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When the Sun and Clock Cycles Agree


One of my daily web stops is EarthSky which reminded me this morning that today is one of the few days in the year when clock time and sun time agree. As someone who has a sundial in the garden, I do pay attention to that shadowy movement.

When the midday sun climbs highest today, if you have a sundial, it will read 12 noon and your local clock will also read 12 noon.

I have always had a sundial in my garden. It keeps me in touch with the movement of the Sun during the day and during the seasons.



Of course that pesky daylight savings time game we play might make your clock say 1 pm today when the sundial says noon. It's all so confusing.

Your local clock time is standard clock time, as long as you live on the meridian that governs your time zone. Denver and Philadelphia, for example, are on the meridian for their respective time zones. East of the time zone line, then your local time runs ahead of standard time and west of the time zone line, local time lags behind standard time.

The sundial and clock agree four times a year: on or near April 15, June 15, September 1 and December 25.

My simple sundial shows a shadow from its style onto a surface marked with lines indicating the hours of the day. The style is the time-telling edge of the gnomon, the straight edge. As the sun moves across the sky, the shadow-edge aligns with hour-lines.

There are plenty of sundials available to you at a wide variety of prices and complexities.

Sundials that directly measure the sun’s hour/angle must have that edge parallel to the axis of the Earth’s rotation to tell the correct time throughout the year. My simple one needs some adjustments during the year and I do play with time and move it to match my clock time every once and awhile.

Isaac Newton had a pretty interesting variation on the sundial. He used a a small mirror placed on the sill of a south-facing window. The mirror would casts a single spot of light on the ceiling and, depending on the geographical latitude and time of year, the light-spot on the ceiling was pretty accurate to the marking he made.

I think it's a good idea to pay attention to the cycles in our lives, both natural and man-made. They are very much a part of us, whether we pass attention to them or not.

Copernicus Armillary,


I would not mind having a Copernicus Armillary, in my home, though I suspect my wife would not think it appropriate to our decor - and might not appreciate me paying about $3000 for it. It is an astronomical instrument that would have been found in libraries and laboratories of the past. I did find some online for less than a hundred dollars so maybe...


More on sundials at: https://paradelle.wordpress.com/2009/08/29/the-time-of-sun-and-shadow/ 

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