Navigation theory

Ten questions with at most 5 mistakes: celestial sphere, coordinates, time and the sextant.

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Celestial sphere

Uranographic coordinates refer to the celestial equator: declination and sidereal hour angle. Horizontal ones refer to the horizon: altitude and azimuth.

The first point of Aries is the reference for locating stars.

Sextant

From sextant to true altitude: index correction, dip (1.76 × √height, in minutes), refraction and semi-diameter.

Activities

• SextantBring the sun to the horizon, read the arc and correct the reading.
• Sight reductionGreat circles, computed altitude, azimuth and meridian latitude, step by step.

Questions in this unit

• The Pole Star is of:
• All bodies visible from an observer’s position that have the same altitude lie on:
• If, sailing from Cape Verde to the Caribbean, we keep zone time all the way, we must:
• When the Sun’s disc cuts the horizon in the west and its upper limb is tangent to it, this is:
• The arc of meridian counted from the observer’s position on the celestial sphere to the elevated pole is:
• The greatest possible difference between zone time and local civil time at any point of the zone we are in is:
• What is the name of the small circle parallel to the apparent horizon of an observer on the celestial sphere?
• The arc of the celestial equator counted from the point of Aries to a star’s hour circle is called:
• Equatorial uranographic coordinates use two arcs of great circles of the celestial sphere. Name them:
• Every meridian, on the terrestrial or the celestial sphere, has:
• The declination of a body on the celestial sphere is an arc of a great circle that corresponds to:
• On the celestial sphere, an observer is at their zenith, and from this point to the elevated pole there is an arc of celestial meridian called:
• The special navigation charts published by the US Navy hydrographic department for each month, covering the North and South Atlantic, North and South Pacific and the Indian Ocean, with detailed nautical and weather information, are known as…
• On the celestial sphere, the axis from our position or zenith, through the centre of the sphere and out 180º away on the other side, is called:
• The two angles formed in the constellation Cassiopeia each have a bisector which, extended across the sky, leads to:
• On the celestial sphere, when we observe a body and find its true altitude, the complementary great-circle arc from the body to our zenith is:
• The elements of the horizontal celestial coordinates are:
• Which celestial coordinates are used for stars?
• The Nautical Almanac is produced by:
• To get the corrections needed to find a body’s observed altitude after a sextant sight, we use:
• In the Indian Ocean south of the equator our latitude is 10º20.6′S, and the Sun is on our meridian with declination +18º23.6′. We see the Sun:
• In the position triangle, the complementary arc from a body being observed to the observer’s zenith is called:
• Bodies rise and set when their parallels of declination cut:
• To get Greenwich hour angle of Aries with the current year’s almanac we use:
• In the spherical triangle, the only side that can exceed 90º is:
• The ellipse the Sun apparently travels around the Earth:
• Applying index error and dip corrections to the sextant altitude gives:
• Extending the line of Orion’s Belt northwards leads to the star:
• When the Sun passes the Vernal Point:
• The nautical azimuth…
• The projection on the celestial sphere of the ellipse of the Sun’s apparent yearly motion around the Earth is known as:
• The sextant measures:
• Legal (zone) time and local civil time coincide:
• Which bodies are never seen from Cádiz (36º32′N 6º18′W)?
• When latitude is 0º:
• The constellation shaped like an M or a W, used to find the Pole Star, is:
• Sextant sights with a hazy horizon:
• For an observer at 25ºS, which bodies never rise (altitude always ≤ 0º)?
• When a star is at the zenith:
• Universal Time equals:
• If a sextant’s index arm is turned 15º, the arc reading changes by:
• The horizon plane is always perpendicular to:
• For an observer in the southern hemisphere, the south cardinal point is the intersection of:
• At 38º50′N, 135º00′W, on 25 July 2020:
• Azimuth can be expressed in three ways:
• The constellation Orion:
• The sides of the position triangle are:
• How do the legal times of two places at 175ºE and 175ºW compare?
• Select the definition of rising (orto).
