Albedo   The ratio of the light reflected to that incident upon a heavenly body. e.g.   The moon has an albedo of 0.07, meaning that 7% of the sun’s incident light is reflected.

Almanac    A book giving comprehensive information about the position and movement of heavenly bodies throughout the year.

Altitude   Angular distance, along a vertical circle from the horizon.

Amplitude  A celestial body’s amplitudeis the arc between the observed body on the horizon and the point where the observer’s horizon intersects the celestial equator. It is a means of checking a steering compass for deviation.

Anomalistic month   A time period averaging 27.5 days, elapsing between one perigee, the closest point of the Moon to the Earth, through its following apogee, its furthest point from the Earth and onward to the next perigee.

Aphelion   The  point in the solar orbit, when Earth is furthest from the Sun, at present in early January, but changing very slowly.

Apogee   The point in its orbit when the Moon is furthest from the Earth.

Apparent Altitude   The altitude of a heavenly body above the horizon after correction for sextant error and dip.

Assumed Position  Same as Chosen Position q.v.

Astronomical Triangle   The navigational triangle solved in sight reduction.

Azimuth   The bearing (direction) of a heavenly body as seen by an observer on earth.

Calculated Altitude   An alternative term used for Tabulated Altitude – the altitude of a heavenly body at a particular moment, as it would be seen by an observer at a Chosen Position (Hc)

Celestial Sphere    An imaginary sphere with the Earth at its centre, and on which – for the purposes of astro navigation- all heavenly bodies are assumed to lie.

Celestial Equator   A projection of the terrestrial equator onto the Celestial Sphere, throwing a Great Circle equidistant at all points from the celestial poles

Chosen Position   An arbitrary position, within about 60 miles of the vessel’s estimated position, used as a reference point when calculating and plotting an astro position line.

Civil twilight   The time at which the sun is 6° below the visual horizon line.

Collimation error   An error in the transmission of light received from the horizon mirror of a sextant as it passes through the optics of the telescope, implying that the telescope is not parallel to the plane of the instrument.

Constellation   A number of stars which appear, to an observer on Earth, to form a recognisable pattern.

Corrected Sextant Altitude   Observed Altitude.

“d” Factor   Hourly rate of change of declination.

Dead Reckoning   An assumed position for navigational purposes arrived at by noting time and distance run

Declination   A term used to define the position of a heavenly body measured as an angle north or south of the Celestial Equator, comparable with latitude on Earth.  Declinations of all heavenly bodies are found in the Almanac for every hour of the day.

Deviation   An error of the magnetic compass, caused by magnets or metal objects distorting the earth’s magnetic field in the vicinity of the compass.

Dip   (or angle of dip) The difference between horizontal (i.e. a tangent to the Earth’s surface) and an observer’s sight to the visible horizon.

1. The path traced by the sun on the Celestial Sphere and which crosses the Equinoctial at the First Point of Aries.

Error   Any discrepancy between an accurate measurement and an approximation.   Most of the calculations involved in astro navigation are concerned with the reduction or elimination of errors.  (See Deviation, Index Error, Perpendicularity, Refraction, Semi-diameter, Side error, Variation)

Ephemerides   (sing: Ephemeris) Tables which give the daily positions of the navigationally useful heavenly bodies positions of the heavenly bodies. (Origin Greek ephemeros, meaning a living day)

Equation of Time.   The mean Sun is the imaginary body moving withy perfect regularity from which GMT is taken.   It represents an average of the motions of the trueor apparent Sun (both words have the same meaning in this instance).   The mean Sun and the apparent Sun are frequently well adrift from one another.   The difference between the two is called the equation of time and details are found for each day in the same box as the time of the Sun’s meridian passage in the daily pages of the Almanac.  Put another way it is the difference between the Sun’s path on the Ecliptic and the apparent path on the Equinoctial.

1. The celestial equator.

Equinox   The time when the sun crosses the equator and day and night are equal (about March 20 and September 22 or 23)

First Point of Aries   The point on the Celestial Sphere at which the Ecliptic cuts the Celestial Equator; it is the reference point from which the Sidereal Hour Angle (SHA) of stars is measured.

Great Circle   The line described on the Earth’s surface by a plane passing through the centre of the Earth.  An arc of a great circle passing through two points on the surface of the Earth represents the shortest distance between those two  points.

Greenwich Hour Angle.  A term used to define the position of a heavenly body measured as an angle west of the Greenwich Meridian.   It is comparable with Longitude on Earth, except that it is never measured eastwards, and can therefore be any value up to 360°. GHA is tabulated for every hour of every day and can be interpolated to the nearest second.

