If you are setting up a commercial dial, you can check it to see if it has been constructed to be a timekeeper and not just a garden ornament. .
If you have an armillary or bowstring equatorial dial, the dial must be positioned so it is level east to west, and the gnomon (usually an arrow), is vertically directly above 12:00. Skip to finding north
If you have a horizontal dial proceed to 1.
For the terms used below, you may wish to refer to the glossary.- 1. The high end of the gnomon should point directly to 12:00 on the dial face. If it doesn't, your commercial dial is incorrect.
If you have a custom made dial, it may include a longitude correction, in which case the gnomon will not point precisely to 12:00. Unless it is designed for your location, setting it up is beyond the scope of this FAQ.
- 2. If the dial includes 6:00 AM and 6:00 PM, they should be at the ends of a single straight line, which is at a right angle to the sub-style - the line where the gnomon meets the dial face.
- 3. Since the gnomon must have a finite width, the indication for 12:00 must have an equivalent width.
A sketch of a dial that meets conditions 1 through 3
- 4. If you want to test further . . .
Measure the style height - the vertical angle the gnomon makes with the dial face.
Measure the angle on the dial face between the sub-style and the 3:00 or 9:00 hourline. (These two angles should be the same.)
The sine of the style height should equal the tangent of the 3:00 hourline.
This trick can also be used to determine the style height, and hence the original latitude, for a dial that is missing its gnomon.
The high end of the gnomon on an ordinary flat dial must be aligned to True North. (In the southern hemisphere to True South.) i.e. The style, or shadow casting edge of the gnomon must be parallel to the earth's axis.
Using the Shadow of a Vertical Object at Solar Noon
Perhaps the most accurate method. The shadow of a vertical object at Solar Noon will point to True North. A string with a weight on the end suspended from a tripod will make a true vertical object.
You can find the time of Solar Noon at your location on the page below. To use this page, do not use the pull-down list of cities. To get the precise time you should use your exact latitude and longitude.
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If you need to convert from decimal degrees to deg/min/sec or vise-versa, go here. |
You will also need to enter your offset to UTC (EST = 5, PST = 8) and indicate if it is currently Daylight Saving Time.
Another method of finding Solar Noon, and much, much, more about your dial and how it operates is to download The Dialist's Companion. It is a DOS program, published by NASS, which calculates a wide range of data elements of interest to dialists and others who are concerned with phenomena associated with solar time.
Using a Compass
Using a compass is not recommended as a compass needle points to Magnetic North, and it is also difficult to get accurate readings with a compass. If you do use a compass, be sure to compensate for the magnetic declination of your location.To find the magnetic declination for your location, go to the link below. Enter your Zip code (plenty close enough), and click [Get Location]. On the next page click [Compute Declination].
In Miami, Florida I get the result:
Declination = 5° 31' W changing by 0° 7' W/year
Which means that True North lies 5 degrees 31 minutes clockwise from the compass needle
A Graphical Method Using Shadows
Draw a diagram with concentric circles as shown below. Place a vertical object (a nail?) at C, and place the diagram on a level surface where it will receive sunlight all day.
Note where the shadow of the object touches one of the circles in the morning, and again in the afternoon as at A and B. Draw a line from A to B and bisect it at D. A line connecting C to D will point to True North.
Using Polaris, the North Star
If you can get a good view of Polaris, it is always within a degree of True North.
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When you have determined True North, proceed to: Correcting a commercial dial to your latitude |