Celestial Navigation
   HOME

TheInfoList



OR:

Celestial navigation, also known as astronavigation, is the practice of
position fixing Geopositioning, also known as geotracking, geolocalization, geolocating, geolocation, or geoposition fixing, is the process of determining or estimating the geographic position of an object. Geopositioning yields a set of geographic coordinates ...
using stars and other celestial bodies that enables a
navigator A navigator is the person on board a ship or aircraft responsible for its navigation.Grierson, MikeAviation History—Demise of the Flight Navigator FrancoFlyers.org website, October 14, 2008. Retrieved August 31, 2014. The navigator's primar ...
to accurately determine their actual current physical position in space (or on the surface of the Earth) without having to rely solely on estimated positional calculations, commonly known as "
dead reckoning In navigation, dead reckoning is the process of calculating current position of some moving object by using a previously determined position, or fix, and then incorporating estimates of speed, heading direction, and course over elapsed time. ...
", made in the absence of
satellite navigation A satellite navigation or satnav system is a system that uses satellites to provide autonomous geo-spatial positioning. It allows satellite navigation devices to determine their location (longitude, latitude, and altitude/elevation) to high pr ...
or other similar modern electronic or digital positioning means. Celestial
navigation Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation, ...
uses "sights", or timed angular measurements, taken typically between a celestial body (e.g. the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
, the
Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
, a
planet A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
, or a star) and the visible horizon. Celestial navigation can also take advantage of measurements between celestial bodies without reference to the Earth horizon, such as when the Moon and other selected bodies are used in the practice called "lunars" or
lunar distance method Lunar most commonly means "of or relating to the Moon". Lunar may also refer to: Arts and entertainment * ''Lunar'' (series), a series of video games * "Lunar" (song), by David Guetta * "Lunar", a song by Priestess from the 2009 album ''Prior t ...
, used for determining precise time when time is unknown. Celestial navigation by taking sights of the Sun and the horizon whilst on the surface of the Earth is commonly used, providing various methods of determining position, one of which is the popular and simple method called "noon sight navigation"—being a single observation of the exact altitude of the Sun and the exact time of that altitude (known as "local noon")—the highest point of the Sun above the horizon from the position of the observer in any single day. This angular observation combined with knowing its simultaneous precise time referred to the time at the prime meridian directly renders a latitude and longitude fix at the time and place of the observation by simple mathematical reduction. The Moon, a planet, Polaris, or one of the 57 other
navigational stars Fifty-seven navigational stars and additionally the star Polaris are given a special status in the field of celestial navigation. Of the approximately 6,000 stars visible to the naked eye under optimal conditions, these selected stars are ...
whose coordinates are tabulated in any of the published
nautical almanac A nautical almanac is a publication describing the positions of a selection of celestial bodies for the purpose of enabling navigators to use celestial navigation to determine the position of their ship while at sea. The Almanac specifies for eac ...
or air
almanac An almanac (also spelled ''almanack'' and ''almanach'') is an annual publication listing a set of current information about one or multiple subjects. It includes information like weather forecasts, farmers' planting dates, tide tables, and othe ...
s can also accomplish this same goal. Celestial navigation accomplishes its purpose by use of angular measurements (sights) between celestial bodies and the visible horizon to locate one's position on the Earth, whether on land, in the air or at sea. In addition, observations between stars and other celestial bodies accomplished the same results whilst in spaceand was used extensively in the Apollo space program and is still used on many contemporary satellites. Equally, celestial navigation may be used whilst on other planetary bodies to determine position on their surface, using their local horizon and suitable celestial bodies with matching reduction tables and knowledge of local time. For navigation by celestial means when on the surface of the Earth for any given instant in time a celestial body is located directly over a single point on the Earth's surface. The
latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...
and
longitude Longitude (, ) is a geographic coordinate that specifies the east– west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek lette ...
of that point is known as the celestial body's
geographic position The geographic coordinate system (GCS) is a spherical or ellipsoidal coordinate system for measuring and communicating positions directly on the Earth as latitude and longitude. It is the simplest, oldest and most widely used of the various ...
(GP), the location of which can be determined from tables in the nautical or air almanac for that year. The measured angle between the celestial body and the visible horizon is directly related to the distance between the celestial body's GP and the observer's position. After some computations, referred to as
sight reduction In astronavigation, sight reduction is the process of deriving from a sight, (in celestial navigation usually obtained using a sextant), the information needed for establishing a line of position, generally by intercept method. Sight is defined ...
, this measurement is used to plot a
line of position A position line or line of position (LOP) is a line (or, on the surface of the earth, a curve) that can be both identified on a chart (nautical chart or aeronautical chart) and translated to the surface of the earth. The intersection of a minimum o ...
(LOP) on a
navigational chart A nautical chart is a graphic representation of a sea area and adjacent coastal regions. Depending on the scale of the chart, it may show depths of water and heights of land (topographic map), natural features of the seabed, details of the coa ...
or plotting worksheet, the observer's position being somewhere on that line. (The LOP is actually a short segment of a very large circle on Earth that surrounds the GP of the observed celestial body. An observer located anywhere on the circumference of this circle on Earth, measuring the angle of the same celestial body above the horizon at that instant of time, would observe that body to be at the same angle above the horizon.) Sights on two celestial bodies give two such lines on the chart, intersecting at the observer's position (actually, the two circles would result in two points of intersection arising from sights on two stars described above, but one can be discarded, since it will be far from the estimated position—see the figure at example below). Most navigators will use sights of three to five stars, if available, since that will result in only one common intersection and minimizes the chance of error. That premise is the basis for the most commonly used method of celestial navigation, referred to as the "altitude-intercept method". At least three points must be plotted. The plot intersection will usually provide a triangle where the exact position is inside of it. Accuracy of the sights is indicated by the size of the triangle. Joshua Slocum used both noon sight and star sight navigation to determine his current position during his voyage, the first recorded single-handed circumnavigation of the world. In addition he used the
lunar distance method Lunar most commonly means "of or relating to the Moon". Lunar may also refer to: Arts and entertainment * ''Lunar'' (series), a series of video games * "Lunar" (song), by David Guetta * "Lunar", a song by Priestess from the 2009 album ''Prior t ...
(or "lunars") to determine and maintain known time at Greenwich (the prime meridian), thereby keeping his "tin clock" reasonably accurate, and therefore his position fixes accurate. Celestial navigation can only determine
longitude Longitude (, ) is a geographic coordinate that specifies the east– west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek lette ...
when time at the
prime meridian A prime meridian is an arbitrary meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°. Together, a prime meridian and its anti-meridian (the 180th meridian in a 360°-system) form a great ...
is accurately known. The more accurately time at the prime meridian (0° longitude) is known, the more accurate the fix indeed, every four seconds of time source (commonly a chronometer or, in aircraft, an accurate "
hack watch A hack watch is a mechanical watch whose movement offers a mechanism for stopping and setting the seconds hand of the watch, then restarting the watch the instant the time setting matches the time displayed by a reference timepiece. Hack watches ...
") error can lead to a positional error of one nautical mile. When time is unknown or not trusted, the
lunar distance method Lunar most commonly means "of or relating to the Moon". Lunar may also refer to: Arts and entertainment * ''Lunar'' (series), a series of video games * "Lunar" (song), by David Guetta * "Lunar", a song by Priestess from the 2009 album ''Prior t ...
can be used as a method of determining time at the prime meridian. A functioning time piece with a second hand or digit, an almanac with lunar corrections and a sextant are used. From no knowledge of time at all, a lunar calculation (given an observable Moon of respectable altitude) can provide time accurate to within a second or two with about 15 to 30 minutes of observations and mathematical reduction from the almanac tables. After practice, an observer can regularly derive and prove time using this method to within about one second, or one nautical mile of navigational error due to errors ascribed to the time source.