Greenwich Meridian   The meridian which passes through the site of the Greenwich observatory, used as a reference point for the measurements of longitude and Greenwich Hour Angle.

Geographical Position.   The geographical position of the sun or a star is the point where a line from the sun or star to the centre of the Earth meets the Earth’s surface.  It is measured by declination and hour angle.   Marked as an “X” on diagrams.

Hc   Astro navigation shorthand for “Calculated Altitude”

Ho   Astro navigation shorthand for “Observed Altitude”.

Horizon Glass/mirror   On a sextant, the half–silvered glass through which the observer can simultaneously see the horizon and the reflected image of a heavenly body.

Horizontal Parallax   Geocentric parallax of an astronomical body on the horizon.

Index Arm   The moving part of a sextant used to adjust the angle of the index glass when taking a sight.

Index Error   An error of the sextant which arises when the index glass is not quite parallel to the horizon glass when the index arm is set to zero.

Index Glass   On a sextant, the mirror which reflects the image of a heavenly body towards the horizon glass.

Intercept    In plotting an astro sight, the distance between the chosen position and the closest point on the position line.

Interpolation   A mathematical process done by eye or calculation to obtain intermediate values between ones given usually in a table.

Inter Tropical Convergence Zone or ITCZ, is the belt of low pressure girdling the Earth, near the equator, where the trade winds of the Northern and Southern Hemispheres come together. It is formed by the vertical ascent of warm, moist air from the latitudes above and below the equator. As the air ascents it cools, releasing the accumulated moisture in an almost perpetual series of thunderstorms.

Latitude   A term used to define a position on Earth in terms of its angle north or south of the equator, measured at the centre of the Earth.

Local Hour Angle    The angle measured westwards around the equator between the observer’s meridian on which the heavenly body is directly overhead: i.e. the difference between the observer’s longitude (measured westwards) and the body’s GHA.

Local Mean Time is a form of solar time that corrects the variations of local apparent time, forming a uniform time scale at a specific longitude. Its uniformity depends only on the accuracy of the clocks used to measure it. Local mean time was used from the early nineteenth century, when local solar time or sundial time was last used, until standard time was adopted on various dates in the several countries. Standard time means that the same time is used throughout some region — usually, it was either offset from Greenwich Mean Time or was the local mean time of the capital of the region. The difference between local mean time and local apparent time is the equation of time.

Longitude   A term used to define a position on Earth in terms of its angle east or west of the Greenwich meridian, measured at the centre of the Earth.

Magnetic Bearing   A direction on the Earth’s surface, measured clockwise relative to the Earth’s magnetic field.

Magnitude   A term used to indicate the brightness of a  heavenly body relative to others.  The brightness as seen from the earth is the body’s apparent magnitude.

Meridian   Any one of an infinite number of imaginary lines on the Earth’s surface, linking the north and south poles.

Meridian Altitude.   The altitude of a heavenly body when it is passing over the observer’s meridian.  This altitude will give a position line running due East and West, in other words, a parallel of latitude.

Meridian Passage.   The time of meridian passage is when the heavenly body  is on the observer’s meridian i.e. when its GHA and the observer’s longitude are the same and when the heavenly  body appears directly north or south of the observer.

Micrometer Screw   On a sextant, a screw used to make small adjustments of the position of the index arm when taking a sight, marked with a scale for measuring angles to a precision of one minute of arc (see Vernier scale).

Nadir   That point of the Celestial Sphere vertically below the observer.

Nautical Almanac   A book giving comprehensive information about the position and movement of heavenly bodies throughout the year.

Nautical Mile   Strictly speaking, an international standard measure of distance equal to 1852metres, bur for most navigational purposes virtually synonymous with a Sea Mile, which is equal to one minute of latitude.  The shape of the Earth means that a sea mile varies in length form 1842.9 metres at the equator to 1861.7 metres at the poles, with a mean value of 1852.3metres.

Nautical Twilight   The time at which the sun is 12° below the visual horizon.

Nutation   Irregularities in precession of the equinoxes.

Noon Sight   The altitude sighting taken of the Sun as it crosses the observer’s meridian of longitude, reaching its highest point in the sky on that day and used in order to establish latitude.

Observed Altitude   The angle of a heavenly body above the observer’s horizontal (Ho).

Observer’s Zenith.   A point where a line projected from the Earth’s centre through the observer’s position on the Earth’s surface meets the celestial sphere.

Parallax   (or angle of parallax)  Difference in apparent position of an object as viewed from different positions.

Perigee   The point in its orbit when the Moon is closest to the Earth.

