Digital sundials
Slashdot is reporting a fabulous device for scientists who want to get back to nature - a digital sundial. And no, it’s not just solar power clock. It uses no electricity, and has no moving parts - just like a “real” sundial.
So how does it work? Using a clever series of slits cut in the screen, sunlight shines through one side and illuminates the shape of numbers on the other. As the sun moves, the beams of sunlight move across and light up different slits (squint your eyes on that image!) creating a new number but still in the same spot! It’s the same principles as some rulers, Tazos, etc that change their picture when you move them back and forwards. The clock works in 5 minute increments, with the numbers fading between each other, so it’s possible to interpolate a little to get better accuracy.
What I (as a physicist!) found most fascinating was how they compensated for the “non-linearity” of the movement of the sunbeams. You can see this by watching the length of a shadow during the day - it changes quickly in the morning and evening, but slowly at noon when the sun is overhead. The sun’s still moving at the same speed across the sky, but the angle the light beams make with a pole (such as a lamp post, or the stick on a normal sundial) changes most when the sun is low in the sky. Think about this - it does make sense!
Therefore, conventional sundials must compensate for this by spacing their hour marks further apart in the afternoon than in the middle of the day. But the digital sundial uses a much neater trick - it uses a layer of refractive medium such as glass or water to compensate. You’ve seen this effect when you look at a spoon in a glass of water - notice how the spoon appears bent? It’s because light from the spoon is being bent by the water. Most importantly, this effect becomes more pronounced when the angle gets shallower, but doesn’t do anything when you’re directly above the water - so we can use it to compensate as the sun gets lower and lower in the sky but not do anything when it’s overhead! Brilliant!
And, best of all, if my earlier suggestions didn’t find you that perfect gift for Christmas, you can pick a tabletop version up for about US$100.