A sextant is a doubly reflecting navigation instrument that measures the angular distance between two visible objects. The primary use of a sextant is to measure. Sextant with beveled scale. Beveled scale sextant Model number: Date manufactured: Jesse Ramsden, London, England, ca Footnotes. An interesting instrument from Ramsden's later period, this is one of the smallest of his known sextants. According to Piazzi.
A sextant is a doubly reflecting navigation instrument that measures the angular distance between two visible objects. The primary use of a sextant is to measure. Footnotes. An interesting instrument from Ramsden's later period, this is one of the smallest of his known sextants. According to Piazzi. Sextant with beveled scale. Beveled scale sextant Model number: Date manufactured: Jesse Ramsden, London, England, ca
Sextant with beveled scale. Beveled scale sextant Model number: Date manufactured: Jesse Ramsden, London, England, ca Footnotes. An interesting instrument from Ramsden's later period, this is one of the smallest of his known sextants. According to Piazzi. Finally, navigators need an angle-measuring instrument, a sextant, .. Here's a small brass sextant that Ramsden made shortly before his death in
The lower part of the index glass is silvered and can be covered by the Maskelyne flap, ramsden the upper part is not silvered. The sextant has three red shades and ramsden red horizon shade. Attached to the sextant sextant a threaded telescope bracket in two parts, fitted for correcting collimation error. It has perpendicular adjustment by ramsden seextant and a sextant knob. The telescope is mm ramsden length with an inverted image, which is possibly not original to this instrument.
The sextsnt has no box. Acquired with the assistance of the National Heritage Memorial Fund. Share your knowledge. Ramsden Encounters Gallery Floor plans. The sextant sextant a polished brass frame and limb sextantt sextant wooden handle. Ramsden, Jesse. National Maritime Museum, Greenwich, London. Sextant NAV Do you know more about this?
To return to homeport, he would sail north or south as needed to bring Polaris to the altitude he'd observed when he left home, then sail down the latitude. Over time, Arab navigators started tying knots in the string at intervals of one issabah. The word issabah is Arabic for finger, and it denotes one degree 36 minutes, which was considered to be the width of a finger.
They even developed a journal of different ports that recorded which knot on the kamal corresponded to the altitude of Polaris for each port they frequently visited. Throughout antiquity, the Greeks and Arabs steadily advanced the science of astronomy and the art of astrology.
About a thousand years ago, in the 10 th century, Arabs introduced Europe to two important astronomical instruments-the quadrant and the astrolabe.
An Astronomers Astrolabe. It's about three and one-half inches in diameter. It was used to find the time of rising and setting of the sun and the altitude of the sun and selected stars. Importantly, it was used to find the direction of Mecca for the devout Moslem's morning and evening prayers. The astronomer's beautiful, intricate and expensive astrolabe was the grandfather of the much simpler, easy to use mariner's quadrant and astrolabe. The mariner's quadrant-a quarter of a circle made of wood or brass--came into widespread use for navigation around , though its use can be traced back at least to the s.
Here is photograph of a mariner's brass quadrant. The scale spans 90 degrees and is divided into whole degrees. A plumb bob establishes a vertical line of reference. The quadrant was a popular instrument with Portuguese explorers. The one you see in the slide is a replica of the type Columbus might have used on his voyages to the New World. Columbus would have marked the observed altitude of Polaris on his quadrant at selected ports of call just as the Arab seaman would tie a knot in the string of his kamal.
Alternatively, the navigator could record the altura , or altitude, of Polaris quantitatively in degrees at Lisbon and at other ports to which he might wish to return. It wasn't long before lists of the alturas of many ports were published to guide the seafarer up and down the coasts of Europe and Africa. During the 's, Portuguese explorers were traveling south along the coast of Africa searching for a route to the orient.
As a seafarer nears the equator heading south, Polaris disappears below the horizon. So, in southern seas, mariners had to have a different way of finding their latitude. The mariner's quadrant was a major conceptual step forward in seagoing celestial navigation.
