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26 THE IDENTIFICATION OF STARS FOR PRACTICAL NAVIGATION
95. THE NAVIGATIONAL STARS
Before any navigational use can be made of a stellar observation it is necessary to know the name of the star observed in order to be able to find its S.H.A. and Declination from ‘The Nautical Almanac’. The experienced navigator usually has no difficulty in identifying any of the selected navigational stars when the sky is cloudless from his knowledge of the Shape of the constellation of which it forms a part. The magnitude and colour of a star may also assist in its identification. Although it is obviously helpful to be able to recognise the major stars at sight, the novice can take comfort from the fact that it is by no means necessary to be able to do so since there is never any difficulty in identifying any particular star by calculation, with the aid of a star-chart, star globe or planisphere, even though it is the only body visible in the sky.
Of the 57 selected navigational stars on the daily pages of ‘The Nautical Almanac’, only about half of these (the brightest) are commonly used for navigational purposes. These are now listed in alphabetical order together with their constellation designation and magnitude, and brief identification details.
ACHERNAR (Ɣ Eridani, mag.0.6) lies midway between Canopus and Fomalhaut on the line joining them.
ACRUX (Ɣ Crucis, mag. 1.1) southern most star of the Southern Cross, about 30° south of Spica.
ALDEBARAN (Ɣ Tauri, mag. 1.1) about 20°north-east of Orion’s belt, distinguished by a reddish tint and at one extremity
of a pronounced ‘V’.
ALPHERATZ (Ɣ Andromedae, mag.2 .2) connecting star between the ‘square’ of Pegasus and the constellations of Andromeda
And Perseus.
ALTAIR (Ɣ Aquilae, mag 0.9) At the end of a line from Capella through Caph in Cassiopeia. Also lies between two less
bright but prominent stars in a line with Vega.
ANTARES (Ɣ Scorpii mag. 1.0) At the end of a line from Regulus through Spica produced about the same distance as
Spica lies from Regulus, distinguished by its reddish tint and the fact that it lies at the centre of a small bow of stars.
ARCTURUS (Ɣ Bootis, mag. 0.2) One of the brightest stars, found by following the curve of the Great Bear’s tail. BELLATRIX
(ᴕ 0rionis, mag.l.7) The north-east ‘shoulder’ of Orion.
BETELGEUSE (Ɣ Orionis mag, 0.5 -1.1 The north-west ‘shoulder’ of Orion, reddish in colour 1.1)
CANOPUS (Ɣ Carinae, mag.0.9) lies about half-way between Sirius and the south celestial pole and on the line joining
Fomalhaut and Achernar. Second-brightest star in the Sky.
CAPELLA (Ɣ Aurigae, mag. 0.2) forms a rough equilateral triangle with Betelgeuse and Castor and is about 40° due
North of Orion’s Belt.
CASTOR (Ɣ Geminorum, mag. 1.6) At the end of a line from Rigel through the middle star of 0rion’s belt.
DENEB (Ɣ Cygni, mag. 1.3) About 20° north-east of Scheat (β Pegasi) forms the ‘Summer Triangle‘ with Altair and Vega.
ELNATH (β Tauri, mag. 1.8) About 20° north of Bellatrix and half-way between Capella and Orion
FOMAL.H.A.UT (Ɣ Piscis Australia, mag. 1.3) About 45° due south of Markab in Pegasus.
HADAR (β Centauri, mag. 0.9) Second brightest star of the two forming Centaurus and the one nearest to the Southern Cross.
HAMAL (Ɣ Arietus, mag. 2.2) Approximately at the end of a line from Scheat through Alpheratz and about 20° from
Alpheratz.
