The Other Sky

Everything about this sky is true except the stars.

Your latitude, your longitude, tonight's date, the real angle of the real Sun below your real horizon, the real phase of the real Moon. Only the stars have been replaced — with a galaxy that has never existed, worked out star by star.

building the galaxy…

What this is

an observatory for a sky that isn't ours

You are looking at the sky above the place you actually are, at the time it actually is, worked out with ordinary astronomy. The horizon is your horizon. The Sun is where the Sun is: when it climbs to within eighteen degrees of your horizon the stars begin to go, in the order they really go. The Moon is in tonight's real phase, and when it is up and full it costs you about three magnitudes — which is why the band vanishes for two weeks out of every four.

The planets are the real planets, from the JPL orbital elements. Jupiter is where Jupiter is. It is the only thing here you can check against your own window.

The stars are not.

what was invented

A galaxy. Not a picture of one — a galaxy, star by star. Every star was drawn from a stellar luminosity function and placed in a two-component exponential disk, given a temperature, a luminosity class, a companion or none, a variability or none, and a three-dimensional velocity around a galactic centre that does not exist.

Nothing was hand-placed. The band across the sky was not drawn; it is what you get when distant luminous stars can only be seen through the plane of a disk, seen through the dust that disk contains. The constellations were not designed; each one was grown outward from the brightest unclaimed star until it ran out of near neighbours — which is, near enough, how the real ones happened. Their names are in a language with a real morphology, so every name can be taken apart and translated, and the translation will be the same for you as for anyone else.

how close to real it is

The catalogue was calibrated against the actual sky's star counts, which is the one number that cannot be fudged:

	here	real sky
brighter than mag 1		15
brighter than mag 2		48
brighter than mag 3		171
brighter than mag 4		513
brighter than mag 5		1,602
brighter than mag 6		4,800
naked eye, dark site		~9,100
constellations		88

things worth doing

Drag the light-pollution slider from a city to a desert and watch the band arrive. It is the only control here you could go outside and act on.
Set the time scale to millennia and pull. The figures come apart, because every star has a real velocity and they do not share it. Five thousand years is survivable. A hundred thousand is not.
Find a star, then come back in a few months. The long-period variables will not be where you left them — some of them will not be there at all.
Turn on the sound. Each voice is a star: pitch from its surface temperature, loudness from how bright it looks tonight. The drift is the Earth turning.

the seam

There is one deliberate inconsistency, and it is the point of the whole thing: the solar system is yours and the galaxy is not. Nobody has to explain how that happened. You are standing where you are standing, at the hour it is, and the sky above you is not the one you were promised.