Sundials and telling the Time

All Sundials generally tell LOCAL TIME. This means that they tell the time at the location where they are situated. So when the sun is directly overhead at its highest point in the sky it is 12 noon, midday, where you are. If you live in Kent it will be noon for you 20 minutes before it is noon by a sundial in Cornwall.

This way of telling the time was fine when everyone worked and lived locally but as soon as we started to develop faster types of transport it became necessary to standardise time. Hence we have Greenwich Mean Time. (GMT)
In the UK we all live by GMT, whether we live on the coast of Norfolk or the far tip of west Wales. Our clocks and watches are set to GMT, or 1 hour different in the summer (BST).
Fortunately for us, the whole world adopted our system and set Greenwich as the prime meridian, 0 degrees longitude. The prime meridian is the imaginary line that passes through Greenwich, North to South from pole to pole. Anyone living on this line shares the same time as GMT, accepting that some countries add hours to suit there own daylight saving time difference.
As the Sun travels from East to West Cities east of the Greenwich meridian are ahead of us in time and cities to the west of us are behind us in time.
At 12 noon in London its 5.30 in the afternoon in Mumbai, India and 7 in the morning in New York.

The closer you live in the UK to the Greenwich meridian the closer your Sundial will run to GMT. Brighton, Cambridge and Hull are all very close to the meridian. In Canterbury it will show noon on a Sundial 4 minutes before it is actually noon at Greenwich. In Cornwall a Sundial will show about 11.40am when it is 12 noon at Greenwich.

All Sundials are generally made to show local time and may have a plate that tells the observer the difference in time between the sundial and GMT. For each degree of longitude a sundial is located east or west of the Greenwich meridian the time difference is 4 minutes.

One small problem is the EQUATION OF TIME. Our clocks and watches put exactly 24 hours into every day and 365 days across the year. Because the earths rotation and orbit around the sun is not absolutely true, a sundial will show time fast or slow on any given day by upto 15 minutes. At the end of the year these pluses and minuses all equal themselves out and we end up with a total of 365  24 hour days. Therefore on each day you have to add or subtract a certain number of minutes and seconds from the time shown on a sundial to correct it to GMT.
Despite the claims of some makers it is unlikely that any sundial will be accurate to much more than a minute as factors like penumbral effects, the equation of time and refraction make setting up a dial perfectly very difficult.
Charts showing the EQUATION OF TIME are widely available and easily found on the web.

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