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Sundial Info



How Does a Sundial Work?


The sun shines on the gnomon (the triangle or device set
perpendicular to the base plate - pronounced NO-men) and casts
its shadow on the appropriate hour line, thus displaying the "time."
 
Information contained on this page:

What Goes Into Accurate Sundial Design?

What Can Be Done About the Time Change Of Daylight Saving Time?

Is One Type Of Sundial “Better” Than the Others?

Can You Purchase a Sundial That Displays Accurate Clock Time Or the Plans and Layout To Build One For Your Location?

Is It Difficult To Build Your Own Sundial?

How Does the Equation of Time Affect Sundial Accuracy?

Why Do Garden Centers and Nature Stores Sell Sundials That Cannot Display Accurate Clock Time?



What goes into accurate sundial design?

#1 - The angle on the gnomon has to be parallel to the earth's axis so as not to require manual adjustment for the changing "height" of the sun in the sky during the changing seasons. This angle must be equal to your latitude, if it is to display accurate clock time.

#2 - The position of the hour lines must be calculated and laid out based upon your latitude, also. Generally speaking, the farther south you live, your accurate sundial's hour lines will be closer together at the hours nearest noon and farther apart at hours nearing sunrise and sunset. The hour lines will not be equally spaced like those dials in the garden shop.






(Same longitude, different latitudes. Notice the difference in the height of the gnomons - laying on their sides - and the positions of the hour lines.)
 
Reason #3 – The position of the hour lines must be calculated and laid out based upon your longitude, also. There can be a difference of almost 2 hours, in some time zones, between two identical sundials, one placed at either East-West end of the time zone.
 





(Same latitude with identical gnomons, different longitudes. Both dials in Eastern time zone.)
 
What can be done about the change of Daylight Saving Time?

All state’s clocks (except Arizona and part of Indiana) “spring ahead” by one hour in early April and “fall back” by one hour in late October. A sundial does not have “hands”, like a clock, which can be set ahead or back and it would be confusing to label a sundial with two sets of hour numbers.

The simple solution is for you to select having your dial labeled for either Standard Time or Daylight Saving Time, depending upon when your dial will experience its heaviest use. The 12-noon hour during Standard Time should be labeled as 1 PM for Daylight Saving Time, and all the other hour labels should be shifted ahead one hour also.

We, being in the northern part of the Central Time Zone, label all our local dials for Daylight Saving Time. (We recommend this option except for Arizona and a portion of Indiana.) We’re out in the yard viewing our dials often in the spring, summer and fall.

You can select either option for a sundial made for you by Accurate Sundials.

 
Is one type of sundial "better" than the others?

Each type of sundial has its advantages:
 



Horizontal Dial – Mounted parallel to earth’s surface
+ Easy to set up and read
+ Familiar to most people
+ Displays time from sunrise to sunset

 



Vertical South Dial - Mounted on a south-facing wall, tree, pole or structure
+ Mounted up and “out-of-the-way”
+ More unusual, a good conversation piece
- The sun can never shine on this dial before 6AM or after 6PM

 
Analemmatic Dial – Mounted parallel to earth’s surface. Gnomon is a “rod,” which must be moved into different "date-holes" in the base plate's Scale of Dates
+ Easy to set up and read
+ This sundial can be laid out in your yard to have rocks, flower beds or bushes as your hour points. Your Scale of Dates can be flagstones laid out in the proper positions that you stand on. You would become the gnomon, and your shadow would cast the correct time on your flowerbed hour-points.
+ Displays time from sunrise to sunset
+ More unusual, a good conversation piece
- Not quite as accurate as the other dials unless it’s made quite large (a smaller dial can’t accommodate 365 holes to move the gnomon into – one for each day of the year).

 



Can you buy a sundial that displays accurate clock time or the plans and layout to build one for your location?
Yes, but…

Caution #1 – Make sure your gnomon and hour lines will be calculated and designed for your specific latitude and longitude.
Caution #2 – Make sure you are asked for your specific latitude and longitude or your address, if the maker will find your latitude and longitude for you. The use of your ZIP code to determine latitude and longitude is not precise enough because many ZIP code areas are too geographically large to insure an accurate sundial at your specific location.

 
Is it difficult to build your own sundial?
No:
+ If you have good instructions and an expert makes the calculations and prepares the layout for your specific latitude and longitude
+ It can be done in a couple of hours using common tools that most people have in their home
+ In fact, it makes an excellent project for school classrooms, Scouting, 4H and other youth organizations
+ It’s a good learning experience for anyone who undertakes it

 
How does the Equation of Time affect sundial accuracy?

It is important that your sundial display as accurate clock time as is possible because there is one correction that can’t be built into these dials, and this correction is for The Equation of Time. All stationary sundials will need to be mentally “corrected” for this factor to achieve accurate clock time.

This variation from clock time changes each day of the year causing the sundial’s time to appear to be “fast” by as much as 16 minutes or “slow” by as much as 14 minutes throughout the year. This variation has two main causes; (1) The earth moves more quickly in it’s elliptical orbit around our sun when closest to it and more slowly when it’s farthest away, and, (2) The sun “travels” across our sky on the ecliptic, as opposed to the equator.

Don’t let this phenomenon alarm you. It has its least effect in the months of April, May, June, July and August when a sundial in North America will likely experience its heaviest use. The maximum deviations during these months are from 4 minutes “fast” to 6 minutes “slow.” Accurate Sundials includes an ‘Adjustment for The Equation of Time Table and Explanation’ with your sundial purchase.

 
Why do garden centers and nature stores sell sundials that are not accurate?

Reason #1 – It is inexpensive to mass-produce sundials which are identical with no concern as to whether they will display accurate clock time.

Reason #2 – The seller doesn’t know that their dials do not display accurate clock time.

Reason #3 – The buyer doesn’t know that these dials do not display accurate clock time.

Reason #4 – Some buyers are looking for a lawn ornament and have no concern as to whether their sundial will display accurate clock time and no desire to properly set up their dial.

 
Please call us at (262)365-3610 for more information.