• The arc of the celestial equator, counted eastwards, between the First Point of Aries and a body’s hour circle is:
• If two places have the same legal (zone) time, then:
• In a position triangle, the side defined by the zenith and the pole:
• At the instant a body sets:
• When a north-declination body’s declination exceeds 90º minus a northern observer’s latitude:
• When measuring the Sun’s altitude with a sextant in the morning:
• The First Points of Aries and Libra both have:
• If circumstances allow, when observing the Sun for the morning line of position:
• The Pole Star is easily found from the stars of the constellations:
• The south cardinal point is always on:
• The angle between the celestial equator and the observer’s horizon is:
• Bodies with the same sidereal hour angle:
• When a body’s local hour angle exceeds 180º:
• When a body’s altitude is 90º, which quantities are 0º?
• When is the day arc shorter than the night arc?
• When does the Sun’s true rising occur exactly at the east cardinal point?
• What must be done to remove the sextant’s index error?
• If an observer has a star at the zenith:
• Observing a body with a sextant in rough seas:
• The celestial coordinate measured along the equator from Aries eastwards to the body’s hour circle is called:
• Which coordinate system can be published in the almanac?
• On nautical azimuth, mark the CORRECT statement:
• If the observer’s latitude and the body’s declination are very similar:
• Mark the correct answer:
• Sextant index error is due to:
• Two places have the same longitude and different latitude. Then:
• The arc of the vertical circle between the zenith and the body is:
• The coordinates of the First Point of Libra are:
• To observe a star’s altitude with the sextant:
• Mark the correct option:
• When a visible body crosses the upper meridian at a place in the southern hemisphere:
• Which bodies always have the same Greenwich hour angle?
• Taking north latitudes positive and south negative, the angular distance from the north celestial pole to the zenith is:
• Select the correct relation:
• The apparent sunset occurs:
• The ecliptic:
• When observing a star with the sextant:
• The observed altitude of a body:
• Local civil time in Madrid (40º30′N, 003º40′W) coincides with:
• In the position triangle, the angle at the zenith is between:
• What is the part of a traditional sextant fixed to the frame, perpendicular to the arc’s plane, half clear glass and half mirror?
• In which case are the celestial equator and the astronomical horizon perpendicular?
• On the First Point of Aries, mark the INCORRECT statement.
• What is the time set by the government to save electricity called?
• Mark the INCORRECT statement about declination.
• On passage planning, before starting the voyage the master must plan:
• One way to find Polaris is from two easily identified stars in the constellation:
• The angular distance along the body’s hour circle from the body to the elevated celestial pole is:
• If a body is visible as it crosses the lower meridian, we can say:
• At sea we take Nunki’s altitude with the sextant. Which horizon did we use to touch the star to?
• What is the arc of equator from the east or west cardinal point to the foot of the body’s hour circle when the body is on the horizon called?
• What is the prime meridian?
• Pilot Charts show:
• What is mean solar time for the observer’s meridian called?
• On official time, mark the CORRECT statement:
• On the sextant “frame”, mark the TRUE statement:
• Right ascension is defined as:
• When planning a route:
• Which star would we recognise by extending the Canopus–Achernar line?
• How is an almucantar defined?
• What is the hour circle?
• The ecliptic plane is inclined to the equatorial plane by:
• On the celestial sphere, the axis from the observer’s position through the sphere’s centre and out the opposite side is known as:
• The great-circle arc on the celestial sphere from a visible body to the observer’s zenith is called:
• When the Sun on the ecliptic reaches the First Point of Libra:
• The Sun’s Greenwich hour angle (GHA) is one of the values of the celestial coordinate system:
• On an observer’s celestial sphere, the arc of the true horizon from the observer’s true meridian to the body is:
• When crossing the 180º meridian heading east:
• The polar distance is given by…
• The star Gacrux belongs to the constellation:
• How does the celestial equator lie in relation to the constellation Orion?
