45 lines
4.6 KiB
Markdown
45 lines
4.6 KiB
Markdown
---
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title: "Backstaff"
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chunk: 2/3
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source: "https://en.wikipedia.org/wiki/Backstaff"
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category: "reference"
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tags: "science, encyclopedia"
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date_saved: "2026-05-05T09:36:31.201916+00:00"
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instance: "kb-cron"
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---
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=== Usage ===
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In order to use the instrument, the navigator would place the shadow vane at a location anticipating the altitude of the sun. Holding the instrument in front of him, with the sun at his back, he holds the instrument so that the shadow cast by the shadow vane falls on the horizon vane at the side of the slit. He then moves the sight vane so that he observes the horizon in a line from the sight vane through the horizon vane's slit while simultaneously maintaining the position of the shadow. This permits him to measure the angle between the horizon and the sun as the sum of the angle read from the two arcs.
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Since the shadow's edge represents the limb of the sun, he must correct the value for the semidiameter of the sun.
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=== Instruments that derived from the Davis quadrant ===
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The Elton's quadrant derived from the Davis quadrant. It added an index arm with spirit levels to provide an artificial horizon.
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== Demi-cross ==
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The demi-cross was an instrument that was contemporary with the Davis quadrant. It was popular outside England.
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The vertical transom was like a half-transom on a cross-staff, hence the name demi-cross. It supported a shadow vane (A in Figure 4) that could be set to one of several heights (three according to May, four according to de Hilster). By setting the shadow vane height, the range of angles that could be measured was set. The transom could be slid along the staff and the angle read from one of the graduated scales on the staff.
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The sight vane (C) and horizon vane (B) were aligned visually with the horizon. With the shadow vane's shadow cast on the horizon vane and aligned with the horizon, the angle was determined. In practice, the instrument was accurate but more unwieldy than the Davis quadrant.
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== Plough ==
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The plough was the name given to an unusual instrument that existed for a short time. It was part cross-staff and part backstaff. In Figure 5, A is the transom that casts its shadow on the horizon vane at B. It functions in the same manner as the staff in Figure 1. C is the sighting vane. The navigator uses the sighting vane and the horizon vane to align the instrument horizontally. The sighting vane can be moved left to right along the staff. D is a transom just as one finds on a cross-staff. This transom has two vanes on it that can be moved closer or farther from the staff to emulate different-length transoms. The transom can be moved on the staff and used to measure angles.
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== Almucantar staff ==
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The Almucantar staff is a device specifically used for measuring the altitude of the sun at low altitudes.
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== Cross-staff ==
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The cross-staff was normally a direct observation instrument. However, in later years it was modified for use with back observations.
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== Quadrant ==
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There was a variation of the quadrant – the Back observation quadrant – that was used for measuring the sun's altitude by observing the shadow cast on a horizon vane.
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== Thomas Hood cross-staff ==
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Thomas Hood invented this cross-staff in 1590. It could be used for surveying, astronomy or other geometric problems.
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It consists of two components, a transom and a yard. The transom is the vertical component and is graduated from 0° at the top to 45° at the bottom. At the top of the transom, a vane is mounted to cast a shadow. The yard is horizontal and is graduated from 45° to 90°. The transom and yard are joined by a special fitting (the double socket in Figure 6) that permits independent adjustments of the transom vertically and the yard horizontally.
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It was possible to construct the instrument with the yard at the top of the transom rather than at the bottom.
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Initially, the transom and yard are set so that the two are joined at their respective 45° settings. The instrument is held so that the yard is horizontal (the navigator can view the horizon along the yard to assist in this). The socket is loosened so that the transom is moved vertically until the shadow of the vane is cast at the yard's 90° setting. If the movement of just the transom can accomplish this, the altitude is given by the transom's graduations. If the sun is too high for this, the yard horizontal opening in the socket is loosened and the yard is moved to allow the shadow to land on the 90° mark. The yard then yields the altitude.
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It was a fairly accurate instrument, as the graduations were well spaced compared to a conventional cross-staff. However, it was a bit unwieldy and difficult to handle in wind. |