Create your skyCreate

Six layers, one sky.

Each artwork is built in layers, from the galaxy to the ground, then graded like an astrophotograph. Here is one of them, taken apart.

Edelweißspitze6 September 201921:00
Build the sky up to

Layer by layer

For each layer: the data it comes from, what in it is true to the sky, and what is drawn by us.

The Milky Way layer alone: a bright band of star clouds crossed by dark dust lanes.

01

The Milky Way

Our own galaxy, seen from inside it.

Its band holds more stars than any catalogue lists one by one. As the template we take NASA's Deep Star Maps, made from the measurements of ESA's Gaia mission: where the galaxy is bright, where dust hides it, and what colour it has.

Then we draw it again as single stars. Their number follows the map, so the dust lanes empty out and the star clouds fill up. A view of the summer Milky Way holds about two million of them.

Data
Deep Star Maps 2020, NASA's Scientific Visualization Studio
True
Where the galaxy is bright, where dust hides it, and where it stood at your minute.
Drawn
The single stars. They are true as a crowd, not one by one.

In this sky

  • 12.5° the centre of our galaxy above the horizon
  • 26,700 light-years to the centre of our galaxy
The deep-sky layer alone: small pink and pale nebulae and clusters scattered on black.

02

Deep sky

Nebulae, star clusters and other galaxies.

Most of them are too faint for the eye. Their positions, sizes and kinds come from the OpenNGC catalogue. We draw the showpieces of the whole sky, a few hundred of them.

The best known are drawn from photographs by the Hubble Space Telescope and the European Southern Observatory, with the stars removed. The others are computed from what the catalogue says about them: the type and the tilt of a galaxy, the kind and the size of a nebula.

All of them are drawn larger than they appear: the small ones two and a half times, the large ones less.

Data
OpenNGC; photographs by NASA/ESA Hubble and ESO
True
Where each object stands, and what kind of object it is.
Drawn
Their size, and one palette of colours for all nebulae.

In this sky

  • 55 nebulae, star clusters and galaxies in this print
The star layer alone: thousands of catalogue stars, a few bright ones in orange and blue-white.

03

Stars

Every star of this layer is a real one.

The Hipparcos and Tycho catalogues of the European Space Agency list two and a half million stars with their measured position, brightness and colour. Every star of this layer comes from them, set where it stood at your minute.

The colour of a star tells its temperature: cool stars are orange, hot ones blue-white. We keep the colours gentle, as a photograph shows them.

Data
Hipparcos and Tycho catalogues, ESA
True
The position, the brightness and the colour of every star.
Drawn
The soft glow around the bright ones.

In this sky

  • 2,267 stars above the horizon bright enough for the naked eye
The layer of the Moon and the planets alone: Saturn with its rings, the half-lit Moon and Jupiter.

04

Moon and planets

The part of the sky that never repeats.

NASA's Jet Propulsion Laboratory has worked out where the Moon and the planets are for every moment from 1900 to 2050. From its ephemeris we take their positions for your minute, as seen from your place.

The Moon is a photograph taken by our founder, lit again for the phase of your minute. Saturn's rings open as far as they did that night. The surfaces of the planets are drawn, not photographed.

To the eye the Moon is half a degree wide and a planet is a point of light. We draw the Moon as it looks through binoculars that magnify eight times, and the planets as through a telescope that magnifies 250 times, so that you can see the phase and the rings. The Almanac names them. Where two of them overlap, the nearer is in front. Neither is moved.

Data
Ephemeris DE421, NASA JPL; the Moon photographed by Christopher Granabetter
True
Position, phase, tilt and ring opening.
Drawn
Their size. The caption on every artwork says so.

In this sky

  • 57% of the Moon lit
  • 25° the opening of Saturn's rings towards us
The atmosphere layer alone: deep blue above, a warm glow low along the horizon.

05

Atmosphere

The air between you and the stars.

It decides how dark the sky was. We work out how far the Sun stood below the horizon: down to 18 degrees there is twilight, brightest in the direction of the Sun. A moment in daylight is shown as day.

Moonlight is added by the phase and the height of the Moon. The warm band low along the horizon belongs to the look of the artwork: it gives the sky its colours, from amber to blue.

Data
The positions of the Sun and the Moon
True
Day, twilight or night, and the side the twilight stood on.
Drawn
The colours and the glow along the horizon. Clouds are left out: we show the sky above the weather.

In this sky

  • 13.8° the Sun below the horizon
The terrain layer alone: the outline of a mountain ridge, paler with distance.

06

Terrain

The skyline of your place, as you would see it standing there.

It comes from the Copernicus elevation model, which knows the height of the land every 30 metres. From your place we look out in every direction, as far as 200 kilometres, and find the ridges that close the view.

The curve of the Earth and the bending of light in the air are part of the calculation. So a far range stands as low as it really does. Each ridge fades with its distance, the way haze fades mountains.

The model includes trees and houses, so we leave out the first kilometre around you.

Data
Copernicus WorldDEM-30 and WorldDEM-90
True
The outline of the land against the sky, seen from your place.
Drawn
A dark silhouette in haze, without the first kilometre.

In this sky

  • 2,571 m above the sea: where this sky was seen from

Then, one grade.

The six layers are kept as light, in the numbers a camera sensor records. A single step turns them into the picture.

It does four things:

  • Sets the black of the sky
  • Lifts the faint light, the way astrophotographers stretch their exposures
  • Lets the bright bodies glow a little
  • Darkens the corners slightly

The preview you see while you create your sky uses the same step as the print. So the print looks like the preview. The same place, minute and view always give the same picture, pixel for pixel.

What we never change

Where things are. The Moon, the planets, every star you can see with the naked eye and every catalogued nebula and galaxy stand at their true positions. We never invent an event.

What we change, and say so

How large things appear. To the eye the Moon is half a degree wide and a planet is a point of light. We draw them larger, so you can see the phase of the Moon and the rings of Saturn. The caption says so. Ask for the true sky and everything keeps its real size.

The data behind every layer, and who made it

Create your sky

Enter the date, the time and the place. Turn the sky to the view you want, add your words, and choose how it should reach you.

Create your sky