Sundial Rock for the Garden

Sundial Rock for the Garden

There are several flat rocks in my backyard.

Why are they just lying around?

They should do something!

They could be useful couldn't they?

.

I know... one of them could be a sundial -

.

I could have a quaint (Massive, Bouldery, Unwieldy Rock - Rock is heavy!) sundial in my backyard!

Sundials have been around for a long time (since at least 1500 BC) but not as long as my lazy rocks.

There are many different types of sundials: Equatorial sundials, Horizontal sundials, Vertical sundials, Polar dials, Vertical declining dials, Reclining dials, Declining-reclining dials/ Declining-inclining dials, Spherical sundials and more according to Wikipedia. Sounds like sundials are overachievers. Not my rocks.

Apparently one of the easiest to make is a horizontal sundial - Easy sounds good -- So off on my sundial odyssey...

The Rock you want to turn into a Sundial

Dark Brown and Gray Spray Paint

Silicone Sealant

Roman Numeral Stencil (self-adhesive or cardboard)

3/8" Oak Dowel

Polyurethane / Stain

Masking Tape

Cardboard and Newspaper

TOOLS:

Drill Motor

12mm diamond drill bit

Spirit Level

Protractor

Magnetic Compass

Drafting Compass

Square

Rule

Angle Gauge (Optional - Cut a cardboard angle template instead)

Utility Knife

Cricut cutting machine - Not Shown. Optional to cut out stencils

Glue gun - Not Shown

Sundials indicate time by casting a shadow. That shadow is affected by many factors.

In order for sundials to work correctly they have to be designed for their specific location (unless you are the self-calibrating one found on Instructables)

A horizontal sundial must be horizontal (level) - go figure!

And a sundial must be oriented North (in the Northern Hemisphere). Not the 'North' a magnetic compass points you to, but TRUE North. Magnetic North and True North are not the same thing. They are actually about 250 miles (400 km) apart right now.

True North is where the North Pole of the Earth is located. It's fixed. Magnetic North is this vagabond location that is not at the North Pole and has the tendency to wander about over the years at its own pleasure - You want True North.

You can use a compass to determine True North, but you will need to know your magnetic declination, which is the difference between True North and Magnetic North for your location. You can go to the NOAA website and enter your street address (right hand box) to find out your magnetic declination.

The declination is added or subtracted from magnetic North depending on whether you are East or West of the Agonic Line (where magnetic North points toward True North) in order to identify True North at your locale. My location required I add 11 degrees to magnetic North to point me toward actual North Pole of the Earth.

Now more geography.... The gnomon is the part of a sundial that casts the shadow. The gnomon must be angled at an angle above horizontal that is equal to your latitude (your location 'between' the equator and the North Pole). The magnetic declination NOAA website will also provide you with your latitude.

Image credit: USGS Magnetic Declination Lines

Gnomon angle image credit: http://www.anycalculator.com/horizontalsundial.htm

I googled "make a horizontal sundial" and read multiple sites full of instructions. Hack-a-Week has a nice tutorial. Look around for a tutorial if you want to know the finer points of sundials.

I took cardboard and cut a triangle for my gnomon with an angle of 40.5 degrees, which is my latitude.

I drew a straight line on my cardboard base and glued the gnomon onto it. I added another piece of cardboard as a 90 degree brace to keep it upright.

I took my magnetic compass and used my magnetic declination to determine True North and aligned my centerline to True North.

During the day I placed a mark on my cardboard sundial on the shadow's location on the hour to lay down my hour markers.

After my cardboard attempt I felt ready to tackle the rock.

The gnomon I will be using is an oak dowel. To mount the dowel I needed to drill a hole in the rock at a 40.5 degree angle (the same angle as my latitude). I used an adjustable angle gauge to keep my drill on target. You could just as well use a cardboard template.

The gnomon, in addition to being angled (at the latitude angle), needs to point to True North. (It actually points toward Polaris, the North Star -- hard to align in the daytime - hence compass and latitude info.)

I drilled my gnomon mounting hole larger than the dowel diameter so that I had room for some adjustment. Rock is pretty unforgiving so I needed some way to correct my errors.

I drilled down until my chuck rubbed on the rock - time to stop. My bit leaves a plug in the stone. I tapped an awl in the gap to snap the rock plug loose.

I temporarily installed an unfinished dowel as my gnomon. I checked that the angle was equal to my latitude. I also leveled the rock in both axis, as much as a naturally 'unflat' rock can be leveled.

The gnomon, in addition to being at your latitude angle, must also be pointed (actually the centerline it is above) to True North. Use your compass skills from the cardboard practice to get your gnomon lined up with True North.

Every hour on the hour I marked the shadow with a piece of tape. The angles between the hours are not equal but vary during the day with the narrowest around noon and the widest at sunrise and sunset.

I took a drawing compass and scribed out an arc of how close I wanted the hour marks to get to the gnomon. I took a ruler and measured out where I wanted the markers to end out on the perimeter where the numerals will be.

I took masking tape and masked out the areas for the markers for the hours. I surrounded the masked area with newspaper to confine the overspray.

After making sure all the masking tape was stuck down as well as I could get it, I sprayed the dark brown paint over the masked areas for the hour markers. I tried to spray as nearly vertical onto the areas in order to minimize any feathering that might come from loose tape.

After a few moments to let the paint dry, I removed the masking to have a look at my markers. Not too bad.

I vacillated between Roman and Arabic numerals. It seemed that more 'traditional' sundials had Roman Numerals so I ultimately decided on Roman Numerals.

It also happens that Roman Numerals have no unattached 'floating' stencil parts like the middle of an Arabic 6, 9, or the loose triangle inside a 4 - YES! A bonus!

I had the Cricut cutting machine cut out Roman Numerals on self-adhesive vinyl to create self-adhesive stencils.

At this point you need to make a decision.... Have your sundial read Standard Time or Daylight Savings Time?

Daylight Savings Time will have your Sundial off an hour for part of the year and Standard time will have it off an hour during Daylight Savings Time. Standard time would put your 12 'o clock on the North axis. Daylight Savings would have you have 12 o' clock on the hour marker earlier.

Daylight Savings Time continues to cause problems - Now for sundials, who knew?

I placed the vinyl stencils down in the appropriate locations, applied masking tape to help hold them in place and surrounded the area with newspaper to keep the overspray in check.

After patting down the vinyl a second time I went ahead and spray painted the Roman Numeral stencils with gray paint.

After a bit of drying time I removed the stencils and newspaper.

I gave my gnomon dowel a slight point - not too pointy as to be a hazard. I finished my gnomon with polyurethane / stain and mounted it in the hole in the rock. I thought silicone sealant would be a better choice than epoxy to fix the dowel in place. Silicone would allow easier replacement of the gnomon or changing to a metal rod, if I want in the future.

My lazy rock has now become useful and serves a utilitarian purpose in my backyard!

Now I just need the other rocks to fall in line.... Maybe stepping stones are an opportunity?...

Enjoy!