How can a sundial reliably work when the sun’s position in the sky changes from day to day and month to month with days getting longer/shorter, etc?
In: Technology
We have one at our local botanical gardens that in on a huge scale with it’s needle at ~2.5m tall. It has each month’s sweep marked so in on time on the first of each month, it also tells you what month you are in (and how far through that month)
Excuse the shit photo
[https://2.bp.blogspot.com/–ft8vXacZtU/UUB1V4bfnXI/AAAAAAAACXo/ohbOyYemH20/s1600/P1010832.JPG](https://2.bp.blogspot.com/–ft8vXacZtU/UUB1V4bfnXI/AAAAAAAACXo/ohbOyYemH20/s1600/P1010832.JPG)
*A Short Guide to the Time Court*
*The Time Court is a combination of a sundial (which tells the time of day) and a solar calendar or ‘Pelekinon’ (which tells the time of year). Using the Time Court you can determine both the date and the time.*
*The metal needle that casts a shadow on the tiles is called a gnomon. As the position of the sun in the sky changes, the position of the shadow of the point of the gnomon moves across the ground in a particular and predictable pattern.*
*The thick blue curved lines trace the path of the shadow on particular days of the year. There is one curve for the first of each month. There are also curves for the Summer Solstice and the Winter Solstice: these are the lines that are closest to and furthest from the gnomon, since on the Summer Solstice the sun is highest in the sky and casts the shortest shadow, whereas on the Winter Solstice the sun is lowest in the sky and casts the longest shadow.*
*When the shadow falls between two of the curved lines then the exact date must be inferred. It is possible to draw a line for every day of the year, however for simplicity’s sake when building a pelekinon it is preferable to select a small number of evenly spaced days.*
*The thin light blue figure-8 shapes are called* *analemmas. Their shape is a result of two features of the Earth’s path around the sun:*
*1. The Earth is tilted relative to the plane of its orbit. In other words, the line between the North and South poles is not at right angles to the direction of the Earth’s travel.*
*2. The Earth does not travel in a circle around the sun. Instead its path is an ellipse.*
*If you marked the sun’s position in the sky at the same time every day for a whole year, the result would be a figure-8 shape. Thus, the shadow that is cast by the sun describes the same shape on the ground. Each analemma shows where the shadow of the point of the gnomon will lie on each hour (the small bronze inlays show how it is corrected for daylight saving time).*
*When the shadow of the point of the gnomon lies on the intersection of an analemma with a thick blue curved line, the current date and time will be ‘on the hour’ on the first of the month.*
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