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A study of sundial calibration for eight selected altitude dials

Mathematical diagrams for the circular altitude dial.

This dial has circular graduations for dates. The gnomon is fixed, and the hour point on a circle is where the nodus shadow falls. A series of calculations for coordinates (X’, Y’) give the calibration for the whole dial. These calculations are by numerical iteration on computer.

Altitude dials are an intriguing type of sundial. There are many individual types, and they mainly date from around the 1600s or earlier. Our own design for a full-scale altitude sundial for the garden is a circular dial. It follows the same working principle as a circular brass dial in The Augustine Museum in Germany. We decided to investigate the calibration of eight different altitude dials, including the one from the museum, and we have now completed a study of their mathematics. We submit the report of our study in full for wider interest here: A Study of Altitude Sundial Calibration.

 

 

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A circular altitude sundial at full-scale for the garden

Altitude sundial in the garden

The altitude sundial in sunshine on 21st July 2026. The dial reads 3:50 pm, which is only 4 minutes difference from clock time, with corrections made for BST, Longitude, and EOT. The reading point is the tip of the shadow, which is the shadow of the gnomon nodus. 

The altitude sundial is an intriguing instrument. It relies on measuring the height of the sun to tell the time. It also requires the user to adjust for the time of year before making a reading. There are many altitude sundials and there is one particular one with a circular dial, from around the year 1600, which we find very interesting. We have now designed and built one like it at full-scale for the garden. Continue reading

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Inverkeithing Mercat Cross – sundial restoration complete

Inverkeithing Mercat Cross, the capital stone, cube sundial, and unicorn fully restored.

The Inverkeithing Mercat Cross is a tall stone column in the centre of the town. The capital stone, cube sundial, and unicorn sculpture are at the top of the column. There has been no ‘Cross’ as a Christian symbol on the column since the Reformation. Photo: Graciela Ainsworth.

The Inverkeithing Mercat Cross is a tall and handsome piece of historic town architecture. The Mercat Cross in Scottish towns was a symbol of licensed authority to trade in goods, and the trading place was at the Cross. The sundial would be a 17th century addition, now fully restored, together with the heraldic capital stone and the symbolic unicorn of Scotland.

The sundial is a cube stone which is ideal for a prominent place in a town. It is visible from all sides and it shows the time at all hours of the day. Continue reading

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Haddo House cube stone sundial restored and reading solar time

Haddo House stone cube sundial

This sundial cube stone has fine carving detail. Each of five faces can read the time. The horizontal dial carved in the stone on top indicates an ornamental piece for a garden. A cube set on a tall pillar or on a wall will have the side dials only. Photo: Graciela Ainsworth.

This cube stone sundial stands in the garden of Haddo House, the National Trust for Scotland property in Aberdeenshire. When we examined it at the workshop, it had broken and missing gnomons and moss obscured the dials. After restoration the dials can now read solar time properly again.

A cube dial has the mathematical attraction of reading time by the sun on south, east, west, and north faces. As a result, as the sun moves round, the cube sundial is an excellent device for showing time Continue reading

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Restoration of Inverkeithing Mercat Cross, finding true North

The azimuth sundial instrument

The azimuth sundial used as a measuring instrument for finding north. The cursor is set for the date, the cursor arm is turned to set the time. The gnomon shadow and cursor are in line.

The Mercat Cross at Inverkeithing in Scotland is a tall stone column that dates from the 14th century. The top of the column has a sundial stone in the form of a cube and a unicorn. Restoration of the whole column is now complete. To re-install the sundial stone correctly we had to find a true north line.

The north line is the direction that shadows will fall at noon, and Noon is the exact time of day when the sun is in the south. Continue reading

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Sundial stones found in a Scottish garden — the cruciform stone

Cruciform sundial stone

The cruciform stone has four concave sides leaving four curving arms between them, thus creating the cross shape. This particular stone has a small concave end on each curving arm [not visible in the photo]. So the single stone has eight dials, on four large concave sides and four small ones. [Note: the shadow in this photo is from indoor lighting not from the sun.]

The cruciform type of sundial stone is a clever idea. During the day, as the sun moves round, each of the dials in turn will show a shadow and indicate the time. The straight dial edges cast the shadows. A cruciform stone can be on a lectern sundial but this one appears unique. Continue reading

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North alignment on a Victorian pedestal

Sun direction measured with an angle finder

The direction of the sun measured against one edge of the octagonal stone pedestal using the angle finder and light slit

A Victorian sundial pedestal stands in the gardens of Riccarton estate outside Edinburgh. It once held a beautiful octagonal dial which is now in safe storage. A replacement dial is to go on show and it must be in a correct orientation. The gnomon on a sundial must point north. A wrong alignment will make the sundial inaccurate, and nothing on the 19th century pedestal actually marks the north. Continue reading

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Looking for a place for our armillary sundial

Looking for a place for our armillary sundial

The armillary sundial captures the dynamics of the earth spinning on its axis and orbiting the sun. The equatorial band is pierced with markers for the hours. A meridian band shows the seasons.

Now that we have moved, we have been looking for a place for our armillary sundial. Before, it was nestling gently in its surroundings with plants in the garden growing up through the rings and wrapping themselves around. For the moment it is out in the open where  we can see it better. It has an air of almost floating on its stand with the lightness of the rings and the delicate shadows on the ground. Continue reading

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A sundial commission unwrapped for an 80th birthday celebration

A sundial commission for an 80th birthday celebration

The design is a polar sundial, which tilts up at the latitude angle. The happy owner has unwrapped his 80th birthday present from the family.

“My father turns 80 this year and we are looking to commission a sundial in celebration. Is this possible? What’s the process? How long does it take?” When this email appeared in our inbox one day in March of course we wanted to follow it up. It sounded such a lovely suggestion from the family. Later when we learned a little more it looked like a perfect plan. Continue reading

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Armillary sundial reads summer solstice by the shadow of a ball at noon

At the summer solstice the armillary sundial casts a shadow of the ball at noon

At summer solstice the ball on the upper tropic ring casts its shadow exactly onto the opposite ring below. The upper tropical ring is the tropic of Cancer and lower ring is Capricorn.

The armillary sphere dates back to ancient times when astronomers created their own vision of the earth and the heavens. The idea still works today and our own design creates a sundial with a special feature of two balls that mark the passage of the seasons by their shadows. One of the balls reads summer solstice. This is the high point of the year when the sun is highest in the sky. Continue reading

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