Sundial

:This article pertains to the astronomical instrument. For the psychedelic rock band, see Sun Dial.

Precision sundials (heliochronometers)

A precision sundial, called a heliochronometer,

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corrects

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apparent solar time

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to mean solar time or another standard time.

Related Topics:
Mean solar time - Standard time

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Heliochronometers usually indicate the minutes to within 1 minute of Universal Timehttp://www.sunlitdesign.com/infosearch/sundialaccuracy.htm.

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Equatorial bow sundial

The classic shape for a heliochronometer is an equatorial bow sundial.

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A bar, slot or stretched wire parallel to the earth's axis forms the style.

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The face is a semicircle with markings on the inner surface.

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This pattern, built a couple of meters wide out of temperature-invariant steel invar, was used to keep the trains running on time in France before World War I.

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One of the simplest sundials that reads clock time is an equatorial bow with a gnomon shaped like two vaseshttp://www.wsanford.com/~wsanford/exo/sundials/ca/claremont/info.html. The vase-shape directly shades the hour line in the correct place as the year passes, and the sun changes elevation.

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The most precise sundials ever made are monumental equatorial bows constructed of masonry, part of the Yantra mandir (Jaipur), in India, built as part of a set of astronomical instruments.

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Precision noonmarks

In some older houses, particularly farmhouses, a noon-mark can be found carved into a floor or windowsill. Such marks act as sundials to indicate local noon, and they provided a simple and accurate time reference for households that did not possess accurate clocks.

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In modern times, some Oriental countries' post offices have set their clocks from a precision noon-mark. These in turn provided the times for the rest of the society. The typical noon-mark sundial was a lens set above an analemmatic plate. The plate has an engraved figure-eight shape. When the edge of the sun's image touches the part of the shape for the current month, it is noon!

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Ancient Greek sundials

The ancient Greeks used a type of sundial sometimes referred to as pelekinon (axe-like, apparently because shape of the hour and day lines suggest the ancient double-headed ax pelekus). The gnomon was a rod or pole upright in a horizontal face or half-spherical face. The shadow of the tip of the rod sweeps out hyperbolic curves on a flat face, or circles on a spherical face. The advantage of these dials is that they can be marked to tell the exact time for all times of year.

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Analemmatic sundials

Analemmatic sundials correct solar time to mean solar time or another standard time. These usually have hour lines shaped like "figure eights" (analemmas) according to the equation of time. This compensates for the slight eccentricity in the Earth's orbit that causes a 15 minute variation from mean solar time.

Related Topics:
Mean solar time - Standard time - Equation of time

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Very accurate dials of this type fit nicely in a public square, using a ball at the tip of a flagpole as the nodus, with the face painted on or inlaid in the pavement.

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A fun, less accurate version of the sundial is to lay out the hour marks on concrete, and then let the user stand in a square marked with the month. The month squares are arranged to correct the sundial for the time of year. The user's head then forms the gnomon of the dial. If the sundial is molded into the concrete, it is almost perfectly immune to vandalism, as well as truly fun and reasonably accurate.

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The geometrical construction of an analemmatic sundial is simple. First imagine an equatorial sundial floating in the air: a vertical bar directed towards the pole and a ring

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in the plane perpendicular to the bar. Label the lowest point of the ring "12", and the other hour marks as usual. At a certain time and date, the shadow of a certain point A on the bar (which falls here or there depending on the time of year) falls on a certain point B of the ring (which depends on the hour, and the position in the Earth's orbit). Now draw the point B' in the ground just below B and the point A' just below A. Now if you stand at A' your shadow will point at B', because the sun is somewhere in the plane A B A' B'.

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In middle latitudes, the ellipse with the hour-marks

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should be about six meters wide, so the shadow of the

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head of the beholder will fall near it most of the time.

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Article of interest: “Analemmatic sundials: How to build one and why they work” by C.J. Budd and C.J. Sangwin

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Reflection sundials

Isaac Newton invented a sundial for a south-facing window. He placed a tiny mirror on the windowsill, and painted the sundial's face in a mirror-image pelekinon on the ceiling and walls. The mirror formed the gnomon by reflecting a spot of light. This provides a large, accurate, perfectly correctable sundial with minimal material, and no wasted space at all. This design could easily be made analemmatic.

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Analog calculating sundials

A last, interesting variation accurately keeps clock time, while still resembling a conventional garden sundial. It is a horizontal sundial with a face cut on a cardioid (a sort of heart-shape). A cardioid is the shape that connects the intersections between the solar-time marks of a conventional sundial, and the equal-angles of a true clock-time face. The place where the shadow crosses the cardioid's edge is the place where clock time can be read on the underlying clock-time dial. The sundial is adjusted for daylight saving time by rotating the underlying equal-angle clock-time face. The sun-time face does not move.

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