• The celestial meridian arc formed between the celestial equator and the observer’s position is named:
• The meridian angle (polar angle P) is one of the angles of the navigational triangle and is formed:
• The parallel of declination of a celestial body is defined as:
• Within standard time zones, the maximum time difference between zone time (Hz) and local mean time (Hcl) is:
• When the line through the stars Merak and Dubhe is extended 5 times their visual separation, it points to Polaris; to which constellation do they belong?
• The arc of the celestial equator measured westward from the First Point of Aries from 0º to 360º to the hour circle of a body is:
• On the celestial sphere, the zenith represents:
• Universal Time (UT) in celestial navigation…
• The ecliptic is inclined to the celestial equator by an angle (obliquity) of:
• The hour coordinates (“coordenadas horarias”) of a celestial body are:
• The points on the Sun’s ecliptic where declination is equal to zero are:
• The Observer’s Upper Meridian is the great semicircle passing through…
• The Sidereal Hour Angle (SHA) of a celestial body is the arc of the celestial equator of…
• Measuring the altitude of a celestial body at the instant of its upper meridian transit is used to obtain:
• Definition of the Greenwich Hour Angle (GHA) of a celestial body:
• The great semicircle passing through the poles and the point where the observer is located is called the:
• The great circle perpendicular to the zenith–nadir axis whose centre coincides with the centre of the Earth, dividing the celestial sphere into visible and invisible hemispheres, is called the:
• The time elapsed between two consecutive transits of the mean Sun across the lower meridian is called a:
• Zone time (“hora legal” or “hora de huso”) is defined as:
• In celestial navigation, what is the azimuth of a body?
• When crossing the 180º meridian (International Date Line) heading west:
• On the celestial sphere, the East and West cardinal points are obtained at the intersection of the Horizon with:
• On the celestial sphere, the North cardinal point (N):
• Universal Time (UT) is defined astronomically as:
• Which celestial circle contains one of the sides of the navigational triangle (triángulo de posición)?
• For an observer in North latitude, when the azimuth of a northern-hemisphere celestial body is 180º:
• If the declination of a celestial body equals the latitude of an observer in North latitude:
• Regarding the First Point of Aries on the celestial sphere:
• An equatorial arc counted 0º to 180º East or West from the observer’s upper meridian to a body’s hour circle is:
• Regarding the Constellation of Orion:
• The sextant’s index error:
• Before observing with the sextant we will take into account:
• The monthly navigation charts of the oceans, with plenty of information to help navigation and passage planning, are:
• On the rising and setting of stars, choose the INCORRECT statement.
• To get horizontal angles between points on the coast with the sextant, the reading needs:
• The dihedral angle between the planes of the first ascension great circle and the body’s, measured directly (eastward), is:
• The sailing directions give:
• The prime or zero meridian:
• When the Great Bear is below the horizon, can we find Polaris using another constellation?
• The day and night arcs of bodies are equal:
• An almucantar is:
• Universal time is defined as the time since the passage of the:
• If we observe the constellation with the most bright stars, we will have identified:
• When a celestial body is on the observer’s upper meridian:
• Where do Universal Time, Local Mean Time and Zone Time all coincide?
• The side of the navigational triangle whose vertices are the celestial body and the zenith is:
• Places located in time zone 12 (huso 12) have:
• For an observer in North latitude, when the azimuth of a celestial body is 0º:
• When two celestial bodies lie on the same almucantar:
• Measured eastward 0º to 360º along the equator from the First Point of Aries to a body’s hour circle is:
• The side of the navigational triangle whose vertices are the celestial body and the elevated pole is:
• Polaris (the Pole Star) can be easily located with the aid of the constellations:
• On the celestial sphere, the elevated celestial pole is:
• What is the arc of the hour semicircle measured from the celestial equator to the body?
• Taking sextant sights in rough seas:
• The great circle obtained by intersecting the celestial sphere with the plane of the Earth’s orbit is:
• For an observer in Bilbao (43° 15.7′ N, 002° 57.2′ W), a celestial body with declination 47º N:
• When calculating an observed position (sight reduction), two angles of the navigational triangle are normally used. Which are they?
