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Navigation Instruments


Foul Anchor Chip log and sand glass
    The chip log was a triangular piece of wood weighted with lead on one side so it would float vertically in the water. From the three corners it had three short lines which joined to the main log line which had knots marking set distances.
    The use required two or three men. The log was heaved over the stern and the line started running out through the loose hand of one man. An initial amount of leader line was let out so the chip would settle in the water and far enough astern from the turbulence of the wake. After this the man would feel the first knot go though his hand and would instantly call "mark!" and the man holding the sandglass would overturn it starting the count of time. When the sand had finished falling the man holding the glass would call "mark!" and the man controlling the line would instantly grab it thereby stopping it. The number of knots in the line paid out during the time the sand was falling indicated the speed of the vessel in knots (nautical miles per hour). The line which goes to the upper corner of the chip is held in place by a peg which releases when subjected to a sudden jerk so the chip will now float horizontally in the water and can be easily retrieved.
    By the end of the 19th century patent logs became common which consisted of a small propeller-like screw which was trailed in the water and turned a clockwork-like device, similar to a car's odometer, which recorded instant speed and accumulated distance. One drawback was that sharks would mistake the device for a fish and would attack it so many propellers were lost and spares had to be carried.
Corredera de Barquilla Corredera de Barquilla Ampolleta
Foul Anchor Sounding lead sonda, sounding lead
    The sounding lead was simply a conical-shaped lead weight with a hollow in the flat bottom where
Foul Anchor Magnetic Compass

brújula, compass
    Magnetic needle or compass. ...

Foul Anchor Bearing Compass bearing compass
    The bearing compass is a magnetic compass, similar to the magnetic compass, which allows the measurement of the magnetic bearing (angle as seen by the observer between magnetic north and the observed object or body) and another ship, landmark or any other observable object or body.
    (Spanish: compás de demoras)
Foul Anchor Pelorus pelorus
    Allows the measurement of the relative bearing between the ship's centerline and the observed body or object.
    (Spanish: taxímetro)

Foul Anchor Quadrant (or astrolabe-quadrant) quadrant, cuadrante
    Consists of a graduated quadrant with a bob weight hanging from its center.  One edge is aligned with the observed body and the angle is read off the graduated circle.  It required two people to use: one to observe the celestial body and one to read the angle.
    (Spanish: Cuadrante)
Foul Anchor Astrolabe

astrolabe, astrolabio
    The basic mariner's astrolabe had an arm and was used to measure the height (or zenital angle) of a celestial body, but with time they became very elaborate and became "calculators" and planispheres that could represent the sky at any given date time and latitude.

Another astrolabe page

    (Spanish: astrolabio)

Foul Anchor Annulo or Ring

anullo astronomico
    It is similar to the astrolabe in its principle although it was developed later.  It is simply a ring with a hole through which the sun would shine and produce a dot of light on the inside graduated scale.  It is easier to use than the astrolabe but it can only be used to measure the sun's altitude so it was necessary to have an astrolabe to take sights of the stars.
    The inherent problem with all intruments which are aligned by gravity is their difficult use when there is significant movement of the ship.
    (Spanish: anillo)

Foul Anchor Kamal

    The kamal is thought to be of Arab origin although it is possible that the Arabs got it from India.  It was brought to Europe by the Portuguese who called it "tablilla india" (Indian table or chip) and also "ballestilla mora" (Moorish staff) as it is based on the same geometric principle as the crossstaff.
    It consists of a small board with a small hole in the center through which passes a string with knots.  The observer holds a knot between his teeth which fixes the distance between the board and his eye and so fixes the angle seen by the eye between the edges of the board.  Each knot on the string corresponded with a certain latitude which could be an even number of degrees or with the latitude of a certain port.  The kamal is extremely simple to build and very easy to use, specially in rough seas when the cross-staff or the astrolabe are impossible to use.  On the other hand it has less precission.
    (Spanish: kamal)

Foul Anchor Cross-staff or "Jacob's Staff" ballestilla, cross staff
    The cross-staff was an instrument with a main staff along which a cross-staff could slide and it was calibrated to read angles directly. Normally the instrument came with three cross-staffs of different lengths so the navigator could choose the one most convenient depending on the angle to be measured.
    In some texts I have seen the use of the cross-staff as in the figure on the left which means the eye has to align simultaneously both ends of the cross-staff, one with the horizon and one with the Sun. This is difficult enough in itself and it it compounded by having to look directly into the sun. Maybe the cross-staff was used this way only when taking sights of the moon or stars.
    It seem more likely that, to take sights of the sun, it would be used as can be seen on the figure on the right where the navigator, with his back to the sun, aligns one side of the triangle with the horizon and lets the shade of the other extreme of the cross-staff coincide with the tip.
    Commonly each instrument had a choice of three cross-staffs of different lengths so that the most convenient could be chosen depending on the height of the body to be observed.
    (Spanish: ballestilla, which is diminutive of "ballesta", crossbow, due to the similarity in shape.)
Foul Anchor Backstaff (AKA Davis' quadrant)

cuadrante de Davis, back staff    The backstaff seems like a natural development of the cross-staff. Look through H and A to align with the horizon. Adjust A so the shade or the sun coincides with A. The altitude of the Sun is the sum of HB plus ID. Advantage: the observer did not have to look directly into the sun. He would take the sight with his back to the Sun by aligning the shadow. The precission was not too good anyway. Note: Because we are shighting the opposite horizon the correction for DIP (depression of the horizon), must be *added* to the reading.