Example

An example illustrating the concept behind the
intercept method In astronomical navigation, the intercept method, also known as Marcq St. Hilaire method, is a method of calculating an observer's position on earth (geopositioning). It was originally called the ''azimuth intercept'' method because the process inv ...
for determining one's position is shown to the right. (Two other common methods for determining one's position using celestial navigation are the
longitude by chronometer Longitude by chronometer is a method, in navigation, of determining longitude using a marine chronometer, which was developed by John Harrison during the first half of the eighteenth century. It is an astronomical method of calculating the long ...
and
ex-meridian Ex-meridian is a celestial navigation method of calculating an observer's position on Earth. The method gives the observer a position line on which the observer is situated. It is usually used when the Sun is obscured at noon, and as a result, a m ...
methods.) In the adjacent image, the two circles on the map represent lines of position for the Sun and Moon at 12:00 
GMT Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon; as a cons ...
on October 29, 2005. At this time, a navigator on a ship at sea measured the Moon to be 56° above the horizon using a sextant. Ten minutes later, the Sun was observed to be 40° above the horizon. Lines of position were then calculated and plotted for each of these observations. Since both the Sun and Moon were observed at their respective angles from the same location, the navigator would have to be located at one of the two locations where the circles cross. In this case, the navigator is either located on the Atlantic Ocean, about west of Madeira, or in South America, about southwest of
Asunción Asunción (, , , Guarani: Paraguay) is the capital and the largest city of Paraguay. The city stands on the eastern bank of the Paraguay River, almost at the confluence of this river with the Pilcomayo River. The Paraguay River and the Bay o ...
, Paraguay. In most cases, determining which of the two intersections is the correct one is obvious to the observer because they are often thousands of miles apart. As it is unlikely that the ship is sailing across South America, the position in the Atlantic is the correct one. Note that the lines of position in the figure are distorted because of the map's projection; they would be circular if plotted on a globe. An observer at the
Gran Chaco The Gran Chaco or Dry Chaco is a sparsely populated, hot and semiarid lowland natural region of the Río de la Plata basin, divided among eastern Bolivia, western Paraguay, northern Argentina, and a portion of the Brazilian states of Mato ...
point would see the Moon at the left of the Sun, and an observer in the Madeira point would see the Moon at the right of the Sun.


