Why every astrology app is purple

Every astrology app is purple, and purple is a promise with a date on it. For most of recorded history it was the colour you could not have. Tyrian purple came from sea snails of the murex family and the yield was very small. The historian David Jacoby put it at twelve thousand snails for about 1.4 grams of pure dye, enough to colour the trim of one garment. Theopompus, writing in the fourth century BCE, reported that purple fetched its weight in silver at Colophon. By the fourth century CE, Roman sumptuary law had narrowed the full purple robe to the emperor alone.

Then it stopped being expensive, and quickly. In 1856 an eighteen-year-old chemistry student named William Henry Perkin, trying to synthesise quinine, oxidised aniline with potassium dichromate and found purple in the flask while cleaning it out with alcohol. By 1859 the colour had a name in England, mauve, and a fashion to go with it; Punch grumbled that the mauve measles were spreading seriously enough that somebody ought to consider how they might be checked. By 1870 the fashion had moved on. The synthetic dye industry it started did not.

That is the mechanism worth carrying forward. The cost went away and the meaning stayed. Purple has been cheap for a hundred and seventy years and it still reads as expensive, because a colour keeps its old promise long after the thing that made the promise true has stopped existing. Anybody choosing a palette is choosing which dead promise to borrow.

Now the palette itself. An astrology app is four colours: indigo close to black for the ground, a violet gradient over it, often running toward magenta at one corner, small white dots standing in for stars, lettering in gold or warm cream with serifs on it. The furniture is nearly as fixed. A thin crescent moon. A hairline circle with figures traced inside it. Capitals spaced wide apart. Somewhere, a soft glow behind an object that nothing is lighting.

None of that is a picture of a result, and that is the whole subject. Underneath the violet there is a table of numbers, and a table of numbers has no colour. The palette is a sentence spoken to you before you have read a word, and money is only the first of the three promises it borrows.

The second promise is the inner cosmos, and it is younger than the first by two thousand years. In 1927 Charles Leadbeater published The Chakras and laid the seven colours of the rainbow onto the seven centres, which is where violet at the crown of the head comes from. Those colours are not ancient. Kurt Leland, who traced how the Western version of that system was assembled, found that its two main elements, the rainbow colours and the modern list of qualities, first appeared together only in 1977.

I am not scoring a point off anybody's practice. What interests me is the route. Violet as the highest colour was assembled in English, largely by Europeans writing about India for readers in Europe and America, and travelled out from there into the world's supply of spiritual design, including design made in India for Indian users.

When jyotisha (ज्योतिष, jyotiṣa) hands out colours of its own, it hands out a different set. Take the navaratna (नवरत्न), the nine gems, one to a planet: ruby for the sun, pearl for the moon, red coral for Mars, emerald for Mercury, yellow sapphire for Jupiter, diamond for Venus, blue sapphire for Saturn, hessonite for Rahu, cat's eye for Ketu. Red, white, red, green, yellow, colourless, blue, orange-brown, yellow-green. No purple anywhere in it. What that list is worth and what it costs at the counter is Nikhil's subject, not mine.

The colour with no wavelength

The third promise is the sky, and here the palette quietly contradicts the thing it is standing for.

There is no purple in a rainbow. Violet is a wavelength, sitting at one end of the visible band. Purple never appears on that band at all. Every purple is a blend of light from the two far ends, red and violet reaching the eye together; no single wavelength makes a purple. On the chart colour scientists use, the purples sit along a straight edge drawn between those two extremes. It is called the line of purples, and it is a straight line because nothing measured falls along it.

The night sky gives the eye very little colour either. In dim light the cone cells that carry colour vision stop working and sight runs on the rods, which record how much light arrived rather than what wavelength it was. That is why stars look white, and why a famous nebula in a small telescope is a pale grey smudge rather than a coloured cloud.

So the purple nebula comes from photographs, and the photographs are coloured on purpose. A telescope takes separate exposures through narrow filters, each admitting the light of one element, and each exposure is then assigned a channel. In the Hubble photograph of the Eagle Nebula taken on 1 April 1995, the picture almost everybody has seen, sulphur was assigned to red, hydrogen to green and oxygen to blue. None of that is concealed. The colours are picked so that structures which would otherwise lie on top of one another come apart on the page, and the result is an instrument's output rather than a view.

Set the three promises side by side. The gradient behind the sign-in button is a colour with no wavelength, standing for a sky very nearly colourless to the naked eye, borrowed from pictures an astronomer coloured deliberately for a different job. Nobody along that chain did anything dishonest. Only the last step is a design decision, and design decisions can be examined.

Written out flat, what the four colours promise is depth, night, age, something a little out of reach, a room you are meant to be quiet in.

Now the product. It takes three fields: a date, a time, a place. It looks up positions in an ephemeris, applies a chosen starting point for the zodiac, draws a diagram, and prints tables underneath. Sunrise for a named city. A list of dated periods. A score out of thirty-six. That is spreadsheet work, done well or done badly, and none of it is secret.

The gap between how the app looks and what it actually does is not quite a lie. It is a mismatch, and a mismatch changes what a reader feels free to question. A screen that looks like a railway timetable invites checking, because a timetable is a claim about the world and everybody knows claims can be wrong. A screen that looks like the inside of a planetarium invites nothing. An atmosphere is not the sort of thing anybody audits. And something here genuinely wants checking: hand two apps the same birth details and they will often put your moon in different signs, for reasons Meghnad has worked all the way out. The arithmetic is public. The surface does not look like a thing with arithmetic in it.

Ornament that is also data

The alternative is not plainness. Instruments have been beautiful for as long as there have been instruments. The question is whether the beauty is load-bearing.

Pick up an astrolabe. The showiest part is the rete, the pierced openwork plate that turns on the front, and the standard descriptions of it read like a list of ornaments: pointers cut in the shape of balls, stars, snakes, hands, dogs' heads and leaves, with star names engraved along them in Arabic or Latin. Every one of those curls ends at a star, and the tip is where the star is. Under the rete lies a flat plate engraved for one latitude and no other, so a working astrolabe carries a small stack of them and you swap plates when you change cities.

That is ornament that is also data. The prettiest thing on the object is the part doing the work, and the object announces its own limits on its face: this plate, this latitude, these named stars. It is decorated to within an inch of its life, and every decoration on it can be questioned.

The rules that govern the pictures on this page are written down, and one line in them refuses the glowing zodiac wheel outright. Stating a preference in advance is the easy half of it.

Where the purple stops

Which brings me to the part you can check in a minute, on your own phone.

The costume is not worn evenly. Open an astrology app and go looking for the edges of it: the account page, the list of subscription options, the terms, the help screen, the store listing the app arrived from. In most of the ones I have gone through, the nebula thins somewhere along that path and then stops. Plain type on a plain ground. Rows with small arrows at the right. A price, a renewal term, a paragraph of ordinary prose in the phone's default typeface.

There is nothing sinister in that seam, and some of it is not the designer's doing. Certain of those screens belong to the operating system rather than the app, and others sit in a default state because dressing them would have cost money for no return. But the seam is worth finding, for where it falls. The half of the product that is meant to be felt gets the violet. The half where a number has to be exactly right gets the system font.

So go and find it, in whatever is already installed. Note the last screen that is still purple and the first one that is not. Then go back to the opening screen, the one with the crescent and the glow behind nothing, and ask which half of the product it has decided you are standing in. The numbers under the gradient did not move by a decimal when the colour did. What moved was your sense that you were allowed to look at them.