TOOLS & EQUIPMENT USED ON THE SHIPS OF OLD: The Mariners Astrolabe and also called the sea astrolabe, was an *1 inclinometer used to determine the latitude of a ship at sea by measuring the sun’s noon altitude (declination) or the *2 meridian altitude of a star of known declinations. Not astrolabe proper, the mariner’s astrolabe was rather a graduated circle with an *3 alidade used to measure vertical angles. They were designed to allow for their use on boats in rough water and/or in heavy winds, which astrolabes are ill equipped to handle. In the sixteenth century, the instrument was also called a ring. Mariner’s astrolabes were made of brass. Since weight was advantageous when using the instrument on the heaving deck of a ship or in high winds, other materials, such as wood or ivory, were not desirable though some wood sea astrolabes were made. Early sea astrolabes were made from sheets of brass. Due to their light weight, they tended to perform poorly at sea. Heavier cast brass frames began to be made in the mid-sixteenth century and were considerably better. As the accuracy of the instrument is related to the radius of the divided circle, these were made as large as practical. Since the large plate form of the *4 planispheric astrolabe makes it sensitive to the wind, the mariner’s astrolabe is made with a frame form. The openings in the frame allow wind to pass through, inducing less motion to the instrument. The essential function of the device was to measure angles. Thus the instrument featured a ring graduated in degrees. Early instruments were all graduated for 90° ; later instruments were graduated for the full 360º circle around the *5 limb. The sole purpose of the spokes was to support the pivot point for the alidade. In order to lower the *6 centre of gravity of the device and thus increase its *7 period of motion as a means of stabilizing it, extra brass was usually added to the bottom of the instrument inside the ring. The alidade was free to rotate about a pin through the centre of the instrument. The vanes of the alidade were either slotted or pierced with a hole to all the user to align the alidade. The astrolabe had a ring attached to the top of the instrument to allow it to hang vertically. The invention of the Cross-staff was inspired by the Kamal used by Arab navigators, which was also known to the Portuguese as the ‘Balestilha of the moors’. The Kamal consisted of a length of cord attached to the centre of a rectangular plaque of wood with knots on the cord that correspond to the latitude of a particular port. The observer will hold onto the appropriate knot to his eye and stretched the plaque he could see the horizon in line with its lower edge. The upper edge will determine if the observer is in the right latitude. Similarly, a Cross-staff (above) was made up of a long staff, usually 36 inches long, and four shorter sliding cross pieces of varying lengths, that can move up and down the longer staff. The four sides of the staff are all graduated differently in degree to suit each cross piece until its upper and lower edge is in line with the *8 celestial body and the horizon respectively. The altitude of the celestial body can then be read off the appropriate scale. Both the Kamal and the Cross-staff can only be used to find altitude of a celestial body. In about 1594 John Davis, an English captain, developed a simple Back-staff which eliminated the problems encountered when using the Cross-staff. Davis’ Back-staff was intended to be an improvement on the Mariners *9 Quadrants, astrolabes and cross-staves. The Davis Back-staff consisted of a graduated staff, a half-cross in the shop of an arc of a circle on the radius of the staff with a fixed vane, a brass horizon vane with a slit in it at the fore-end of the staff. The observer places the staff on his shoulder and stands with his back to the sun. With the horizon vane lined up with the horizon, he slides the half-cross back and forth until the shadow of its vane falls across the slit in the bottom vane while the horizon remains visible through the slit. By doing this, the observer is able to sight both the sun and the horizon while his back is towards the sun. Davis improved upon this design in a model with two half-crosses, which divided an accurate scale into two parts, had the appearance of a large triangle equipped with a 30° arc at one end and a small 60º arc at the other. One scale was engraved on its upper side towards the front of the staff, the other was on its underside and at the back. By Henny Crijns-Coenen...
Posted on: Fri, 05 Sep 2014 16:10:28 +0000
Trending Topics
Recently Viewed Topics
© 2015