MARKAB (Ɣ Pegasi, mag. 2.6) South-east corner of square of Pegasus, diagonally opposite Alpheratz
MIRFAK (Ɣ Persei, mag. 1.9) Lies at end of ‘handle’ to the square of Pegasus and midway between Cassiopeia and Capella
POLARIS (Ɣ Ursa Minoris, mag.2.1) The Pole star, at the end of a line from Merak through Dubhe (the “pointers” in the Great Bear)
POLLUX (β Geminorum, mag. 1.2) Close to the other ‘twin’ of Gemini, Castor
PROCYON (Ɣ Canis Minoris, mag.0.S) Lies to the west of 0rion’s shoulders and forms an equilateral triangle with Betelgeuse And Sirius
REGULUS (Ɣ Leonis, mag. 1.3) About 60° from Betelgeuse on a line through Betelgeuse from Bellatrix
RIGEL (β Orionis, mag. 0.3) The south-east ‘foot’ of Orion
RIGIL KENTARUS (Ɣ Centauri, mag. 0.1) The most westerly of the two stars forming Centaurus, about 10° west of the Southern Cross and on the line joining Antares and Canopus
SIRIUS (Ɣ Canis Majoris, mag.1.6) The brightest star in the heavens, to the south-east of and approximately in a line with 0rion’s Belt
SPICA (Ɣ Virginis, mag. 1.2) at the end of arc formed by the tail of the Great Bear through Arcturus, which lies about
midway between the tail and Spica
VEGA (Ɣ Lyrae, mag. 0.1) Found by extending the line joining Capella to Polaris for an equal distance on the opposite side of the Pole. Near Vega is a district ‘W’ of small stars.
96. THE IDENTIFICATION OF STARS
In the practice of navigation, star sights are usually taken at morning and evening twilight, when the horizon and only a few bright stars are visible at the same time. This means that there is usually no “background” of constellations to assist the navigator in star identification, and he must therefore employ other methods. The identity of a star is known when its sidereal hour angle and declination are known, and the observer’s task is to find these quantities from the star’s bearing and altitude.
In fig. 26.1, for example, it is required to deduce ♈ K (the S.H.A.) and KX (the declination) from AX (the altitude), the angle PZX (the azimuth) and the angle ZPX (which is the L.H.A. found from the deck watch or chronometer time).
Since ♈PK = QK – Q♈ = L.H.A. – L.H.A.♈ the first step is to find the local hour angle of Aries. The second step is to find a quick solution to the triangle PZX to determine PZ and therefore KX, the declination.
This is most easily done with the aid of a star globe, star chart, planisphere or star identifier, but a simple computation using the star’s approximate altitude and azimuth and the observer’s latitude, or a graphical solution by projection will assist a navigator in star identification when the more sophisticated devices are not available.
In the practice of navigation, star sights are usually taken at morning and evening twilight, when the horizon and only a few bright stars are visible at the same time. This means that there is usually no “background” of constellations to assist the navigator in star identification, and he must therefore employ other methods. The identity of a star is known when its sidereal hour angle and declination are known, and the observer’s task is to find these quantities from the star’s bearing and altitude.
In fig. 26.1, for example, it is required to deduce ♈ K (the S.H.A.) and KX (the declination) from AX (the altitude), the angle PZX (the azimuth) and the angle ZPX (which is the L.H.A. found from the deck watch or chronometer time).
Since ♈PK = QK – Q♈ = L.H.A. – L.H.A.♈ the first step is to find the local hour angle of Aries. The second step is to find a quick solution to the triangle PZX to determine PZ and therefore KX, the declination.
This is most easily done with the aid of a star globe, star chart, planisphere or star identifier, but a simple computation using the star’s approximate altitude and azimuth and the observer’s latitude, or a graphical solution by projection will assist a navigator in star identification when the more sophisticated devices are not available.
