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Sundials

Sundial Corrections

There are two primary corrections to consider when comparing clock time with sundial time:
  1. correcting for the sun's variable motion
    and
  2. correcting for the observer's longitude


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Sundial Solar Motion Correction

Due to the non-circular orbit of the Earth and the tilt of the Earth's rotation axis, the sun's motion as seen from the Earth is much like a car moving along a highway. While a car may maintain an average speed (say 70 mph for example), it sometimes is a little faster and sometimes a little slower. In the case of the sun, these variations are well documented and relatively easy to calculate if one is interested in that sort of thing.

This page allows the interested dial owner the opportunity to quickly calculate a correction to the time displayed by their sundial that will correct for the variable motion associated with the apparent sun. This particular correction is often referred to as the Equation of Time and does not depend on the location of the sundial.

NOTE: The code which calculates corrections for solar motion uses javascript and has not been tested on platforms other than Microsoft. It should work for the most commonly used browsers (Firefox and Internet Explorer).

Simply select the year, month, and day for which you would like to know the sundial correction. The result will be calculated and displayed.

Please see notes for more details.

     

You may also view the variation of the sun's motion over an entire calendar year.

 

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• The correction given is not exact and can vary by approximately ± 0.25 minutes (In time I hope to refine the code which performs the calculations in order to make the calculations more exact). In spite of this, it none-the-less will give a reasonably reliable correction and serve to reinforce the non-exact nature of the time-keeping associated with a typical sundial.

• Also, this correction is for variable solar motion and does not correct for the observer's longitude (see the note below on Longitude Corrections for details), which can contribute an additional several minute correction, depending on where one is located within a particular time zone.

• Finally, the placement of the dial (e.g. is it level?, oriented due north?) will also affect the reading. If not oriented properly, this too will alter the resulting reading.

• If you have any questions, feel free to get in touch.

 

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Sundial Longitude Correction

If you are not located on or close to your time zone meridian, adjust your dial reading by adding (if west) or subtracting (if east) four minutes of clock time for each degree of longitude you are separated from from your time zone meridian. You may then compare the adjusted dial reading to your clock time. Be sure to correct for the sun's variable motion as well.

EXAMPLE:

I am located at 93.3° W longtitude. So when the sun is directly overhead someone located at 90° W (noon to them, as measured by the sun), the sun appears to me as still slightly to the east. This translates into 3.3° × 4 minutes of time/° = 13.2 minutes of clock time. Therefore, I need to add 13.2 minutes to the reading of my sundial if I want to compare it to my clock time (which is based on time zones and specific meridians).