Friday, 3 August 2007

How Do I Get My Sundial To Tell The Time?

Hello fellow sundial lovers,

I get asked this question alot via my website so i thought i would just post it in a blog so anyone and everyone interested can read up and ask me questions if they would like to.

How to set up your sundial so it will work
The set up:
· Make sure that the gnomon is facing true north and is at the same angle as your latitude.
· Make sure that the flat part or dial of the sundial is level. Use a builders level if need.
· Find out your latitude from or an atlas. This will be a number of degrees between 0 and 90. For example, Dublin is about 56° north and London is 51° north. Places such as New York and Washington are 40° north and Vancouver 49° north. Make sure the angle between the gnomon and the base is the same number of degrees. If it were 90 degrees then the gnomon would be vertical. When using Multimap make sure you use the first two letters for N as shown, 49:15:42N (49.26178) for Vancouver.
· Point the gnomon towards true north if you live in the northern hemisphere or towards true south if you live in the southern hemisphere. True north means the North Pole, not the magnetic north that is shown by a compass.
· You can also find true north at night by finding the pole star, Polaris, and making sure the gnomon is lined up with it. Unfortunately, there is no 'southern pole star', so this method does not work in the southern hemisphere. Here it best to use your compass for true south or use the sun and horizon and measure the distance between the two at noon.
How to make your sundial show the same time as your clock or watch
Once your gnomon is pointing in the right direction, you can mark your sundial so that it shows the right time. In many countries, daylight saving time is used in the summer and clocks are set one hour forward. Since the sun shines most in the summer, it is best to set up your sundial to show daylight saving time, rather than the local time that most sundials show. Mark the shadow at exactly 12 o'clock by your watch. Now mark out the other hours on either side, so that 6am and 6pm are at 90° to 12 o'clock. Subdivide the hours into 5 minute intervals. Of course to be more accurate you will need to use the calculations needed in order to get the exact angles for the hour lines.
Seasonal Corrections
You will find that your sundial is accurate to within a couple of minutes on many days of the year, such as 15 April or 2 September. At other times, it will be wildly inaccurate, such as on 4 November, when it will be nearly 17 minutes fast. To overcome this, it is possible to use a correction chart if you so wish.

Thanks, Vicki.