97. STAR IDENTIFICATION WITH A STAR GLOBE
A Star Globe provides by far the best method of identifying a star. It consists of a globe, usually about eight inches in diameter, marked with the navigational stars in their correct relative positions in the celestial sphere, and with a graticule formed by parallels of declination and hour circles. The celestial equator and the ecliptic are also marked on the globe, while the celestial meridians (hour circles) are marked with their angular distance from the First Point of Aries, thus providing a scale of Sidereal Hour Angle. The globe is housed in a box and may be rotated within a brass meridian ring which lies on a vertical plane, and which is graduated in degrees. The lower hemisphere of the globe lies within the box and is invisible. The top surface of the box within which the globe is housed is also graduated in degrees, ad the intersection of this surface with the globe represents the observer’s horizon. In some instruments (but not all), a hemispherical cage can be fitted over the globe, the arms of which are graduated in degrees and correspond to circles of altitude. The observer’s zenith lies at their point of intersection. The star globe is used as follows:-
- Set the meridian ring to the latitude by making the elevation of the pole equal to the observer’s latitude.
- Revolve the globe within the ring until the meridian marked with the local hour angle of Aries appears under the meridian ring (the L.H.A.♈ is calculated from the G.M.T. and the longitude in order to do this)
- Turn the brass cage until one of the altitude circles lies along the observed true bearing of the body to be identified, then move the small pointer along (the altitude circle to the observed altitude.
The pointer will now indicate the star which has been observed. In those instruments, without a cage, the bearing and altitude can usually be estimated with sufficient accuracy by the eye to identify the correct star from the globe. If no star is indicated in the observed position this would immediately suggest that the body observed was a planet, in which case note its S.H.A. in degrees along the scale at the celestial equator and estimate its declination by eye. Inspection of the daily planet columns in ‘The Nautical Almanac’ will show which planet has been observed (the S.H.A. Planets are given under the selected stars list).
98. STAR IDENTIFICATION WITH A PLANISPHERE OR STAR IDENTIFIER
A useful instrument which serves the same function as, but replaces the more costly star globe, is a star identifier or planisphere. A very popular star identifier (available from most Admiralty Chart Agents) is that invented by Captain G. T. Rude of the U.S. Coast Guard. The Rude Star Finder consists of a thin white plastic disc about eight inches in diameter and having a small pin at its centre. Navigational stars having north declination are marked on one side of this disc, the centre of which represents the north celestial pole. The other side of the disc is marked with the navigational stars of the southern celestial hemisphere. The circumferential edge of this disc is graduated in degrees of L.H.A. Aries from 0° to 359°.
Supplied with the white opaque base plate, as the disc is called, is a series of transparent discs each having the same dimension as the base plate so that they can be fitted over the central pin of the base plate. Each of nine transparent discs is marked with families of altitude and azimuth curves covering a range of 10° of latitude. A tenth transparent disc is printed in red with concentric circles representing parallels of declination and intersecting diametrical lines representing hour circles. The ‘Rude Star Finder’ is used as follows:-
- Select the transparent disc nearest to the observer’s latitude and fit this over the base plate.
- Calculate the L.H.A.♈ by applying the observer’s longitude to the G.H.A.♈ extracted from the ‘Nautical Almanac’.
- Orientate the transparent disc so that the north-south azimuth line on the transparent disc coincides with the L.H.A. of Aries on the base plate.
- Using the curves appropriate to the altitude and azimuth of the observed body, the star to be identified will be indicated on the base plate. The red printed template can be used for plotting the declinations and S.H.A.‘s of the navigational planets to enable the observer to estimate their altitudes and azimuths at any given time.
A similar planisphere is published by the British Hydrographer of the Navy under the title ‘Star Finder and Identifier N.P. 323′ consisting of a cardboard base plate plus eight transparent templates. It is used in exactly the same way as the ‘Rude Star Finder‘, an is also available from Admiralty Chart Agents.