Angular measurement

Accurate angle measurement evolved over the years. One simple method is to hold the hand above the horizon with one's arm stretched out. The angular width of the little finger is just over 1.5 degrees elevation at extended arm's length and can be used to estimate the elevation of the Sun from the horizon plane and therefore estimate the time until sunset. The need for more accurate measurements led to the development of a number of increasingly accurate instruments, including the kamal, astrolabe, octant and sextant. The sextant and octant are most accurate because they measure angles from the horizon, eliminating errors caused by the placement of an instrument's pointers, and because their dual-mirror system cancels relative motions of the instrument, showing a steady view of the object and horizon. Navigators measure distance on the globe in degrees,
arcminute A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The n ...
s and arcseconds. A nautical mile is defined as 1,852 meters, but is also (not accidentally) one arcminute of angle along a meridian on the Earth. Sextants can be read accurately to within 0.1 arcminutes, so the observer's position can be determined within (theoretically) , or about 203 yards. Most ocean navigators, measuring from a moving platform under fair conditions, can achieve a practical accuracy of approximately , enough to navigate safely when out of sight of land or other hazards.


Practical navigation

Omega_Marine_Chronometer#4.19_MHz_Ships_Marine_Chronometer, Ships Marine Chronometer giving an accuracy of less than ±5 seconds per year, French Navy issued, 1980 image:180423-N-DL434-149 (27894845758).jpg, Quartermaster#U.S._Navy, U.S. Navy quartermaster 3rd class, practices using a sextant as part of a navigation training aboard the amphibious assault ship USS Bonhomme Richard (LHD 6), 2018. Practical celestial navigation usually requires a marine chronometer to measure time, a sextant to measure the angles, an
almanac An almanac (also spelled ''almanack'' and ''almanach'') is an annual publication listing a set of current information about one or multiple subjects. It includes information like weather forecasts, farmers' planting dates, tide tables, and othe ...
giving schedules of the coordinates of celestial objects, a set of sight reduction tables to help perform the height and
azimuth An azimuth (; from ar, اَلسُّمُوت, as-sumūt, the directions) is an angular measurement in a spherical coordinate system. More specifically, it is the horizontal angle from a cardinal direction, most commonly north. Mathematical ...
computations, and a chart of the region. With sight reduction tables, the only calculations required are addition and subtraction. Small handheld computers, laptops and even scientific calculators enable modern navigators to "reduce" sextant sights in minutes, by automating all the calculation and/or data lookup steps. Most people can master simpler celestial navigation procedures after a day or two of instruction and practice, even using manual calculation methods. Modern practical navigators usually use celestial navigation in combination with
satellite navigation A satellite navigation or satnav system is a system that uses satellites to provide autonomous geo-spatial positioning. It allows satellite navigation devices to determine their location (longitude, latitude, and altitude/elevation) to high pr ...
to correct a
dead reckoning In navigation, dead reckoning is the process of calculating current position of some moving object by using a previously determined position, or fix, and then incorporating estimates of speed, heading direction, and course over elapsed time. ...
track, that is, a course estimated from a vessel's position, course and speed. Using multiple methods helps the navigator detect errors and simplifies procedures. When used this way, a navigator from time to time measures the Sun's altitude with a sextant, then compares that with a precalculated altitude based on the exact time and estimated position of the observation. On the chart, one uses the straight edge of a plotter to mark each position line. If the position line indicates a location more than a few miles from the estimated position, more observations can be taken to restart the dead-reckoning track. In the event of equipment or electrical failure, taking Sun lines a few times a day and advancing them by dead reckoning allows a vessel to get a crude running fix sufficient to return to port. One can also use the Moon, a planet, Polaris, or one of 57 other
navigational stars Fifty-seven navigational stars and additionally the star Polaris are given a special status in the field of celestial navigation. Of the approximately 6,000 stars visible to the naked eye under optimal conditions, these selected stars are ...
to track celestial positioning.


