This is a horoscope app that keeps getting distracted. Ask for your reading and it’ll tell you which stars your sign is really made of, how many hundreds of light years apart they are, where to look for them tonight, and why the sun stopped matching your birthday about two thousand years ago. It does the reading too. That part goes badly for the reading.
Eight of the twenty-four modules are playable on this page. The rest are being written.
No sign-up, no birth time, no account. It couldn’t use them anyway.
Pick your sign, see its real constellation
The one you’d say out loud at a party. Module 2 shows you what’s up there. Module 3 turns it side on. We come back to whether it’s yours in module 14.
From modules 2 and 3 of the course
The one you weren’t offered
Leo
the lion
Nearest to farthest, this “shape” is spread across 1,780 light years of empty space.
The pattern everyone gets taught. Nothing up there is joined to anything. The lines are a habit about four thousand years old.
The stars
| Star | Brightness | Distance | Its light left |
|---|---|---|---|
| Regulus (α) | 1.40 | 79 ly | 1947 |
| Algieba (γ) | 2.08 | 130 ly | 1896 |
| Denebola (β) | 2.14 | 36 ly | 1990 |
| Zosma (δ) | 2.56 | 58 ly | 1968 |
| Ras Elased (ε) | 2.98 | 227 ly | 1799 |
| Chertan (θ) | 3.32 | 161 ly | 1865 |
| η Leo | 3.51 | 1,816 ly | 210 |
Lower is brighter. The eye stops at about 6 from a dark field, and nearer 4 from a lit street.
You never see a constellation as it is. You see each star at a different moment in history, all arriving tonight at once.
What’s up there in the constellation of Leo
One of the few that looks like its animal. Regulus is 79 light years away and spinning so fast it is visibly squashed. And η Leonis, sitting quietly in the same sickle, is roughly 1,800 light years off, give or take a few hundred: its parallax is so small that nobody can pin the number down closer than that. Its light has been travelling for something like two thousand years, and the two stars appear side by side only because of where you happen to be standing.
The part of astrology that is just a map
You picked Leo, so this is Leo, tonight, from wherever you’re sitting. This is the useful half: a constellation is a direction you can point in, and working out where it is tonight is arithmetic. Sidereal time, your latitude, done. No interpretation needed, and it’s right every time.
From module 6 of the course
Working out where it is from your clock…
Best in the evening around April, when it stands highest at a civilised hour.
Position comes from your device clock, using your time zone as a stand-in for your longitude. That’s good to about half a degree, and nothing leaves your browser. Latitude is the slider, because that one your time zone can’t tell us.
Why your sign isn’t up on your birthday
Still Leo. Drag through the year and that line is how high Leo stands at nine in the evening on every night of it. The dip lands on the month everybody celebrates in.
From module 7 of the course
Up at nine, about 29° above the horizon.
The sun moves in front of Leo around August and takes it out of the evening sky for weeks either side. Your sign is the one constellation you can’t see on your birthday.
A day is about 4 minutes longer than the sky’s own day, so every star reaches the same spot 4 minutes earlier each night. That’s roughly 118 minutes a month, near enough two hours by the end of a season, and a completely different evening sky by the end of the year. Heights are drawn for 40° north.
The brightest star in your sign is rarely the nearest one
Leo one more time, sorted twice. On the left, the order your eye puts its stars in. On the right, how much light each one actually makes, once the distance is taken back out. Follow a line across and watch it change places.
From module 8 of the course
η in Leo moves the furthest. It’s 1,816 light years away, and that distance is doing most of the work on how bright it looks to you.
Brightness at your eye is output divided by distance squared, so a modest star nearby and a monster a long way off can arrive looking the same. Your eye can’t separate the two, and for a few thousand years nobody could, which is why the constellations were drawn as though every star in them sat at the same depth.
Across all 77 stars in the 13 constellations the sun crosses, the relationship between how bright a star looks and how close it is comes out at 0.06, on a scale where 1 would be a perfect match. There isn’t one. That number is computed in your browser from the same table you scrolled past, not typed into this sentence.
“How much light it makes” is the standard astronomers use: how bright each star would look if they were all moved to the same distance, 32.6 light years away.
Find your birth star: the light that left the year you did
Light takes a year to cross a light year, so somewhere up there is a star far enough away that the light arriving tonight set off the year you did. Put your birth year in. It will find the closest match and tell you where to look for it.
From module 9 of the course
Your star
Denebola
36 light years away, in Leo.
The light hitting your eye tonight left Denebola the year you were born.
This one happens to sit in Leo. That is a coincidence of direction rather than anything to do with you, but it does hand you a word people recognise to point with.
Best in the evening around May.
Look south, about 65° up, which is roughly 7 fists above the horizon at arm’s length.
Near misses
- Zavijava, 1990
- Arcturus, 1989
- Zeta Herculis, 1991
Distances are published parallax measurements, rounded, and every star here is bright enough to see without a telescope. That rules out some closer ones: Barnard’s Star is six light years away and far too faint to find with your eyes.
Where the sun was on your birthday
Give it a date. It’ll check that date against the sky, which are two different things and have been since the Roman republic.
From module 14 of the course
Your horoscope says
Leo
July 23 to August 22
The sun was actually in
Cancer
the crab
Off by one
You’ve been told Leo your whole life. On that date the sun was in front of Cancer, and had been for days. The gap is about 18 days. Leo doesn’t begin in the sky until then.
Twelve slices from Babylon, and why Ophiuchus never got one
The dates aren’t arbitrary and they aren’t mystical. They’re an administrative decision from the first millennium BCE, and you can follow every step of it.
