Sundials

Applied Gnomonics

Sundials have a long history and are amongst the earliest forms of timekeepers. They have the advantage of requiring no mechanical parts or maintenance - but the obvious disadvantage of only working on a sunny day. For this reason the use of sundials was more prevalent in some countries than in others. Despite this, historic sundials are found around the world and modern ones are still produced and sold today.

Sundial
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How Does a Sundial Work?

A sundial is in essence simply any form of stick - known as a style or gnomon - which casts a shadow. The position of the shadow can then be used to determine the current solar time.

In theory a stick stuck in the ground could form the basis of a sundial - in reality it's not that simple. The problem is that the earth's axis is tilted, which means that the apparent movement of the sun through the sky changes every day. If this isn't accounted for then a sundial that tells perfect time today will be slightly wrong next week and very wrong next month.

There are two basic ways of compensating for the earth's tilt, both of which involve aligning the gnomon with the earth's axis.

On a horizontal sundial, the base plate is kept level with the ground whilst the gnomon is tilted to keep it in line with the earth's axis. On an equatorial sundial, the base plate is tilted at an angle according to latitude and the gnomon placed perpendicular to this. In fact these two methods are in essence the same, with the horizontal sundial face simply being a projection of the equatorial dial onto the horizontal plane.

Both these methods mean that the proper alignment of the sundial depends on latitude - it is not enough to simply pick up a sundial from one location and move it to another. Installing a sundial always requires adjustment, a fact often overlooked by potential buyers.

Another, less common, form of sundial is based on the armillary sphere - a model of the heavens. In these dials, several interlocking rings outline a sphere and a carefully aligned gnomon at the centre casts a shadow on the equatorial ring.

A sundial is a lot more sophisticated than most people realise!

Calibration

Even when your sundial has been set up properly for latitude it is still necessary to calibrate it to indicate the time. Midday is easy - mark when the sun is overhead - but what about the other hours?

If using a horizontal sundial it's not enough to just divide the dial into equal parts. The angles involved mean that the hour intervals are unequal. These can be calculated using trigonometry or - more easily in this day and age - can be calibrated by reference to another time source.

Another complexity with the horizontal sundial is Summer Time. Since the hours are not equally spaced it is not enough to simply read the time and add one. Instead a new set of markings will be required. One loose convention in modern sundials is to have winter markings in blue and summer ones in red.

Sundials Today

Sundials remain extremely popular today. Many have wonderful designs and inscriptions. High quality modern sundials are still being made and are available to buy on the market from many suppliers.

Rather than buying one, you might like to build your own. Building a sundial is an interesting project and a good way to learn about time, astronomy and trigonometry and is sometimes used in schools as a class project.

External Links:
The British Sundial Society
The North American Sundial Society

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Sundials Applied Gnomonics Sundials have a long history and are amongst the earliest forms of timekeepers. They have the advantage of requiring no mechanical parts or maintenance - but the obvious disadvantage of only working on a sunny day. For this reason the use of sundials was more prevalent in some countries than in others. Despite this, historic sundials are found around the world and modern ones are still produced and sold today. Sundial Buy this Giclee Print at AllPosters.com How Does a Sundial Work? A sundial is in essence simply any form of stick - known as a style or gnomon - which casts a shadow. The position of the shadow can then be used to determine the current solar time. In theory a stick stuck in the ground could form the basis of a sundial - in reality it's not that simple. The problem is that the earth's axis is tilted, which means that the apparent movement of the sun through the sky changes every day. If this isn't accounted for then a sundial that tells perfect time today will be slightly wrong next week and very wrong next month. There are two basic ways of compensating for the earth's tilt, both of which involve aligning the gnomon with the earth's axis. On a horizontal sundial, the base plate is kept level with the ground whilst the gnomon is tilted to keep it in line with the earth's axis. On an equatorial sundial, the base plate is tilted at an angle according to latitude and the gnomon placed perpendicular to this. In fact these two methods are in essence the same, with the horizontal sundial face simply being a projection of the equatorial dial onto the horizontal plane. Both these methods mean that the proper alignment of the sundial depends on latitude - it is not enough to simply pick up a sundial from one location and move it to another. Installing a sundial always requires adjustment, a fact often overlooked by potential buyers. Another, less common, form of sundial is based on the armillary sphere - a model of the heavens. In these dials, several interlocking rings outline a sphere and a carefully aligned gnomon at the centre casts a shadow on the equatorial ring. A sundial is a lot more sophisticated than most people realise! Calibration Even when your sundial has been set up properly for latitude it is still necessary to calibrate it to indicate the time. Midday is easy - mark when the sun is overhead - but what about the other hours? If using a horizontal sundial it's not enough to just divide the dial into equal parts. The angles involved mean that the hour intervals are unequal. These can be calculated using trigonometry or - more easily in this day and age - can be calibrated by reference to another time source. Another complexity with the horizontal sundial is Summer Time. Since the hours are not equally spaced it is not enough to simply read the time and add one. Instead a new set of markings will be required. One loose convention in modern sundials is to have winter markings in blue and summer ones in red. Sundials Today Sundials remain extremely popular today. Many have wonderful designs and inscriptions. High quality modern sundials are still being made and are available to buy on the market from many suppliers. Rather than buying one, you might like to build your own. Building a sundial is an interesting project and a good way to learn about time, astronomy and trigonometry and is sometimes used in schools as a class project. External Links: The British Sundial Society The North American Sundial Society Clocks-Watches-Time / Clocks / Types / Sundials / All original material copyright © Trevor Mendham 2005-2008. All trademarks are the property of their respective owners Please read the site terms of use		Clocks Types Alarm Clocks Atomic Banjo Bracket Carriage Cuckoo Kit Cat Lantern Longcase Mantel Mystery Parliament Skeleton Sundials Pocket Tide Tower Wall Water Makes Mechanics Famous Watches Time Time & Motion More...