Latitude

Latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...
was measured in the past either by measuring the altitude of the Sun at noon (the "noon sight") or by measuring the altitudes of any other celestial body when crossing the meridian (reaching its maximum altitude when due north or south), and frequently by measuring the altitude of Polaris, the north star (assuming it is sufficiently visible above the horizon, which it is not in the Southern Hemisphere). Polaris always stays within 1 degree of the
celestial north pole The north and south celestial poles are the two points in the sky where Earth's rotation around a fixed axis, axis of rotation, indefinitely extended, intersects the celestial sphere. The north and south celestial poles appear permanently dire ...
. If a navigator measures the angle to Polaris and finds it to be 10 degrees from the horizon, then he is about 10 degrees north of the equator. This approximate latitude is then corrected using simple tables or almanac corrections to determine a latitude theoretically accurate to within a fraction of a mile. Angles are measured from the horizon because locating the point directly overhead, the
zenith The zenith (, ) is an imaginary point directly "above" a particular location, on the celestial sphere. "Above" means in the vertical direction ( plumb line) opposite to the gravity direction at that location ( nadir). The zenith is the "high ...
, is not normally possible. When haze obscures the horizon, navigators use artificial horizons, which are horizontal mirrors or pans of reflective fluid, especially mercury historically. In the latter case, the angle between the reflected image in the mirror and the actual image of the object in the sky is exactly twice the required altitude.


Longitude

If the angle to Polaris can be accurately measured, a similar measurement to a star near the eastern or western horizons would provide the
longitude Longitude (, ) is a geographic coordinate that specifies the east– west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek lette ...
. The problem is that the Earth turns 15 degrees per hour, making such measurements dependent on time. A measure a few minutes before or after the same measure the day before creates serious navigation errors. Before good chronometers were available, longitude measurements were based on the transit of the Moon or the positions of the
moons of Jupiter There are 82 known moons of Jupiter, not counting a number of moonlets likely shed from the inner moons. All together, they form a satellite system which is called the Jovian system. The most massive of the moons are the four Galilean moons: ...
. For the most part, these were too difficult to be used by anyone except professional astronomers. The invention of the modern chronometer by
John Harrison John Harrison ( – 24 March 1776) was a self-educated English carpenter and clockmaker who invented the marine chronometer, a long-sought-after device for solving the problem of calculating longitude while at sea. Harrison's solution revol ...
in 1761 vastly simplified longitudinal calculation. The
longitude problem The history of longitude describes the centuries-long effort by astronomers, cartographers and navigators to discover a means of determining the longitude of any given place on Earth. The measurement of longitude is important to both cartography ...
took centuries to solve and was dependent on the construction of a non-pendulum clock (as pendulum clocks cannot function accurately on a tilting ship, or indeed a moving vehicle of any kind). Two useful methods evolved during the 18th century and are still practiced today: lunar distance, which does not involve the use of a chronometer, and use of an accurate timepiece or chronometer. Presently, lay-person calculations of longitude can be made by noting the exact local time (leaving out any reference for
daylight saving time Daylight saving time (DST), also referred to as daylight savings time or simply daylight time (United States, Canada, and Australia), and summer time (United Kingdom, European Union, and others), is the practice of advancing clocks (typicall ...
) when the Sun is at its highest point in the sky. The calculation of noon can be made more easily and accurately with a small, exactly vertical rod driven into level ground—take the time reading when the shadow is pointing due north (in the northern hemisphere). Then take your local time reading and subtract it from GMT (
Greenwich Mean Time Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon; as a con ...
) or the time in London, England. For example, a noon reading (12:00) near central Canada or the US would occur at approximately 6 pm (18:00) in London. The 6-hour difference is one quarter of a 24-hour day, or 90 degrees of a 360-degree circle (the Earth). The calculation can also be made by taking the number of hours (use decimals for fractions of an hour) multiplied by 15, the number of degrees in one hour. Either way, it can be demonstrated that much of central North America is at or near 90 degrees west longitude. Eastern longitudes can be determined by adding the local time to GMT, with similar calculations.


Lunar distance

An older but still useful and practical method of determining accurate time at sea before the advent of precise timekeeping and satellite-based time systems is called "lunar distances", or "lunars", which was used extensively for a short period and refined for daily use on board ships in the 18th century. Use declined through the middle of the 19th century, as better and better timepieces (chronometers) became available to the average vessel at sea. Although most recently only used by sextant hobbyists and historians, it is now becoming more common in celestial navigation courses to reduce total dependence on
GNSS A satellite navigation or satnav system is a system that uses satellites to provide autonomous geo-spatial positioning. It allows satellite navigation devices to determine their location (longitude, latitude, and altitude/elevation) to high pr ...
systems as potentially the only accurate time source aboard a vessel. Destined for use when an accurate timepiece is not available or timepiece accuracy is suspect during a long sea voyage, the navigator precisely measures the angle between the Moon and the Sun, or between the Moon and one of several stars near the
ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic agains ...
. The observed angle must be corrected for the effects of refraction and parallax, like any celestial sight. To make this correction, the navigator measures the altitudes of the Moon and Sun (or star) at about the same time as the lunar distance angle. Only rough values for the altitudes are required. A calculation with suitable published tables (or longhand with logarithms and graphical tables) requires about 10 to 15 minutes' work converting the observed angle(s) to a geocentric lunar distance. The navigator then compares the corrected angle against those listed against the appropriate almanac pages for every three hours of Greenwich time, using interpolation tables to derive intermediate values. The result is a difference time between the time source (it being of unknown time) used for the observations, and the actual prime meridian time (that of the "Zero Meridian" at Greenwich also known as UTC or GMT). Now knowing UTC/GMT, a further set of sights can be taken and reduced by the navigator to calculate their exact position on the Earth as a local latitude and longitude.