A useful instrument which serves the same function as, but replaces the more costly star globe, is a star identifier or planisphere. A very popular star identifier (available from most Admiralty Chart Agents) is that invented by Captain G. T. Rude of the U.S. Coast Guard. The Rude Star Finder consists of a thin white plastic disc about eight inches in diameter and having a small pin at its centre. Navigational stars having north declination are marked on one side of this disc, the centre of which represents the north celestial pole. The other side of the disc is marked with the navigational stars of the southern celestial hemisphere. The circumferential edge of this disc is graduated in degrees of L.H.A. Aries from 0° to 359°.
Supplied with the white opaque base plate, as the disc is called, is a series of transparent discs each having the same dimension as the base plate so that they can be fitted over the central pin of the base plate. Each of nine transparent discs is marked with families of altitude and azimuth curves covering a range of 10° of latitude. A tenth transparent disc is printed in red with concentric circles representing parallels of declination and intersecting diametrical lines representing hour circles. The ‘Rude Star Finder’ is used as follows:-
Select the transparent disc nearest to the observer’s latitude and fit this over the base plate.
Calculate the L.H.A.♈ by applying the observer’s longitude to the G.H.A.♈ extracted from the ‘Nautical Almanac’.
Orientate the transparent disc so that the north-south azimuth line on the transparent disc coincides with the L.H.A. of Aries on the base plate.
Using the curves appropriate to the altitude and azimuth of the observed body, the star to be identified will be indicated on the base plate. The red printed template can be used for plotting the declinations and S.H.A.‘s of the navigational planets to enable the observer to estimate their altitudes and azimuths at any given time.
A similar planisphere is published by the British Hydrographer of the Navy under the title ‘Star Finder and Identifier N.P. 323′ consisting of a cardboard base plate plus eight transparent templates. It is used in exactly the same way as the ‘Rude Star Finder‘, an is also available from Admiralty Chart Agents.
99. STAR IDENTIFICATION WITH A NAUTICAL STAR CHART.
A rough and ready method of identifying stars is provided with ‘Brown’s Nautical Star Chart‘ supplied with the Course. To identify a particular star it is only necessary to plot the observer’s position on the chart using the latitude and, instead of the Longitude, the local hour angle of Aries (calculated as previously shown and measured on the scale along the bottom of the chart. From this position and estimating by eye the approximate bearing of the unidentified body the most likely star will be indicated (it is not necessary to use a protractor or parallel rule because on a Mercator chart of such small scale azimuths cannot be accurately measured).
Next look up the declination of the chosen star in ‘The Nautical Almanac‘ and find the-mean or mid-latitude‘ between the observer’s latitude and the star’s declination. Using this mid-latitude‘ and the scale of altitude in the bottom right-hand corner of the Star Chart, and measuring from the right hand side of the scale, set a pair of dividers to the approximate observed altitude of the unidentified body.
Finally, with one point of the dividers on the plotted observer‘s position, swing the other point to the approximate azimuth. This point should now coincide with the previously selected star and confirm the identification.
If it does not, another star on the sane bearing should be chosen and the process repeated until the identification is confirmed. If a star cannot be identified it is most likely to be a planet. The most likely planet can be selected by inspection of the daily page of ‘The Nautical Almanac in conjunction with the Planet Notes and Planet Diagram (see §4). The position of a planet can be plotted on the Star Chart by using the planet’s declination for latitude and (36O°- S.H.A. Planet) for longitude along the L.H.A. scale on the star chart, the S.H.A. of the planet being found at the foot of the list of selected stars on the daily pages of ‘The Nautical Almanac‘.
On the star chart, plot the observer’s position at Lat. 40° N., L.H.A♈ 226°.
From this position, it will be seen that there are two possible bodies on the approximate bearing 205°, these being Arcturus and Spica. From ‘The Nautical Almanac‘ it will be seen that Arcturus has an approximate declination of 19° N., while Spica has an approximate declination of 11°S. The ‘mean lat.‘ between the observer‘s lat. and the Dec. of Arcturus would be 30° N.,’and between the observer’s lat. and the Dec. of Spica would be 14½° N.
