Ice Spike Sundial

I like sundials. I like ice. I like mechanisms, in the sense of simple arrangements of parts that produce clever results without human intervention.

I had an idea that combines all these. I spent about a month fiddling with it, before reality reminded me that I am an intellectual dilettante, with barely the manual dexterity to build a cup of iced water. However, as it happens, I have an audience which includes people who are not so encumbered.

So, I present to you my idea for an ice sundial. There is probably a more elegant way to do it, but probably not a cuter way to do it.

Let’s talk about ice spikes.

Here’s the basic idea: Suppose you have some warm water. Then, suddenly the water starts freezing very rapidly. Now and then, on the surface, there will be a small hole. If things are just right, as the water cools and expands, it’ll be forced up through that hole. As a result, the hole starts naturally extruding a “spike.”

If you look on the wikipedia page, the spike in birdbaths naturally resembles the gnomon of a sundial.

So I got to thinking – could you mechanically recreate the conditions in which a spike happens? More charmingly, to my mind, could you have it happen spontaneously by having the right simple setup. I’m not really sure, since all my experimenting failed. But, here’s my idea:


Have one container (let’s call it Container A) half-filled with water. This container should be tall and thin. Imagine a long test tube. The purpose of Container A is to capture some motion. As the water in container A freezes, it will expand, thus raising its surface.

Just above the surface of A, when in liquid form, you have a plunger. As the surface freezes and rises, the plunger goes up.


Atop the plunger is a long arm. The plunger pushes up the base of the arm, so it rotates a few degrees. At the far end of the arm, you have a plastic needle that points downward.


The needle dips into a larger reservoir of water, Container B. The bottom of Container B is such that the container retains heat well, so long as there is light from the sun. The needle from the arm should be such that it just dips below the surface of Container B’s water when no freezing has occurred.


Thus, as water freezes, Container A’s water expands and lifts the plunger, which lifts the arm. The plastic needle slowly rises out of container B, leaving a hole. As this happens, the temperature in Container B drops very rapidly, and (if this idea actually works) an ice spike should form exactly where you had the needle.

Here’s the really cute part – *if* (and it’s a big if) you could somewhat reliably control the shape of the spike/gnomon, you could use Container B’s rim as the surface of a sundial.

Now, you ask, what use is a sundial that only forms at night? Well, yes, fair enough. My hope is that once it formed it’d stick around for much of the day. Alternatively, perhaps there’s some way you could alter the way Container B retains heat after the gnomon is formed. By this means, you could keep or melt the sundial.

That said, there’s something very ghostly and charming about a sundial that appears briefly then melts away.


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