Glossary

62 terms

Amplitude

The Sun’s bearing from east or west as it rises or sets: a check of the compass. Checking the compass by the Sun: Where the Sun rises

Apparent altitude (Ha)

Hs with index correction and dip applied. Sextant corrections: Hs to Ho: Index correction and dip give Ha

Assumed position (AP)

A position near the DR chosen to give whole degrees of latitude and LHA, to enter the tables. The assumed position and LHA: The assumed position

Azimuth (Zn)

The true bearing of the body, 000° to 360°: the intercept is drawn along it. The line of position: Where the line goes: Ho against Hc

Azimuth angle (Z)

The tables’ angle at the zenith, turned into Zn by the rules printed on each page. The intercept method: Reading Pub. 249

Celestial equator (equinoctial)

The Earth’s equator projected onto the celestial sphere; declination is measured from it. Where the Sun is: GHA and declination: The celestial sphere

Celestial horizon

The great circle 90° from your zenith; altitudes are measured up from it. The circle of equal altitude: Zenith, horizon and altitude

Celestial meridian

The hour circle through your zenith; LHA is measured west from it. The assumed position and LHA: LHA on the celestial sphere

Celestial poles

The Earth’s axis extended to the celestial sphere; the sky turns about them once a day. Where the Sun is: GHA and declination: The celestial sphere

Celestial sphere

The sky drawn as a huge sphere with the Earth at its center; each body is placed where its direction meets it. Where the Sun is: GHA and declination: The celestial sphere

Circle of equal altitude

Every place that sees a body at the same altitude: a circle centered on its GP. The circle of equal altitude: One altitude, a whole circle

Civil twilight

Sun 6° below the horizon: bright stars visible and the horizon still sharp. The time for star sights. Planning a twilight fix: Twilight: the window for star sights

Cocked hat

The small triangle three lines make; the fix is taken inside it. The fix: How good is the fix?

Compass error

True bearing minus compass bearing: variation plus deviation. Checking the compass by the Sun: Compass error and deviation

Computed altitude (Hc)

The altitude you would observe at a chosen position, from the tables or by calculation. The line of position: Where the line goes: Ho against Hc

Course to steer (CTS)

The heading that, with the current, makes good the track you want: head up into the current. The fix: Currents: set, drift and the course to steer

d correction

The change in declination over the hour, scaled to the minutes past the hour. Where the Sun is: GHA and declination: Minutes and seconds past the hour

Date line

The 180th meridian, where zone descriptions +12 and −12 meet: crossing it westward, the ship's calendar jumps a day forward. Angles, time and UT: Crossing the date line

Dead reckoning (DR)

Position worked out from the last fix, the course steered and the distance run. Latitude and the nautical mile: Dead reckoning: where you think you are

Declination (Dec)

The GP’s latitude: how far north or south of the equator the body is. Where the Sun is: GHA and declination: The Sun's ground position

Deviation

Compass error caused by the boat’s own iron and electrics; it changes with heading. Checking the compass by the Sun: Compass error and deviation

Dip

Correction for height of eye: the sea horizon is below true horizontal, so subtract dip. Sextant corrections: Hs to Ho: Index correction and dip give Ha

Ecliptic

The Sun’s yearly path among the stars, tilted 23.44° to the celestial equator; it crosses the equator going north at Aries. Latitude from Polaris: Aries and SHA on the celestial sphere

Estimated position (EP)

A DR with the current (and leeway) allowed for. The fix: Currents: set, drift and the course to steer

First point of Aries (♈)

The reference point among the stars; a star’s GHA is GHA Aries plus its SHA. Latitude from Polaris: The stars, and the first point of Aries

Fix

Where two or more lines of position cross: your position at the time of the sights. The fix: Two lines make a fix

Greenwich hour angle (GHA)

The GP’s angle west of Greenwich, 0° to 360°: a longitude measured westward. Where the Sun is: GHA and declination: The Sun's ground position

Ground position (GP)

The point on the Earth directly under a body: the body is straight overhead there. Where the Sun is: GHA and declination: The Sun's ground position

Horizontal parallax (HP)

The Moon’s parallax at the horizon; sets the second Moon altitude correction. Planets and the Moon: The Moon

Hour circle

A half great circle on the celestial sphere from pole to pole through a body: the sky’s meridian. Hour angles are measured between them. Where the Sun is: GHA and declination: The celestial sphere

Increments and corrections

The almanac’s tables of GHA gained in the minutes and seconds past the hour. Where the Sun is: GHA and declination: Minutes and seconds past the hour

Index correction (IC)