Use of time

The considerably more popular method was (and still is) to use an accurate timepiece to directly measure the time of a sextant sight. The need for accurate navigation led to the development of progressively more accurate chronometers in the 18th century (see
John Harrison John Harrison ( – 24 March 1776) was a self-educated English carpenter and clockmaker who invented the marine chronometer, a long-sought-after device for solving the problem of calculating longitude while at sea. Harrison's solution revol ...
). Today, time is measured with a chronometer, a
quartz watch Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
, a shortwave radio time signal broadcast from an
atomic clock An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwe ...
, or the time displayed on a satellite time signal receiver. A quartz wristwatch normally keeps time within a half-second per day. If it is worn constantly, keeping it near body heat, its rate of drift can be measured with the radio and, by compensating for this drift, a navigator can keep time to better than a second per month. When time at the
prime meridian A prime meridian is an arbitrary meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°. Together, a prime meridian and its anti-meridian (the 180th meridian in a 360°-system) form a great ...
(or another starting point) is accurately enough known, celestial navigation can determine longitude, and the more accurately latitude and time are known, the more accurate is the longitude determination. The angular speed of Earth is latitude-dependent. At the poles, or latitude 90°, the rotation velocity of Earth reaches zero. At latitude 45° one second of time is equivalent in longitude to , or one-tenth of a second means . At the slightly bulged-out equator, or latitude 0°, the rotation velocity of Earth or its equivalent in longitude reaches its maximum at . Traditionally, a navigator checked their chronometer(s) from their sextant, at a geographic marker surveyed by a professional astronomer. This is now a rare skill, and most
harbormaster A harbourmaster (or harbormaster, see spelling differences) is an official responsible for enforcing the regulations of a particular harbour or port, in order to ensure the safety of navigation, the security of the harbour and the correct opera ...
s cannot locate their harbor's marker. Ships often carried more than one chronometer. Chronometers were kept in
gimbals A gimbal is a pivoted support that permits rotation of an object about an axis. A set of three gimbals, one mounted on the other with orthogonal pivot axes, may be used to allow an object mounted on the innermost gimbal to remain independent of ...
in a dry room near the center of the ship. They were used to set a
hack watch A hack watch is a mechanical watch whose movement offers a mechanism for stopping and setting the seconds hand of the watch, then restarting the watch the instant the time setting matches the time displayed by a reference timepiece. Hack watches ...
for the actual sight, so that no chronometers were ever exposed to the wind and salt water on deck. Winding and comparing the chronometers was a crucial duty of the navigator. Even today, it is still logged daily in the ship's deck log and reported to the captain before eight bells on the forenoon watch (shipboard noon). Navigators also set the ship's clocks and calendar. Two chronometers provided
dual modular redundancy In reliability engineering, dual modular redundancy (DMR) is when components of a system are duplicated, providing redundancy in case one should fail. It is particularly applied to systems where the duplicated components work in parallel, particu ...
, allowing a backup if one ceases to work, but not allowing any error correction if the two displayed a different time, since in case of contradiction between the two chronometers, it would be impossible to know which one was wrong (the
error detection In information theory and coding theory with applications in computer science and telecommunication, error detection and correction (EDAC) or error control are techniques that enable reliable delivery of digital data over unreliable communi ...
obtained would be the same of having only one chronometer and checking it periodically: every day at noon against
dead reckoning In navigation, dead reckoning is the process of calculating current position of some moving object by using a previously determined position, or fix, and then incorporating estimates of speed, heading direction, and course over elapsed time. ...
). Three chronometers provided
triple modular redundancy Triple is used in several contexts to mean "threefold" or a " treble": Sports * Triple (baseball), a three-base hit * A basketball three-point field goal * A figure skating jump with three rotations * In bowling terms, three strikes in a row * I ...
, allowing error correction if one of the three was wrong, so the pilot would take the average of the two with closer readings (average precision vote). There is an old adage to this effect, stating: "Never go to sea with two chronometers; take one or three." Vessels engaged in survey work generally carried many more than three chronometers for example, HMS ''Beagle'' carried 22 chronometers.


