# What is the best paint for optimal visibility and contrast on a sundial face?

• Optical
• sophiecentaur
In summary, the person is making a sundial based on an armillary. The image shows the principle and it's fixed to a sunny wall; it's only half finished as yet. The bar is aligned with the polar axis and the arc with the horizontal plane. The gnomon throws a shadow on the arc (a nice straight line). The angle of the arc is adjustable to compensate for the Equation of Time, using a calibration graph. It certainly works but
sophiecentaur
Gold Member
I am making a sundial, based on an armillary. The image shows the principle and it's fixed to a sunny wall; it's only half finished as yet. The bar is aligned with the polar axis and the arc with the horizontal plane. I have omitted the higher parts of the arc so it doesn't get in the way around the equinox. That bit isn't functional of course but it would adds strength. The bar is the gnomon and throws a shadow on the arc (a nice straight line).
I intend to make the angle of the arc adjustable to compensate for the Equation of Time, using a calibration graph.
It certainly works but I want to optimise the visibility / contrast of the shadow. Given time, I could improve, step by step but it could involve a lot of spray paint cans.

So I'm torn between white and silver and between matt, silk and gloss. (Gloss is not really a starter I think because I don't want a mirror image of the gnomon). That wide reflective band could produce extra light which could fill the shadow. I expect some lenticular design could give the best result but that's too hard guv.

So what should I go for, do you think? I can't afford gold leaf.

Matt white = glass enamel, like an old cast iron bathtub.
One problem to consider is rust staining of the white during later life.

Have you computed the diameter of the gnomon rod, to be slightly greater than the 0.53° angular diameter of the Sun, to maximise the blackness of the shadow centre, while minimising the shadow width ?

hutchphd
I plan to use many coats of paint! But matt seems like a good idea - except for the scatter from other parts of the arc?

My rod is, at the moment, 11mm , to be strong enough to support the arc. But I do take your point about the optimum shadow, [Edit:It's about 2 degrees (not 12!! - which would be the limit of the penumbra perhaps), which would be 1/7 hour wide]. But I can always look for the centre of the shadow . . . . It's obvs work in progress.

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I have a black chain link fence in back yard and much of the rail is galvanized rail spray painted semigloss black with rustoleum enamel. About four very light coats...I touch it up every couple of years (takes ten minutes!). It looks great. I've used very little paint over 15 yrs (five cans at the most including initial painting).
It strikes me that having some optical figure in the shadow screen will enhance the visual contrast in the shadow. The figure should be maybe X10 smaller than the shadow width. Either applied or maybe stencil ?

hutchphd said:
It strikes me that having some optical figure in the shadow screen will enhance the visual contrast in the shadow.
I'm planning for black vinyl lines and Roman numerals of the same. Easy to re- apply and very long lasting, as I found when I used that system for name and number markings on my boat. Out in all weathers without losing a good black surface. Unless I were to use engraving in white enamel, I don't think it could be improved on easily. The vinyl letters and symbols are dead cheap, too.

Thing is, I'm hoping to produce some sort of shadow, even with some finite cloud cover. As I look out of my window at the moment (10.33 solar time) the sun is actually visible but what I see is about 10 degrees across. I can make a shadow with my finger on my hand and, although it's diffuse, I can locate the centre of it. I'd bet those ancients had all sorts of techniques to get the best out of their sundials. I will test my estimating abilities when it's installed. But the quality of that shadow needs to be as good as possible - hence the need for an optimal surface.

hutchphd
Not an expert, but a quick 1/2 Hr. on the Internet brought up these possibilities.

If you have a full-sky view, which you don't, this would be my first attempt.
Since cloudy skies tilt the ambient spectrum towards Blue, I suspect a light blue (egg shell blue) would be a good choice for paint color.

In your case with mounting on a wall, maybe paint in a tint to block the wall color.

(In other words, Experiment. )

sophiecentaur
For best precision, you'll need a 'second' coat. I crack me up.

sophiecentaur, hutchphd and berkeman
It'll give me hours of fun and I'll go into minute details.

Dullard and hutchphd
We'll all learn something - It will be in hour best interest.

The white spray paint I used seems to be pretty good and, on the rare occasions when the Sun is almost present, the shadow is very visible.

sophiecentaur said:
So what should I go for, do you think? I can't afford gold leaf.

Because gold is very malleable, gold leaf may be less expensive than you expect:
Booklet of 25 3.4" squares for \$57

Gold leaf looks lovely, I agree. But it looks too opulent. All the local villains would assume we were rich and be climbing in all over the house.
And I did already cost it out, along with gold paint. Essex has enough bling, in any case.