Like the knots-in-a string method of the Arab kamal , the quadrant provided a quantitative measure, in degrees, of the altitude of Polaris or the sun, and related this number to a geographic position - the latitude - on the earth's surface.
But for all its utility, the quadrant had two major limitations: On a windy, rolling deck, it was hard to keep it exactly vertical in the plane of a heavenly body.
And it was simply impossible to keep the wind from blowing the plumb bob off line. The solution was the mariner's astrolabe. Here is a beautiful mariners' astrolabe made in Lisbon by J. Mariner's astrolabes are now very rare and expensive - less than one hundred are known to survive and most of these are in poor condition having been recovered from ship wrecks.
The seagoing astrolabe was a simplified version of the much more sophisticated Middle Eastern astronomer's astrolabe that we saw a moment ago. All the complex scales were eliminated, leaving only a simple circular scale marked off in degrees. A rotatable alidade carried sighting pinnules. Holding the instrument at eye level, the user could sight the star through the pinnules and read the star's altitude from the point where the alidade crosses the scale.
For a sun sight, the astrolabe was allowed to hang freely and the alidade was adjusted so that a ray of sunlight passed through the hole in the upper vane and fell precisely on the hole in the lower vane. The astrolabe was popular for more than years because it was reliable and easy to use under the frequently adverse conditions aboard ship. The next step in the evolution of celestial navigation instruments was the cross-staff, a device resembling a Christian cross. This one is a modern reproduction in the style popular with Dutch navigators in the eighteenth century.
Interestingly, its operating principle was the same as that of the kamal. The vertical piece, the transom or limb, slides along the staff so that the star can be sighted over the upper edge of the transom while the horizon is aligned with the bottom edge.
The Persian mathematician Avicenna wrote about a cross-staff in the eleventh century. This drawing, from a Spanish book on navigation published in , shows how the cross-staff was used to determine the altitude of Polaris. Early cross-staffs had only two pieces - the staff and one transom.
Over time they became more elaborate. Each transom corresponds to the scale on one of the four sides of the staff. These scales mark off 90, 60, 30, and 10 degrees, respectively.
In practice, the navigator used only one transom at a time. The major problem with the cross-staff was that the observer had to look in two directions at once - along the bottom of the transom to the horizon and along the top of the transom to the sun or the star. A neat trick on a rolling deck! One of the most popular instruments of the seventeenth century was the Davis quadrant or back-staff. Captain John Davis conceived this instrument during his voyage to search for the Northwest Passage.
It was described in his Seaman's Secrets published in It was called a quadrant because it could measure up to 90 degrees, that is, a quarter of a circle. The observer determined the altitude of the sun by observing its shadow while simultaneously sighting the horizon. Relatively inexpensive and sturdy, with a proven track record, Davis quadrants remained popular for more than years, even after much more sophisticated instruments using double-reflection optics were invented.
An English craftsman named Walter Henshaw made the one you see in the slide in It's made of rosewood with a diagonal scale on boxwood. One of the major advantages of the Davis back-staff over the cross-staff was that the navigator had to look in only one direction to take the sight - through the slit in the horizon vane to the horizon while simultaneously aligning the shadow of the shadow vane with the slit in the horizon vane. The major problem with back-sight instruments was that it was difficult if not impossible to sight the moon, the planets or the stars.
Thus, toward the end of the 's and into the 's, the more inventive instrument makers were shifting their focus to optical systems based on mirrors and prisms that could be used to observe the nighttime celestial bodies. The critical development was made independently and almost simultaneously by John Hadley in England and by Thomas Godfrey, a Philadelphia glazier, about The fundamental idea is to use of two mirrors to make a doubly reflecting instrument-the forerunner of the modern sextant.
How does such an instrument work? Hold the instrument vertically and point it toward the celestial body. Sight the horizon through an unsilvered portion of the horizon mirror.
Adjust the index arm until the image of the sun or star, which has been reflected first by the index mirror and second by the silvered portion of the horizon mirror, appears to rest on the horizon. The altitude of the heavenly body can be read from the scale on the arc of the instrument's frame.
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