrise
Sunset
Time, the time of an event that is recorded, usually in a log.
True Course, plotted or calculated for a mercator chart from point A to point B.
Universal Coordinated Time, the time from NIST broadcast by WWV. Same as GMT, Zulu, UTC.
Zone Time, the time on the meridian of a vessel's either DR or FIX, the longitude DR/FIX divided by 15.