Modern celestial navigation

The celestial line of position concept was discovered in 1837 by
Thomas Hubbard Sumner Thomas Hubbard Sumner (20 March 1807 – 9 March 1876) was a sea captain during the 19th century. He is best known for developing the celestial navigation method known as the Sumner line or circle of equal altitude. Biography Thomas Hubbard ...
when, after one observation, he computed and plotted his longitude at more than one trial latitude in his vicinity – and noticed that the positions lay along a line. Using this method with two bodies, navigators were finally able to cross two position lines and obtain their position – in effect determining both latitude and longitude. Later in the 19th century came the development of the modern (Marcq St. Hilaire)
intercept method In astronomical navigation, the intercept method, also known as Marcq St. Hilaire method, is a method of calculating an observer's position on earth (geopositioning). It was originally called the ''azimuth intercept'' method because the process inv ...
; with this method the body height and azimuth are calculated for a convenient trial position, and compared with the observed height. The difference in arcminutes is the nautical mile "intercept" distance that the position line needs to be shifted toward or away from the direction of the body's subpoint. (The intercept method uses the concept illustrated in the example in the “How it works” section above.) Two other methods of reducing sights are the
longitude by chronometer Longitude by chronometer is a method, in navigation, of determining longitude using a marine chronometer, which was developed by John Harrison during the first half of the eighteenth century. It is an astronomical method of calculating the long ...
and the
ex-meridian Ex-meridian is a celestial navigation method of calculating an observer's position on Earth. The method gives the observer a position line on which the observer is situated. It is usually used when the Sun is obscured at noon, and as a result, a m ...
method. While celestial navigation is becoming increasingly redundant with the advent of inexpensive and highly accurate satellite navigation receivers (
GNSS A satellite navigation or satnav system is a system that uses satellites to provide autonomous geo-spatial positioning. It allows satellite navigation devices to determine their location (longitude, latitude, and altitude/elevation) to high pr ...
), it was used extensively in aviation until the 1960s, and
marine navigation Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation, ...
until quite recently. However; since a prudent mariner never relies on any sole means of fixing their position, many national maritime authorities still require deck officers to show knowledge of celestial navigation in examinations, primarily as a backup for electronic/satellite navigation. One of the most common current usages of celestial navigation aboard large merchant vessels is for compass calibration and error checking at sea when no terrestrial references are available. In 1980, French Navy regulations still required an independently operated timepiece on board so that, in combination with a sextant, a ship’s position could be determined by celestial navigation. The
U.S. Air Force The United States Air Force (USAF) is the air service branch of the United States Armed Forces, and is one of the eight uniformed services of the United States. Originally created on 1 August 1907, as a part of the United States Army Sign ...
and
U.S. Navy The United States Navy (USN) is the maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage o ...
continued instructing military aviators on celestial navigation use until 1997, because: * celestial navigation can be used independently of ground aids. * celestial navigation has global coverage. * celestial navigation can not be jammed (although it can be obscured by clouds). * celestial navigation does not give off any signals that could be detected by an enemy. The
United States Naval Academy The United States Naval Academy (US Naval Academy, USNA, or Navy) is a federal service academy in Annapolis, Maryland. It was established on 10 October 1845 during the tenure of George Bancroft as Secretary of the Navy. The Naval Academy ...
(USNA) announced that it was discontinuing its course on celestial navigation (considered to be one of its most demanding non-engineering courses) from the formal curriculum in the spring of 1998. In October 2015, citing concerns about the reliability of GNSS systems in the face of potential hostile hacking, the USNA reinstated instruction in celestial navigation in the 2015 to 2016 academic year. At another federal service academy, the US Merchant Marine Academy, there was no break in instruction in celestial navigation as it is required to pass the US Coast Guard License Exam to enter the Merchant Marine. It is also taught at Harvard, most recently as Astronomy 2. Celestial navigation continues to be used by private yachtsmen, and particularly by long-distance cruising yachts around the world. For small cruising boat crews, celestial navigation is generally considered an essential skill when venturing beyond visual range of land. Although
satellite navigation A satellite navigation or satnav system is a system that uses satellites to provide autonomous geo-spatial positioning. It allows satellite navigation devices to determine their location (longitude, latitude, and altitude/elevation) to high pr ...
