Introduction: Unique 4 Season Vertical Sundial on Slate
Sundials promote wonder and awareness. They are small devices that connect us to very big events.
A Brief Description
This very unique sundial focuses on the 4 seasons only by marking the winter and summer solstice and the spring and summer equinox, major celestial events that define the 4 seasons. A solar noon mark is also included.
(For sundial traditionalist, hour lines and a unique date and time line are an option. The practicality of a 4 season only sundial will be explained in detail later.)
My intent was a high quality sundial project that used easy, low cost construction methods available to everyone, yet also allowed modern fabrication methods for further idea development and creativity.
A 12” x 12” x 3/8” natural slate tile is hand marked with a carbide scribe. Markings on slate produce a contrasting light gray line. Prehistoric technology at its best.
21st century sundial software technology (available online) ensures a math free experience (apologies to all math people). A rigid pattern is used for marking to ensure easy and accurate results.
Want a total upgrade from Neolithic to 21st century? The sundial software allows exporting in file formats to CAD/CAM software such as Autodesk. You can then cnc engrave, laser engrave, or generate stencils for sandblasting, all modern fabrication methods used to mark slate, marble, granite, or similar stone materials.
Stone has an unequaled durability compared to most materials. In the right location, your sundial could be around for a long time.
Solar powered, no batteries required.
1/8" Masonite Hardboard
#8-30 Brass Machine Screw and Nut. (Length as Needed).
Tungsten Carbide Marking Pen
5/32” Masonry Drill Bit
Drill or Drill Press
Coping Saw or Power Jig Saw
Metal Ruler or Straightedge
Light Color Pencil
File and/or sandpaper
! Safety ! Follow all standard procedures to protect eyes, lungs and other body parts.
Protect what you value.
Step 1: "One Who Knows"
The 23.5 degree tilt of our planet’s axis gives unequal sun exposure throughout the year. We experience this as a high summer sun, and a low winter sun. (a nice animation here).
Three lines on the dial define the seasons. A short rod, called a gnomon (Greek – “one who knows”), throws a slowly moving shadow on the face of the slate dial throughout the day.
Our focus is where the tip or very end of the shadow falls on the dial in relation to the three very important lines.
- Winter Solstice -- Upper Curved Line.
- Equinox(s)-- Middle Horizontal Line.
- Summer Solstice-- Lower Curved Line.
The tip of the gnomon’s shadow will trace the above 3 lines on the respective dates. As the year progresses similar lines (unmarked, as the dotted line in the illustration) will be traced daily on the dial in between the three major lines. 182 day lines in all, 1/2 of a year from solstice to solstice.
The shadow's tip will have a corresponding location for every day, and day light hour of the year, within the two solstice lines on the dial.
Using the dial software a specific date line and/or time can be drawn and marked for any day of the year (birthday, anniversary, memorial, etc.) for a very unique personalized dial. Note that the line or mark will be traced twice in the year, on the date and then 182 days opposite that date.
Yes, hour time lines are included in the program and can be used on your dial if desired. I used a simplified 4 season only sundial as I feel it reflects the true nature of solar time as described below.
Solar Time vs Modern "Clock" Time. The key to understanding sundials.
The vertical center line on the dial is the noon line; however this marks solar noon, the sun at its true zenith (maximum height or directly overhead) at a specific location. Solar or sun time and modern “clock” (standard or universal) time that we use every day are slightly different ways of measuring time.
Why two different methods of marking time? Solar time by its nature is very local; meaning solar noon (sun directly overhead) at one town is not exactly solar noon at a town 100 miles east or west because of the constantly revolving earth . During the age of sundials this was not an issue. As technology made the world smaller (trains, telephones, airplanes), everyone had to be on the same page time wise. A universal system of averaging local time was devised using 24 time zones, one time zone for each hour of the day (360/24 = 15 degree segments). All clocks within each of the 24 time zone segments show the same averaged local time for that zone, with one hour jumps between zones. Time zones
Understanding the above, you can now see why sundials can tell very accurate local solar time but not modern standard (clock) time.
The hour lines, if used, in the sundial program (program discussed later) are adjusted to standard clock time for your location, however be aware they are still off a bit and don’t forget daylight saving time adjustments! Once again, two slightly different types of time measurement.
The unique 4 season dial constructed here focuses on solar time exclusively eliminating the complexities of trying to adjust for modern (clock) time.
I chose a vertical sundial as opposed to the more standard horizontal format for practical reasons. The wall of a house meets the rigid placement required for accuracy, and I had a nice south facing wall, optimal for sun exposure.
Step 2: Prepare
Gather your materials. The slate tile was purchased at a very reasonable price on EBay (search "slate tile sample"). Another source could be recycled roof slate, but note the larger the sundial, the more accurate it will be.
Tungsten Carbide tipped pens are very hard marking tools used in metal fabrication, available at major hardware stores or online. Other material/tools should be readily available.
Step 3: Location Data
Determine the latitude, longitude and wall declination for your dial location. Yikes! Sounds difficult but keep reading, you can find the information online in a few minutes using Google Maps!
Few walls face true dead south (mine did! who knew). Most will be positioned, or “decline” east or west by various degrees. To easily determine the three important measurements for your location (latitude, longitude, wall declination) go to sundialzone and follow the directions (you can print out a unique folded paper sundial at this site).
I found the Google Maps wall declination measurement for my location taken from this site very accurate when I compared it with a standard compass. If you wish to double check your wall declination measurement note that compass magnetic north must be adjusted to true north for your location (see NOAA). True north points to earth's axis, compass magnetic north is slightly affected by earth's magnetic field. Google Maps use true north.
Note that my sundial layout is perfectly symmetrical because its location faces exactly due south, for walls declining east or west the dial layout will skew right or left as in the above illustration.