Correction for the sextant reading not quite 0° when the mirrors are parallel: “on the arc, take it off”. Sextant corrections: Hs to Ho: Index correction and dip give Ha

Intercept

Ho − Hc in minutes, which are miles: toward the GP if Ho is greater (“Ho Mo To”), away if smaller. The line of position: Where the line goes: Ho against Hc

Latitude

Angle north or south of the equator, measured at the Earth’s center: 0° to 90°. Latitude and the nautical mile: Latitude is an angle

Line of position (LOP)

A short stretch of a circle of equal altitude, drawn as a straight line: you are somewhere on it. The line of position: Close up, the circle is a straight line

Local apparent noon (LAN)

The moment the Sun crosses your meridian and reaches its highest altitude of the day. Latitude from the noon Sun: Watch the Sun cross your meridian

Local hour angle (LHA)

The GP’s angle west of your meridian: GHA plus east longitude, or minus west. The assumed position and LHA: LHA: the hour angle from your meridian

Local mean time (LMT)

Time by the mean Sun at your meridian; the almanac lists sunrise and twilight in LMT. Planning a twilight fix: When is twilight by my watch?

Longitude

Angle east or west of the prime meridian through Greenwich: 0° to 180°. Latitude and the nautical mile: Longitude, and why distance uses latitude

Main correction

Refraction, semi-diameter and parallax combined in one almanac table. Sextant corrections: Hs to Ho: The main correction gives Ho

Nautical mile (nm)

One minute of latitude: 1,852 meters. Distances are measured on the latitude scale. Latitude and the nautical mile: A minute of latitude is a nautical mile

Nautical twilight

Sun 12° below the horizon: the horizon is too dark for sights. Planning a twilight fix: Twilight: the window for star sights

Navigational triangle

Pole, zenith and GP on the celestial sphere: solving it gives Hc and the azimuth. The intercept method: The navigational triangle

Observed altitude (Ho)

The fully corrected altitude: the true angle of the body’s center above the horizon. Sextant corrections: Hs to Ho: The main correction gives Ho

Polaris

The pole star, within a degree of the celestial pole: its altitude is close to your latitude. Latitude from Polaris: Polaris sits almost over the pole

Pub. 249

Sight reduction tables for air navigation: Hc, d and Z for whole-degree latitude, LHA and declination. The intercept method: Reading Pub. 249

Running fix

An earlier line advanced along the course by the distance run, crossed with a later one. The fix: The running fix

Set and drift

A current's direction (set, the way it flows) and the distance it carried you (drift); drift ÷ hours is its rate. The fix: Currents: set, drift and the course to steer

Sextant altitude (Hs)

The angle read off the sextant, before any corrections. Sextant corrections: Hs to Ho: What the sextant reads

Side error

Sextant error from the horizon mirror not being square to the frame; adjusted out with its screw. Sextant corrections: Hs to Ho: Adjusting the sextant

Sidereal hour angle (SHA)

A star’s angle west of Aries; nearly constant, listed on the daily pages. Latitude from Polaris: The stars, and the first point of Aries

Star finder

A star base and latitude templates (the 2102-D, or the Admiralty’s NP 323) that show every navigational star’s altitude and bearing for a given LHA of Aries. Planning a twilight fix: The star finder

Universal plotting sheet (UPS)

A blank sheet you set up for your latitude, with the longitude scale shrunk by cos(latitude). The fix: Making a plotting sheet

Universal Time (UT)

The time at Greenwich, used for every almanac lookup. Angles, time and UT: Everything runs on UT

Upper and lower limb

Top and bottom edges of the Sun or Moon: the edge you bring down to the horizon. Planets and the Moon: The Moon

v correction

Extra GHA for a planet or the Moon moving at a rate other than the table’s, scaled to the minutes. Planets and the Moon: Planets: the wandering stars

Variation

The angle between true north and magnetic north at a place; printed on the chart. Latitude and the nautical mile: Dead reckoning: where you think you are

Watch error

How fast or slow the watch is against UT. Each second of time is 15″ of longitude. Angles, time and UT: Why the seconds matter

Watch rate

How much the watch gains or loses a day, used to work out the error since the last time check. Angles, time and UT: Why the seconds matter

Zenith

The point on the celestial sphere straight above you; its declination is your latitude. The opposite point is the nadir. The circle of equal altitude: Zenith, horizon and altitude

Zenith distance (z)

90° minus the altitude: the angle from straight overhead, and the distance to the GP. Latitude from the noon Sun: Working a noon sight

Zone description (ZD)

Hours to add to zone time to get UT: +5 in the US east coast winter zone, for example. Angles, time and UT: Everything runs on UT