technology is reliable, offshore yachtsmen use celestial navigation as either a primary navigational tool or as a backup. Celestial navigation was used in commercial aviation up until the early part of the jet age; early Boeing 747s had a "sextant port" in the roof of the cockpit. It was only phased out in the 1960s with the advent of
inertial navigation An inertial navigation system (INS) is a navigation device that uses motion sensors ( accelerometers), rotation sensors ( gyroscopes) and a computer to continuously calculate by dead reckoning the position, the orientation, and the velocity ...
and Doppler navigation systems, and today's satellite-based systems which can locate the aircraft's position accurate to a 3-meter sphere with several updates per second. A variation on terrestrial celestial navigation was used to help orient the
Apollo spacecraft The Apollo spacecraft was composed of three parts designed to accomplish the American Apollo program's goal of landing astronauts on the Moon by the end of the 1960s and returning them safely to Earth. The expendable (single-use) spacecraft ...
en route to and from the Moon. To this day, space missions such as the
Mars Exploration Rover NASA's Mars Exploration Rover (MER) mission was a robotic space mission involving two Mars rovers, '' Spirit'' and '' Opportunity'', exploring the planet Mars. It began in 2003 with the launch of the two rovers to explore the Martian surface ...
use
star tracker A star tracker is an optical device that measures the positions of stars using photocells or a camera. As the positions of many stars have been measured by astronomers to a high degree of accuracy, a star tracker on a satellite or spacecraft may ...
s to determine the
attitude Attitude may refer to: Philosophy and psychology * Attitude (psychology), an individual's predisposed state of mind regarding a value * Metaphysics of presence * Propositional attitude, a relational mental state connecting a person to a pro ...
of the spacecraft. As early as the mid-1960s, advanced electronic and computer systems had evolved enabling navigators to obtain automated celestial sight fixes. These systems were used aboard both ships and US Air Force aircraft, and were highly accurate, able to lock onto up to 11 stars (even in daytime) and resolve the craft's position to less than . The SR-71 high-speed reconnaissance aircraft was one example of an aircraft that used a combination of automated celestial and inertial navigation. These rare systems were expensive, however, and the few that remain in use today are regarded as backups to more reliable satellite positioning systems. Intercontinental ballistic missiles use celestial navigation to check and correct their course (initially set using internal gyroscopes) while flying outside the Earth's atmosphere. The immunity to jamming signals is the main driver behind this seemingly archaic technique. X-ray pulsar-based navigation and timing (XNAV) is an experimental navigation technique whereby the periodic
X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10  picometers to 10  nanometers, corresponding to frequencies in the range 30&nb ...
signals emitted from
pulsars A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward E ...
are used to determine the location of a vehicle, such as a spacecraft in deep space. A vehicle using XNAV would compare received X-ray signals with a database of known pulsar frequencies and locations. Similar to GNSS, this comparison would allow the vehicle to triangulate its position accurately (±5 km). The advantage of using X-ray signals over
radio waves Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz ( GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (s ...
is that
X-ray telescopes An X-ray telescope (XRT) is a telescope that is designed to observe remote objects in the X-ray spectrum. In order to get above the Earth's atmosphere, which is opaque to X-rays, X-ray telescopes must be mounted on high altitude rockets, balloon ...
can be made smaller and lighter. On 9 November 2016 the
Chinese Academy of Sciences The Chinese Academy of Sciences (CAS); ), known by Academia Sinica in English until the 1980s, is the national academy of the People's Republic of China for natural sciences. It has historical origins in the Academia Sinica during the Republi ...
launched an experimental pulsar navigation satellite called
XPNAV 1 X-ray pulsar-based navigation and timing (XNAV) or simply pulsar navigation is a navigation technique whereby the periodic X-ray signals emitted from pulsars are used to determine the location of a vehicle, such as a spacecraft in deep space. A ve ...
. SEXTANT (Station Explorer for X-ray Timing and Navigation Technology) is a
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...
-funded project developed at the Goddard Space Flight Center that is testing XNAV on-orbit on board the
International Space Station The International Space Station (ISS) is the largest modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos (Russia), JAXA ( ...
in connection with the
NICER The Neutron Star Interior Composition ExploreR (NICER) is a NASA telescope on the International Space Station, designed and dedicated to the study of the extraordinary gravitational, electromagnetic, and nuclear physics environments embodied by ...
project, launched on 3 June 2017 on the
SpaceX CRS-11 SpaceX CRS-11, also known as SpX-11, was a Commercial Resupply Service mission to the International Space Station, launched successfully on 3 June 2017. The mission was contracted by NASA and was flown by SpaceX. The mission utilized a Falcon ...
ISS resupply mission.


