created by Carl Sabanski
Standard Time Equatorial Sundial Kit
Equatorial Sundial: a dial in which the dial plate is parallel to the equatorial plane and the polar-pointing gnomon is perpendicular to it.
Vernier: a small moveable scale for obtaining fractional parts of the subdivisions of a fixed scale. Invented by Pierre Vernier in 1631 (published Brussels, 1638). For circular scales, a Type A vernier has a central zero.
The equatorial sundial is a universal dial and can be used anywhere in the world. All you have to do is position it correctly and it will tell you the time. It is quite easy to make as the hour lines on the sundial are placed at intervals of 15° for every full hour. Because of the equal hour intervals it is possible to correct the sundial for longitude and the Equation of Time (EoT) just by rotating it. The longitude correction is constant but the EoT changes each day. The adjustment must be made every day for maximum accuracy. Once done the sundial will indicate standard or clock time.
Figure 1 shows the Kit that you will use to make your own standard time equatorial sundial. This kit only include the upper dial plate of the equatorial sundial. It will only work in the summer, from the spring to the fall solstices.
Don't forget to get a "Correct-A-Dial" or "Correct-A-Dial II" to help your with the correction for longitude and the EoT.
The sundial is also different as it is not made completely from paper. The design is the result of a request for a sundial that could be used for outdoor events but could tell clock time. If you wish to build one for this purpose it is recommended that all the paper parts be laminated. All the structural parts of the sundial were made from 1/2 inch rigid styrofoam board. To give the edges a clean finish you can use metal foil duct tape. The tape has an adhesive backing and bonds well to the styrofoam. To glue all the parts together I recommend you use a silicone rubber adhesive called "RTV". It provides an excellent bond and does not warp.
Figure 1: Standard Time Equatorial Sundial Kit (CAD)
is designed so that you can build one for the
Northern or Southern Hemisphere. If you would like to download a
PDF file of the "Standard Time Equatorial Sundial Kit" just open the
door that works for you.
There is another version of this sundial that can be used all year. The one kit will allow you to build a "Standard Time Equatorial Sundial" for either the Northern or Southern Hemisphere. One difficulty with this sundial is keeping the lower dial plate from moving away from the back plate. A solution for this is to use a "Post-It" to hold the dial plate in position. Perhaps you can come up with a better idea.
There is a unique feature incorporated into this sundial...a vernier scale. A vernier scale allows you to measure or set linear or angular measurements much more accurately than you could do by eye. The measurement scale must be evenly divided whether it is straight or circular. This sundial has three vernier scales. One vernier scale is use to set longitude and EoT corrections to 1 minute on a sundial that has hour lines incremented at 5 minute intervals. A second vernier scale is used to set the angle of the dial plate to the nearest 1/4º for a latitude scale spaced at intervals of 1º. The third vernier scale is used to estimate the time to the nearest minute but has the limitation of only being able to work on the circular portion of the sundial.
How To Use A Vernier Scale.
Let's look at a general example that will illustrate how to use the dial plate vernier scale to adjust the sundial for longitude correction and the Equation of Time. This applies to any other time that may need to be set.
Figure 2 illustrates the dial plate adjusted so the red arrow points to +20 minutes. This is a sundial that has been determined to be slow on a particular day. Now what if the sundial is 21, 22, 23, 24 or 25 minutes slow. By aligning the vernier +1, 2, 3 and 4 minute lines with the first line to the left on the ±60 minute measurement scale the sundial will be adjusted by 21, 22, 23 and 24 minutes respectively. The 25 minute adjustment results in the red arrow now pointing to the +25 minute line. This adjustment is easily and accurately made for any 1-minute time interval on the ±60 minute measurement scale. Be sure to use the + vernier scale when adjustments are made on the + side of the measurement scale and the - vernier scale on the - side.
Figure 2: Vernier Scale (CAD)
Figure 3 is a close-up of Figure 2 to better illustrate the use of the vernier scale. The spacing between the lines is equivalent to 1, 2, 3 and 4 minutes. When the 21 minute lines are aligned the spacing of next 3 sets in the sequence are reduced by 1 minute. And so on.
Figure 3: Vernier Scale Close-Up (CAD)