Foul Anchor Nocturnal nocturlabio, reloj nocturno, nocturnal
    Translation pending of this section.
    El nocturlabio o nocturnal es un artilugio que sirve para saber la hora mediante la observación de la posición de las estrellas. Tiene un disco exterior, de unos 7 cm de diámetro, fijo a un mango de modo que el usuario sujeta el mango en posición vertical y el disco queda orientado siempre en la misma posición. Este disco lleva grabados alrededor del borde los días y meses del año y lleva otro disco de diámetro alque más pequeño que puede girar alrededor del mismo eje central. Además lleva una alidada que sobresale de los discos y que se hace coincidir con las estrellas elegidas.
    Es de uso relativamente sencillo. El observador gira el disco central hasta que la marca de la estrella elegida quede enfrentada con la fecha del día. Entonces sujeta el nocturnal con el mango en posición vertical y, mirando a la estrella polar por el agujero central del eje del nocturnal, hace girar la alidada hasta que coincida con la estrella. En ese momento puede leer la hora marcada por la alidada en el círculo interior.
    En el hemisferio norte las estrellas aparentan girar alrededor del polo norte sideral con velocidad constante y un período igual a 23 h y 56 minutos aproximadamente. Esto es debido al giro de la tierra alrededor del sol. Mientras que el sol ha girado 365.25 veces alrededor de la tierra en un año, la boveda celeste ha girado 366.25. Es decir, la tierra ha girado sobre sí misma 366.25 veces pero el sol ha perdido un giro debido a la órbita de la tierra alrededor del sol y solo han transcurrido 365.25 dias solares.
    Realmente el cálculo puede hacerse de forma manual con relativa facilidad pero el nocturnal sirve para hacer la observación y el cálculo de la hora simultáneamente.
    Breve explicación del cálculo manual: Dubhe y Merak son las dos estrellas de la Osa Mayor que apuntan directamente a la estrella polar y giran alrededor de la polar en sentido contrario a las agujas del reloj. Llamemos alpha al angulo girado por estas estrellas desde la posición superior (90º si están a la izquierda, 180º si están directamente debajo, 270º a la derecha). Toma alfa (en grados) y restale el número de días transcurridos desde el último 6 de marzo pasado. Divide por 15 para obtener el número de horas transcurridas desde la medianoche pasada.  ¡Así de simple!  Si puedes medir el ángulo alfa con cierta precisión puedes conocer la hora con precisión de algunos minutos. Un error de 5 grados produce un error de 20 minutos de tiempo. Con un astrolabio o círculo graduado puedes medir con una precisión de un grado lo que produce un error no superior a 4 minutos de tiempo.
    He aquí la explicación para aquellos con curiosidad científica: El 6 de marzo Dubhe y Merak cruzan el meridiano por encima de la estrella polar a medianoche y por cada día que pasa lo hará unos 4 minutos antes o, lo que es lo mismo, a medianoche estará 360/365.25 grados adelantada respecto a la noche anterior. Podemos redondear esto y simplemente restar un grado por cada día transcurrido. Para más precisión podemos restar un grado adicional por cada dos meses transcurridos: uno en mayo y junio, dos grados en julio y agosto, tres en septiembre y octubre cuatro en noviembre y diciembre, cinco en enero y febrero. La esfera celeste gira 15.04 grados en una hora de modo que dividimos por 15 para determinar las horas transcurridas desde media noche.
    En esta página puede verse explicado (en inglés) cómo construir un reloj nocturno.
    (Inglés: nocturnal)
Foul Anchor Sextant

    Previously described in principle by Sir Isaac Newton and even foreshadowed by Robert Hooke in 1644, the octant, incorporates the idea of using a mirror to bring a target object into coincidence with another, and then noting the inclination of the mirror, from which a value of the arc between objects could be obtained. A description of this new instrument was first published (1731) in London by the astronomer John Hadley (1682-1744), and about the same time by Thomas Godfrey (1704-1749) in Philadelphia.
    The sextant (shown in diagram) is the same instrument in everything except the arc is 60 degress rather than 45 as in the octant. This allows the measuring of angles greater than 90 and up to 120.
    The angle measured by the arm (blue-blue) is one half of the angle between the horizon (green) and the body (red). But the arc is graduated so the angle can be read directly. Note: the correction for DIP (depression of the horizon), must be *subtracted* from the reading.

This page last updated 2011-06-12