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
By the way we define time now, they aren’t. But before mechanical clocks were widespread, the time you got from the sun was accepted as the actual time. Early mechanical clocks (the very fancy ones anyway) sometimes had modules built that would convert to the time the sundial said it was.
The sun’s position changes throughout the year, but that is the vertical position (altitude from the horizon). One every single day of the year at noon, the sun will be exactly dead north or south depending on where you are in the and what day it is. In summer, the sun might be dead south of you, and high in the sky. In winter however, the sun will still be dead south, but low in the sky. It is only the vertical position of the sun that changes day to day.
This means that a sundial will stay accurate, but the vertical position does not matter. Sundials track the horizontal movement of the sun. The sun will cast longer and shorter shadows throughout the year, but the shadow will always be at the same horizontal angle.
Now, /u/Skusci makes a good point about the Equation of Time, which does offset the times by give or take 14-16 minutes throughout the year. However, that is due to a different phenomenon than the one you seem to be addressing.
From an outside perspective, 24 hours is how long it takes for one point of the earth to spin around and directly face the sun again.
From an earthling’s perspective, 24 hours is how long it takes for the sun to reach the high point of its arc through the sky.
This high point is called “noon.” It happens at essentially the same time every day, whether it’s January or July.
The days get longer and shorter not because the sun is moving at different speeds or getting to its high point at different times, but because the arc it carves through the sky is getting higher and lower. In winter, more of the arc is below the horizon, so the sun is up less.
So the sun is still at the same *direction* every day at noon, just higher or lower in the sky, and so its shadow points the same direction at that time, just longer or shorter. So 11am and 1pm also cast the same shadow year round, as do 10am and 2pm, and so on. The only difference is the arc will be high enough where you’ll get a shadow at say 6am in June, but not in December, so the extremes of the sundial can indeed be useless in winter.
They aren’t actually accurate with clock time. They are only going to be exactly synced 4 times a year, and at the most inaccurate is going to be off by about 16 minutes. Which if you are living in times before mechanical timekeeping is generally good enough. You can add an offset based on the date though if you really need more accuracy using based on the date and a chart like this one for your location.
https://earthsky.org/upl/2010/04/10apr14_430.jpg
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.*
Sundials tell you the “true” time. Early clocks used complex systems to show the same time as sundials. But it was then decided it was easier to have clocks show what’s called “mean time”, where you assume every day has the same length of 24h.
Sundials show you where the Earth is in its rotation about it’s axis, clocks show you what the agreed on average time is, if all days were the same length.
According to a sundial, the highest point the Sun makes on any given day is Noon, and is due South. According to a clock, that isn’t always the case.
See more at https://en.m.wikipedia.org/wiki/Solar_time
You can get sundials with either the months marked in some way, or very fancy ones with a wiggly shape to compensate.
But nobody is using sundials for precise time measurements. They’re for a rough idea of what hour it is. They only work with clear skies, for starters.
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