Step 4: Sundial Software
Sundial software is free but most must be downloaded to use. See sundial software. I used the Orologi Solari e Merdiane program found at that site, all following instructions refer to this program. Follow download safety protocol and download from the original site only, using appropriate security scans. I scanned with Microsoft Security Essentials and Malwarebytes and found no issues. Additional dial software can be found online.
Once the program loads, enter your location data in “parameters” under “file” and explore the many possibilities available with the program. A lot of the inputs do not apply to vertical sundials. If you get stuck you can check out the help section.
Step 5: Pattern
When satisfied with the size and selections (tip - the sizing unit takes decimals), print out your dial image. Use the mosaic print feature to print a dial larger than standard paper size, and then tape the pages together. Glue (I used spray adhesive) the paper image on a piece of 1/8” Masonite or suitable hardboard. Note the pattern hour lines I chose not to use.
Now is a good time to give the paper pattern dial a trial run before committing to stone. I taped a temporary pipe cleaner gnomon to the pattern (note the exact length of the gnomon given on the printed paper pattern). Position the paper pattern level on the wall and check for true solar noon (see how in Step 9).
Very carefully cut out along the upper and lower curved solstice lines using a power jig saw or hand coping saw. Important: Use sandpaper or a file to get a smooth, precise, accurate edge. Cut off the patterns outer edges that define the square of the slate.
Step 6: Marking
Position the pattern on top of the slate then c-clamp so it will not move. Now take your carbide scriber and using the rigid edge of the Masonite pattern as a guide, mark the curved upper and lower solstice lines on the slate. Many medium to light pressure passes will give you the best control. It took 20 - 30 or so passes to get a good line.
Focus… one slip and time will be altered for all eternity !! (Actually any slips can be covered with black marker).
Before removing the pattern very carefully transfer/mark the positions of the straight noon and equinox lines (and hour lines if desired) from the pattern edges onto the slate with a sharp light colored pencil. Remove the pattern, then with a ruler and pencil draw the straight lines by connecting those marks.
If you are including a special date/time curved line you will require an additional curved pattern saw cut and scribe at this time. A single mark can also suffice for date/time, perhaps a drilled hole with a slightly raised brass rod inserted.
All of the remaining straight lines are then scribed. The same process as above is used except a rigid metal straight edge (ruler) is c-clamped to the slate as a guide.
Step 7: Gnomon
Transfer and mark the location for the hole to be drilled for the gnomon. I did this by carefully taking measurements from an additional paper pattern I had printed out. Use a 5/32” carbide masonry drill bit. A drill press is best used as the gnomon must be at a true 90 degrees to the dial plate. Use the scribe to make a small guide/starter hole, accuracy matters. The two mounting holes can also be drilled at this time, sized for your mounting hardware.
The gnomon is a common #8 – 32 round head, solid brass machine screw (length to suite) with a brass nut. Non-ferrous metal must be used to avoid rust. Brass all thread (fully threaded rod) can also be used if you can't find a machine screw of adequate length.
The head is filed flat and the backside of the slate hole slightly enlarged and recessed so the screw head will sit flush on the dial back and not interfere when mounting the dial to the wall. I used a small Dremel abrasive stone in a drill press to accomplish this.
Insert the bolt from the back and screw down the nut on the face side. The measurement of the gnomon height shown on the paper pattern must match the height of the screw above the face of the slate exactly. Trim the screw to size. Double check for a true 90 degree orientation to the slate face. If leaning, shim under the head of the brass screw with thin metal (cut from an aluminum soda can or aluminum foil) for a true upright position.
Step 8: Fun
Enough science, now for some creativity. I decided to embellish with the letter N and a stylized arrow pointing to the Noon line which also happens to be true North. PDF file below. The design uses mostly straight lines that are easily scribed with a straight edge. I used a circle template for the circles. I noted that all scribed lines could be carefully free handed after the initial groove depth was established.
Tips - Note the light colored pencil guide lines used. Also masking tape on the back of a metal ruler will provide grip and prevent slippage. Remember, many light to moderate strokes.
I considered painting the lines with a high quality enamel, then carefully wiping the surface so the paint would remain in the grooves, but decided to keep the natural look.
21st Century Technology Upgrade
As previously mentioned, the sundial software pattern can be exported in different file formats for graphic editing and enhancement. Unique and elaborate designs are possible by adding free vector and/or raster graphics. A digital sundial file then allows modern slate, marble, and granite marking processes to be used. CNC engraving, laser engraving, and stencils for sandblasting create a diverse range of creative possibilities. A few ideas:
- House street numbers.
- House built dates.
- Names/dates/times, if special day lines or marks are used.
- Vintage sun images.
- A bee graphic (bees use the sun to navigate).
- Ancient solar symbols
- House longitude/latitude.
- An appropriate motto or quote ("Light, God's eldest daughter."' T. Fuller).
Step 9: Install
Mount using a level along the outer horizontal straight lines of the upper winter solstice line, also ensure the face of the dial is plumb. Use non-ferrous fasteners, I used brass screws with plastic expanding plugs. Drilling exact holes in a masonry wall is difficult, so I drilled the mounting holes in the slate oversize, then shimmed to level using small copper shims in the holes as needed.
You can check your accuracy by determining the exact standard "clock" time for true solar noon occurring at your location, which can be found here. Your sundial is correct if it shows exact solar noon at the standard clock time given for that day and location. Check the solstice and equinox lines on the respective dates. Epoxy non-ferrous metal shims on the backside of the slate if slight adjustments are needed, the length of the gnomon can also be adjusted.
Step 10: Observe
Enjoy a personal connection to a very special star.
Illustration: 19th Century unknown author, public domain image. The Flammarion engraving.
Runner Up in the
Stone Concrete and Cement Contest