How the sky is worked out

For the curious: what the dial computes, what was checked, and what was simply decided.

Noon is an event.

The top of the dial is not twelve o’clock. It is the moment the sun stands highest over the place where you are. The dial learns where that is from GPS and works out the rest from the sun.

The sun keeps its own time

Against a steady clock, the real sun runs a little fast for part of the year and a little slow for the rest, by up to about a quarter of an hour.

The dial follows the real sun.

Keyed to the sun’s height, not the clock.

Every color on the dial is tied to how high the sun is above or below the horizon, measured in degrees. The clock never enters into it.

The whole sky, not one point of it

Each color is the average of the entire dome of sky at that moment. Not the color straight overhead, and not the color at the horizon.

It is the reason the sunsets on the dial look soft. The softness is correct.

Dusk comes in three stages.

The dial spends its precision where the sky spends its.

The sun does not know which way it is going

Take the moment when the sun is six degrees below the horizon. It happens twice a day. Once on the way down. Once on the way up. Physically, those two moments are identical.

And yet nobody in the history of the world has ever confused a sunrise with a sunset.

The difference is real. It just isn’t in the sun. It is in the air. Evening air is not morning air, and that is what your eye is actually reading.

So the difference on this dial is a decision. Evening holds its warmth a little longer; morning clears a little sooner.

That asymmetry was chosen rather than computed, and I would rather say so here than have someone find it later.

They are the real ones.

Each is placed where it truly is for the date, the hour and the place where you are standing.

The sky comes out one star at a time

A star is drawn only when it is truly above your horizon, and only once the sky is dark enough to show it. So at dusk the brightest arrive first and the faint ones last, as they do outside.

Each in its own color

A star’s color on the dial comes from its measured color in the catalogue. Of the 553, 551 were matched star by star. Two had to be estimated.

Painting a light that isn’t there yet

Go outside an hour before sunrise and the sky is black. Half an hour before, it isn’t — but you would struggle to say what has changed. You simply know the night is ending.

The obvious approach is to add a glow along the horizon where the sun is going to come up. Try it and you get a lamp. Not a dawn — a lamp, switched on behind a hill.

Dawn is not something arriving. Dawn is the darkness letting go of something it had been holding.

36,576 comparisons.

The dial’s solar arithmetic was run against a full astronomical reference for every one of them.

Within twenty seconds, every time

The worst case is the last stage of dusk, far north, in high summer, when the sun slides under that line so shallowly that the moment is hard to pin down. Every other event is under five seconds.

Asking the question at the wrong moment

There is a standard way to calculate where the sun is. It is in textbooks, it is correct, and used the obvious way it can be wrong by up to six minutes. The fix is to do it twice.

And the part I find genuinely funny: doing it a third time makes it worse again. Like over-correcting a steering wheel.

The timing on this dial has been measured. The brightness has not been measured, and is not claimed to be.

It was built to look like what happens, using color drawn from the real sky, and it does. It is not a photometric simulation, and a dial on a wrist is not a light meter.

The figures above test the arithmetic. Each Skydial is then checked on the bench against the original dial before it ships.