• The observer’s vertical (the line perpendicular to the horizon passing through the observer) intersects the celestial sphere at the points called:
• In the spherical navigational triangle (triángulo de posición), it always holds that:
• On the celestial sphere, the North cardinal point is the intersection of:
• What is the difference between the sidereal hour angle and the local hour angle of a star?
• The durations of the diurnal and nocturnal arcs of a celestial body depend on:
• The arc of the celestial equator measured westward from the observer’s upper meridian to the First Point of Aries is:
• Identify the correct statement regarding time scales in navigation:
• Identify the correct option concerning the equatorial uranographic coordinates of a celestial body:
• What is meant by official time (“hora oficial”)?
• Universal Time (UT) coincides with:
• The Pole Star (Polaris) can be located directly from the stars of:
• Which lines on the celestial sphere coincide with each other?
• For an observer at latitude 45º S, the South cardinal point lies:
• When a celestial body is on the observer’s lower meridian:
• The dihedral angle (from 0º to 180º) formed by the body’s hour semicircle and the observer’s upper meridian is:
• On the celestial sphere, the altitude of a body is measured:
• The constellation Crux (“Southern Cross”) indicates where South lies by means of the stars:
• The crossing of the apparent horizon by the Sun’s upper limb as it descends is known as:
• Greenwich Hour Angle (GHA) is determined from Local Hour Angle (LHA) by means of:
• The marine sextant is an optical instrument:
• The time taken by the Moon to return to the same phase is known as a:
• Regarding the constellation Cassiopeia, identify the TRUE statement:
• With respect to Universal Time (UT), identify the FALSE statement:
• Standard definition of the declination of a celestial body:
• What is meant by the rising (“orto”) of a celestial body?
• In nautical astronomy, the ecliptic is defined as:
• If the altitude of a celestial body is 33º (select the correct answer):
• Regarding the altitude of a celestial body, select the INCORRECT statement:
• Concerning Sidereal Hour Angle, select the CORRECT answer:
• In a traditional marine sextant, how is the “horizon glass” (“visor de horizonte”) defined?
• If a sextant’s index error (Ei) exceeds 3' and you wish to reduce it, which adjustment screw should mainly be turned?
• With reference to horizon coordinates, what is the complementary angle of altitude called?
• At 03:30 UTC on the 15th, what will be the zone time (hora legal) at a place of longitude 100° 10' W?
• How is “Sidereal Hour Angle (SHA / AS)” defined?
• The celestial semicircle of the observer’s meridian that contains the Zenith is called the:
• In navigation, the “prime meridian” (“primer meridiano”) is also known as the:
• In the navigational triangle, the angle formed between the colatitude side and the codeclination side is:
• The time of transit of the Sun across the lower meridian is determined approximately by:
• To ensure proper care of the sextant, movements of the index arm (alidada) while taking sights should be made:
• The angle formed between the Ecliptic and the Celestial Equator is:
• In angular units, the value of “Right Ascension” (RA / AR) is equivalent to:
• Starting from the Pole Star (Polaris), which constellation can we locate using alignments from Ursa Major?
• The intersection between the prime vertical and the astronomical (rational) horizon contains:
• At sea, if we measure with the compass azimuth ring (alidada) the horizontal angle between North and the star Dubhe, what have we obtained?
• The crossing of the apparent horizon by the Sun’s upper limb as it ascends is the definition of:
• Using Orion’s Belt (“Las Tres Marías”) as a reference and extending its line toward the Southern Hemisphere, we can easily recognize:
• In celestial navigation, the “nocturnal arc” (“arco nocturno”) is defined as the:
• Regarding the Bayer designation of stars within a constellation, identify the CORRECT statement:
• The “International Date Line” (“línea de cambio de fecha”) is defined as the:
• Is it possible to determine and correct the index error in a sextant reading?
• On the celestial sphere a yacht is on a given celestial meridian and, at that instant, a body is on its hour circle. What is the arc of the celestial equator between the observer’s celestial meridian and the body’s hour circle called?