Training

Celestial navigation training equipment for aircraft crews combine a simple
flight simulator A flight simulator is a device that artificially re-creates aircraft flight and the environment in which it flies, for pilot training, design, or other purposes. It includes replicating the equations that govern how aircraft fly, how they rea ...
with a planetarium. An early example is the Link Celestial Navigation Trainer, used in the
Second World War World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposi ...
. Housed in a high building, it featured a cockpit accommodating a whole
bomber A bomber is a military combat aircraft designed to attack ground and naval targets by dropping air-to-ground weaponry (such as bombs), launching torpedoes, or deploying air-launched cruise missiles. The first use of bombs dropped from an air ...
crew (pilot, navigator, and bombardier). The cockpit offered a full array of
instruments Instrument may refer to: Science and technology * Flight instruments, the devices used to measure the speed, altitude, and pertinent flight angles of various kinds of aircraft * Laboratory equipment, the measuring tools used in a scientific lab ...
which the
pilot An aircraft pilot or aviator is a person who controls the flight of an aircraft by operating its directional flight controls. Some other aircrew members, such as navigators or flight engineers, are also considered aviators, because they a ...
used to fly the simulated airplane. Fixed to a dome above the cockpit was an arrangement of lights, some
collimated A collimated beam of light or other electromagnetic radiation has parallel rays, and therefore will spread minimally as it propagates. A perfectly collimated light beam, with no divergence, would not disperse with distance. However, diffraction p ...
, simulating constellations from which the navigator determined the plane's position. The dome's movement simulated the changing positions of the stars with the passage of time and the movement of the plane around the earth. The navigator also received simulated radio signals from various positions on the ground. Below the cockpit moved "terrain plates" – large, movable aerial photographs of the land below – which gave the crew the impression of flight and enabled the bomber to practice lining up bombing targets. A team of operators sat at a control booth on the ground below the machine, from which they could simulate
weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloud cover, cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmos ...
conditions such as wind or cloud. This team also tracked the airplane's position by moving a "crab" (a marker) on a paper map. The Link Celestial Navigation Trainer was developed in response to a request made by the
Royal Air Force The Royal Air Force (RAF) is the United Kingdom's air and space force. It was formed towards the end of the First World War on 1 April 1918, becoming the first independent air force in the world, by regrouping the Royal Flying Corps (RFC) an ...
(RAF) in 1939. The RAF ordered 60 of these machines, and the first one was built in 1941. The RAF used only a few of these, leasing the rest back to the US, where eventually hundreds were in use.


See also

* Air navigation * Astrodome (aeronautics) *
Astronautics Astronautics (or cosmonautics) is the theory and practice of travel beyond Earth's atmosphere into outer space. Spaceflight is one of its main applications and space science its overarching field. The term ''astronautics'' (originally ''astron ...
*''
Bowditch's American Practical Navigator ''The American Practical Navigator'' (colloquially often referred to as ''Bowditch''), originally written by Nathaniel Bowditch, is an encyclopedia of navigation. It serves as a valuable handbook on oceanography and meteorology, and contains usef ...
'' *
Celestial pole The north and south celestial poles are the two points in the sky where Earth's axis of rotation, indefinitely extended, intersects the celestial sphere. The north and south celestial poles appear permanently directly overhead to observers a ...
*
Circle of equal altitude The circle of equal altitude, also called circle of position (CoP), is defined as the locus (geometry), locus of points on Earth on which an observer sees a celestial object such as the sun or a star, at a given time, with the same observed altitud ...
* Ephemeris *
Geodetic astronomy Geodetic astronomy or astronomical geodesy (astro-geodesy) is the application of astronomical methods into geodetic networks and other technical projects of geodesy. Applications The most important applications are: * Establishment of geodetic d ...
*GNSS
Satellite Navigation A satellite navigation or satnav system is a system that uses satellites to provide autonomous geo-spatial positioning. It allows satellite navigation devices to determine their location (longitude, latitude, and altitude/elevation) to high pr ...
*
History of longitude The history of longitude describes the centuries-long effort by astronomers, cartographers and navigators to discover a means of determining the longitude of any given place on Earth. The measurement of longitude is important to both cartography ...
*
List of proper names of stars These names of stars that have either been approved by the International Astronomical Union or which have been in somewhat recent use. IAU approval comes mostly from its Working Group on Star Names, which has been publishing a "List of IAU-approv ...
*
List of selected stars for navigation Fifty-seven navigational stars and additionally the star Polaris are given a special status in the field of celestial navigation. Of the approximately 6,000 stars visible to the naked eye under optimal conditions, these selected stars are am ...
*
Polar alignment Polar alignment is the act of aligning the rotational axis of a telescope's equatorial mount or a sundial's gnomon with a celestial pole to parallel Earth's axis. Alignment methods The method to use differs depending on whether the alignment ...
*
Polynesian navigation Polynesian navigation or Polynesian wayfinding was used for thousands of years to enable long voyages across thousands of kilometers of the open Pacific Ocean. Polynesians made contact with nearly every island within the vast Polynesian Triangl ...
*
Radio navigation Radio navigation or radionavigation is the application of radio frequencies to determine a position of an object on the Earth, either the vessel or an obstruction. Like radiolocation, it is a type of radiodetermination. The basic principles a ...
* Spherical geometry * Star clock


References


External links


Celestial Navigation Net
* ttp://sv-inua.net/the-nautical-almanac InuaComplete nautical Almanac and more
Calculating Lunar Distances
Almanac, Sight Reduction Tables and more. {{Authority control