Using the ‘Scale of Altitude‘ on the star chart and a ‘mid-lat.‘ of 30° the distance measured between the observer’s position and Arcturus would give an altitude of 66° therefore this cannot be the body observed. However, transferring the distance between the observer’s position and Spica to the Scale of Altitude on a ‘mid-lat‘ of 14½° gives an altitude of 31°, therefore the body observed must be the star Spica.
100. STAR IDENTIFICATION BY GRAPHICAL PROJECTION
It has been shown above that extreme accuracy is not necessary for identifying celestial bodies, because if it should happen that several stars are close together, the one observed will most certainly be the one of greatest magnitude. An ordinary diagram on the plane of the horizon may therefore be used to give the required arguments.
Referring to fig. 26-2, WNES is the horizon and Z the observer’s’ zenith. Q is placed so that if ZS is 90 units ZQ is SO units (corresponding to the latitude, 50°N.) PZ must therefore be 40 units. A circle drawn through q WQE is taken as the celestial equator and divided into 360 spaces representing degrees of sidereal hour angle, starting from Q on the observer’s meridian. QW, QE, Q’W and Q’E each represent angular distances of 90°.
Between Q and W the spaces are equal, but between W and Q’ they increase in length as shown, because the arc WQ’E is really below the plane of the horizon and in the figure it is ‘pulled out‘ in order to show the position of ♈. In fig. 26-2, the celestial equator is shown divided into arcs of 15° and ♈ is marked so that its local hour angle, measured westward from the meridian is 183°, as calculated for the given time above.
From Z, the body’s bearing ZA is drawn, the angle PZA being equal to 130. X is placed so that XA is 51 units (corresponding to the altitude of the observed body X), and XZ (the zenith distance) is 39, making 90 units for AZ. PX is drawn as a curved line and produced to cut the celestial equator at right angles at K. Since KP is 90°, XX (the declination) is therefore about 2O° N., while ♈K (the sidereal hour angle, measured clockwise) is about 147°. Examination of the selected list of stars on the daily page of ‘The Nautical Almanac’ for 20th January shows that the star Arcturus, with a declination of 19°20.3 N. and an S.H.A. of 146°26′.3 must be the observed body.
101. STAR IDENT. BY SIGHT REDUCTION TABLES FOR MARINE NAVIGATION N.P.401 (H.D. 605)
Sight Reduction Tables NP 401 may be use or star identification through an exchange of arguments. The procedure consists of entering the tables with the observers latitude but using the observed bearing as L.H.A. and the observed altitude as declination, and extracting from the tables, for these arguments, the altitude and azimuth respondents; the extracted altitude becomes the body’s declination, the extracted azimuth the body’s local hour angle. The tables should always be entered with an integral declination of the same name, nearest the observed altitude, although the respondents may come from either same or contrary name areas; in the latter case, the required declination is of contrary same. The star’s S.H.A. is found from S.H.A.* = L.H.A.* – L.H.A.♈
This method is fully described with examples and solutions on introductory pages xxiv and xxv of the tables, but we will describe the method using the data, for EXAMPLE, No.2 above, i.e. Lat. 50° N., Long 44° W, observed body’s altitude 51° and bearing 130°, L.H.A.♈ 183°.
The tables must be entered with the observed bearing as L.H.A., so L.H.A. 130° is found in the first half of the Tables. The observed altitude must be used for declination, so running up the Lat. 50° column, opposite 51° we find 19° in the HC column and this is the declination of the required body.
We also find 31° in the Z column and this, taken from 360° in accordance with the azimuth ‘rules’ printed at the top of the page, give the body’s L.H.A. as 329°. Since S.H.A.* = L.H.A.* – L.H.A.♈, the body’s S.H.A. must be 329° – 183° = 146°. As above, an inspection of the list of selected stars in ‘The Nautical Almanac‘ shows Arcturus to be the body with S.H.A. 146° and Declination 19° N.
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