• The local upper meridian and the Greenwich meridian cut the Equator in an arc equal to the:
• The vertices of the position triangle are:
• Regarding “legal time”, which statement is CORRECT?
• When the Sun is at the First Point of Aries:
• The East and West cardinal points on the celestial sphere are defined by:
• The arc of the equator from the First Point of Aries to the body’s hour circle, counted from 0° to 360° opposite to the Sun’s apparent motion on the ecliptic (clockwise), is:
• The observer’s upper meridian:
• At sea, during evening twilight, we take a body’s altitude with the sextant. After applying the corrections, we obtain:
• The small circles parallel to the true horizon and perpendicular to the zenith-nadir axis (Z-Z’) are called:
• The semicircle joining the North and South Poles of the celestial sphere and passing through the body is called:
• The complement of the true altitude of a body on the celestial sphere (90º − true altitude) is:
• The inclination of the plane of the ecliptic to the celestial equator is:
• Uranographic coordinates measured on the celestial equator are:
• The time zone gives zone time (Hz), or legal time (Hl). What is the greatest possible difference between that zone time and the local civil time (HcL) at any point within the zone?
• The horizontal coordinates take as their reference great circle the:
• The arc of a body’s hour semicircle on the celestial sphere, between the celestial equator and the centre of the body, always less than 90º, is called:
• The greatest altitudes we can get from all visible bodies with the sextant occur at:
• Through the centre of the constellation of Orion (Orion’s Belt) passes:
• The constellation of the Southern Cross lies:
• The small circles of the celestial sphere parallel to an observer’s visible horizon are:
• On the celestial sphere, the sides of any position triangle will always be:
• An observer’s colatitude on the celestial sphere corresponds to:
• The International Date Line on the Earth corresponds to the meridian:
• The values at a given moment of the Sun’s hour circle and declination correspond to its:
• The sidereal hour angle of a given star is measured on the celestial equator from:
• If, on observing a body for a position line, we are on the circle of equal altitudes, we are on…
• An observer on the celestial sphere is at latitude 40º N. What is their elevated pole, and what is their colatitude?
• Evening twilight occurs...
• The arc of a great circle of the terrestrial sphere passing through two points of a direct route is known as…
• If we are on celestial parallel 40º N, what is the arc of our celestial meridian from this parallel to our depressed pole?
• The horizontal coordinates of a body on the celestial sphere are:
• The arc of a body’s hour semicircle on the celestial sphere, counted from the body’s position on that circle to the elevated pole, is called... :
• The Northern Hemisphere spring equinox on the ecliptic coincides with the zodiacal constellation of...
• The arc of the equator on the celestial sphere between the local upper celestial meridian and the body’s hour semicircle is called:
• The plane of the ecliptic is inclined to the plane of the celestial equator by:
• The star Alkaid belongs to the constellation... :
• For an observer on the Earth, the greatest altitude of the Sun they can take with the sextant comes at:
• The sextant’s index error is...
• When the Sun, on its parallel of declination for the time of year, cuts the observer’s horizon:
• The arcs that define equatorial uranographic coordinates are:
• Every day there is a moment when all places in the world have the same date. This happens when:
• If we find the Sun’s Greenwich hour angle and its declination at that moment, we have:
• Through every body on the celestial sphere passes an arc of a great circle running from celestial pole to celestial pole. This arc is known as:
• If we know a star’s Greenwich hour angle and our longitude (east or west), we can calculate an angle of the position triangle, which is:
• The time since the Sun passed an observer’s lower meridian corresponds to:
• The arc of the hour semicircle between the celestial equator and the centre of a given body corresponds to the celestial hour coordinate:
• Equatorial uranographic celestial coordinates are referred to:
• The imaginary axis from our position on the celestial sphere (zenith), through the centre of the sphere, cuts a point at the opposite end. This point is:
• The plane of the great circle perpendicular to the zenith-nadir axis at the centre of the celestial sphere is:
• Two bodies on the same almucantar of an observer’s oblique celestial sphere have:
• How is the Pole Star identified using constellations?