Perpendicularity   On a sextant, refers to the fact that the index glass must be perpendicular to the index arm.

Polar Distance   The angular distance from a celestial pole.

Polaris – the Pole Star   A moderately bright star lying very close to the north pole of the Celestial Sphere.

Point of Aries   The point on the Celestial Sphere from which all star GHAs are calculated is called the First Point of Aries.

Position Circle   A circular position line is one drawn from a particular position and forms a circle round that position.

Position Line   The line on a chart drawn across the azimuth of an observed body at the point of the intercept

Precession   Conical rotation of the Earth’s rotational axis about the vertical to the plane of the Ecliptic, caused by the attractive force of other bodies of the solar system on the equatorial bulge of the Earth and resulting in a slow drift in the Equinoxes and Solstices.

PZX  triangle   The astronavigational spherical triangle on the surface of the celestial sphere; PZX  denoting the Pole (P), zenith of the  observer (Z), and celestial position (X) of an observed body given by its Greenwich Hour Angle and Declination.

Refraction   Distortion caused by light waves being “bent” as they pass from empty space into the Earth’s atmosphere.

Rhumb Line   An imaginary line on the Earth’s surface which intercepts all meridians at the same angle.

Semi Diameter   Half the diameter of a heavenly body, expressed as an angle.   Used to correct for the fact the sextant is actually used to measure the altitude of the top or bottom of the visible disc of the sun or moon.

Sextant   An optical and mechanical device used to measure angles very precisely.

Sextant Altitude   Altitude of a heavenly body as measured by a sextant.

Shadow Pin   A pin or spike protruding from the centre of a compass card and intended to cast a shadow to allow the Azimuth of the sun to be measured.

Side Error   An error of the sextant which arises when the horizon glass is not quite perpendicular to the frame of the sextant.   It has the effect of making the reflected image of a heavenly body appear to one side of the direct image.

Sidereal   Of, or pertaining to stars.

Sidereal Hour Angle   (SHA) The angle between the Meridian through the First Point of Aries and the Meridian through a heavenly body, measured westwards.   Usually used (in conjunction with declination) to define the position of stars on the Celestial Sphere.

Sight Reduction Tables   A book of tables which simplify the calculation of an astro fix by allowing the Chosen Position, Declination and LHA to be quickly “reduced” to an Azimuth and Intercept.

Solstice   Either Summer or Winter (about June 22 or December 22) when the sun is farthest from the Equator and appears to pause before returning.

Sun-run-sun    A technique using transferred position lines whereby a navigational fix may be arrived at using forenoon and afternoon sun sights with or without a midday meridian passage to establish latitude.

True Altitude   The corrected Observed Altitude.

True Zenith Distance.   For a meridian passage, the sextant angle obtained is corrected for dip  and refraction to obtain the true altitude of the object above the horizon. This is then subtracted from 90° to obtain the angular distance from the position directly above, the zenith. This is referred to as the True Zenith Distance. The true zenith distance of the object is also the distance (in arc) on the Earth’s surface from the observer to where that object is overhead, the geographical position of the object.

Tabulated Altitude   An alternative term for Calculated Altitude – the altitude of a heavenly body at a particular moment in time, as it would be seen by an observer at a Chosen Position (Hc).

Tiny Tab Towards   A mnemonic to remember the direction in which the Intercept should be applied when plotting an astro fix, based on the fact that if the Tabulated Altitude is less (i.e. Tinier) than the Observed Altitude, the intercept has to be applied towards the heavenly body.

Twilight   The time between day and night, when the sun is below the visual horizon but the sky is still light.

“V” factor   In the Nautical Almanac, “v” is a small correction applied to the GHA to account for the fact that the heavenly bodies do no appear to move around the celestial sphere at a constant rate.

Variation   The difference between True North(the direction of the Meridian) and Magnetic North( the horizontal component of the Earth’s magnetic field, detected by a magnetic compass)

Vernier Scale   A series of graduations of the micrometer screw and on the index arm, which allow measurements to be taken to a precision of a tenth of a minute of arc.

“Z”   The Azimuth of a heavenly body measured in degrees clockwise or anti-clockwise from North or South.   Conversion rules in the Sight Reduction Tables allow it to be converted to the more useful “Zn”.

Zenith   The point on the Celestial Sphere which lies directly overhead from an observer on Earth.  Marked as “Z” on diagrams.

Zenith Distance.   The angular measurement on the celestial sphere from an observer’s zenith to the position of the heavenly body.

“Zn”   Astro navigation shorthand for Azimuth, measured in degrees clockwise from North.