• The points where the ecliptic cuts the celestial equator are:
• The Sun’s apparent path on the celestial sphere as seen from Earth, whose plane is inclined 23º27′ to the plane of the celestial equator, is called:
• Equatorial uranographic coordinates:
• The Sun’s “diurnal arc” relative to an observer on Earth means:
• When under way we use legal or zone time:
• The Pole Star is a star:
• Sunset, for an observer on Earth, occurs:
• Within one time zone, the greatest possible difference between zone time and local civil time is:
• In the position triangle, the meridian angle is formed:
• Sidereal hour angle means:
• The definition of a body’s parallel of declination on the celestial sphere is the:
• An observer’s depressed pole on the celestial sphere is the pole:
• Set out from an observer’s zenith on the celestial sphere and parallel to the true or rational horizon are the small circles called:
• The Sun’s yearly passage through the first point of Aries on the ecliptic corresponds to:
• The Sun’s twilight after sunset is called:
• The star Rigel belongs to the constellation:
• The right chart publication for planning an ocean passage is a chart of the type:
• In equatorial uranographic coordinates, the components are:
• The projection of an observer’s position on the celestial sphere occupies a point called:
• If we sail along a great circle of the terrestrial sphere, we are following:
• On the celestial sphere, the point diametrically opposite an observer’s zenith is called:
• The great semicircles of the celestial sphere that run from pole to pole through the centre of a body are:
• From an observer, the arc of the horizon counted clockwise from the north cardinal point to the foot of the vertical semicircle through the body is:
• The arc of a great semicircle counted from the body to the observer’s zenith is:
• The arc of the celestial equator counted from the vernal point westward (0º to 360º) to a star’s hour circle is called:
• The small circles of the celestial sphere parallel to an observer’s apparent horizon are:
• For an observer on the celestial sphere at latitude 20º N, the depressed pole will be:
• The arc of a body’s hour circle, counted from the body to its elevated pole, is:
• The meridian angle forms between the body’s hour circle and the observer’s meridian, and is always referred to:
• The point on the ecliptic from which the Sun passes from the southern to the northern celestial hemisphere is called:
• The two stars of Ursa Major, Merak and Dubhe, extended in a straight line about 5 times the distance between them, point to the star:
• The celestial coordinates whose elements are sidereal hour angle and declination are:
• The constellation of Cassiopeia is in the hemisphere:
• The greatest altitude we can obtain with the sextant from a visible body occurs at the moment of:
• On the ecliptic, the point where the Sun passes from the southern to the northern hemisphere, coinciding with the start of northern spring, is called:
• When we observe a body, the vertices of the corresponding position triangle on the celestial sphere form angles which are:
• The great semicircle of the celestial sphere that runs from pole to pole through the body’s position is called:
• The ocean charts that give plentiful data for ocean navigation and are published for each month of the year are called:
• Sextant altitudes of bodies are turned into true altitudes with the:
• The Sun’s hour circle value and its declination are the elements of the celestial coordinates:
• Zone time coincides with local civil time:
• If we sail west from El Hierro (Canaries) to St Lucia (Caribbean), regarding time zones we must:
• Greenwich Civil Time is related to Local Civil Time through:
• Where are chart corrections announced?
• The term “meridian altitude” refers to a sight of any celestial body when:
• What is the time of a Time Zone called?
• If we get a Sun’s declination of 23º 27’ S we will be sailing in the month of:
• In sextant sights, which error refers to the angle between the sea horizon and the apparent horizon?
• The cardinal points North and South lie at the intersections of:
• Uranographic coordinates take as their fundamental reference plane the:
• In the constellation of the Plough or Great Bear (northern celestial hemisphere), the tail points as a reference line to the star:
• The codeclination arc of a star is defined as:
• When we say “first point of Aries”, we are referring to...
• The greatest altitude of a body observable with the sextant occurs at its...