From part III, modules 10 to 13 of the course
01
A road for the sun
Watch long enough and the sun, the moon and the planets all keep to one narrow band around the sky. That band is the plane the solar system got flattened into. Babylonian scribes had the stars along it written down by about 1000 BCE, in a tablet series called MUL.APIN.
02
Cut into twelve
Around the fifth century BCE, that band was divided into twelve equal slices of exactly 30 degrees. Twelve because the moon goes round roughly twelve times a year, and because Babylonian arithmetic ran on sixties, in which twelve is a friendly number. The same two habits gave us sixty minutes, sixty seconds and 360 degrees.
03
Names borrowed from nearby
Each 30-degree slice took the name of a constellation near it. Near, not equal to. The star patterns were never the same size. The sun crosses Virgo for about forty-four days and Scorpius for about six, so the slices and the pictures only ever roughly agreed.
04
Pinned to spring, not to the stars
Greek astronomers, Ptolemy above all, fixed slice one to the March equinox: wherever spring starts, that’s 0° Aries. It was a sensible choice for a calendar. It also quietly made the zodiac a seasonal grid that had stopped pointing at its own namesakes.
05
And then the sky moved
Earth wobbles like a slow top, one turn every 25,772 years, so the equinox slides backwards along that band at about one degree every 72 years. Hipparchus caught it around 130 BCE by comparing his star positions to records 150 years older. Nobody has moved the dates since.
Since the dates were written down, the equinox has slipped about 30 degrees. A sign is 30 degrees wide. That’s the whole discrepancy: your horoscope is running roughly one sign behind the sky, and drifting a further degree every seventy-two years.
The constellations got their final, exact edges in 1930, when the International Astronomical Union drew formal boundaries around all 88 of them. Those boundaries are what the dates on this page use, and they’re why the sun crosses thirteen constellations rather than twelve.
Fine. Here’s your reading.
Generated for your sign, from your sign, the way every one of these is. Read it honestly and rate how well it describes you before you tap the second button.
From module 18 of the course
Is astrology real? It’s been checked, properly, more than once.
Not argued about. Measured, under conditions the astrologers agreed to in advance. This is the short version of a long literature, and it points one way.
From part IV, modules 17 to 21 of the course
1985
The test astrologers helped design
Shawn Carlson ran a double-blind trial published in Nature. Twenty-eight astrologers, chosen with the help of astrological societies, were given a natal chart and three personality profiles, and asked which profile belonged to the chart. They had agreed the method was fair beforehand and predicted they would get better than half right. They scored one in three: exactly what guessing gets you.
2003
Two thousand people born at the same moment
Astrology says a chart is fixed by the minute. Geoffrey Dean and Ivan Kelly followed more than two thousand “time twins”, babies born in London within minutes of each other in March 1958, through more than a hundred measured traits: IQ, anxiety, sociability, ability, occupation. If the moment of birth wrote anything, these people should resemble each other. The correlations came out flat.
Since
And the sun-sign effects that do turn up
Small, and explained by the calendar rather than the stars: your birth month shifts your age relative to your classmates, your first year’s daylight and illnesses, and what your parents were doing that season. Those leave faint traces in the data. They belong to the month, not to Mars, and they don’t line up with what any sign is supposed to mean.
And the force it would have to travel on
The usual defence is gravity: the planets tug on you. They do. Here’s every pull in the delivery room, per kilogram of you, and the two comparisons are more interesting than either side usually admits.
| On a newborn | Straight pull (m/s²) | Tide across your body (m/s²) |
|---|---|---|
| Jupiter | 3.7 × 10⁻⁷ | 2.5 × 10⁻¹⁹ |
| Mars | 1.4 × 10⁻⁸ | 9.9 × 10⁻²⁰ |
| the Moon | 3.7 × 10⁻⁵ | 4.1 × 10⁻¹⁴ |
| the midwife | 1.9 × 10⁻⁸ | 1.5 × 10⁻⁸ |
Jupiter does out-pull the midwife, by about 20 to one. That much is true and usually gets left out of the debunkings. Mars at its closest is beaten by an adult standing half a metre away, and only by 1.4×. The catch is that a straight pull does nothing to you at all. It moves you and the whole room together, which is what falling is, and you feel none of it. The only part of gravity that can act on a body is the difference in pull across it, the tide. There the midwife beats Jupiter by about 60 billion to one, and always will, because tides fall away with the cube of the distance and Jupiter is very far away.
So there’s no force, no mechanism, no measurable effect, and the predictions fail when someone checks them. That’s four different kinds of nothing, and they all have to be wrong at once for a forecast to work.
Everything astrology tells you about yourself
Three claims. Three findings, printed here in full.
From part IV of the course
Impact on your fortunes
This section is empty.
Impact on your disposition
This section is empty.
Impact on your future
This section is empty.
These are supposed to be blank. That’s the joke. Hope the rest of it was educational…
24 modules, from “what’s my sign” to finding it yourself
Every module is four five-minute lessons, about twenty minutes, with something to play with in each one and a Sky Card at the end. Eight of them are playable on this page right now.
Part I
The sign
What You Were Told
The sign, the dates and the personality, and what’s genuinely up there behind all three.
- 1Somebody Drew Lines Between DotsWhat is a constellation
- 2The Stars in Your SignWhat stars are in my zodiac sign
- 3Nothing Up There Is Near AnythingHow far apart are the stars in a constellation
- 4A Sign Is a Direction, Not a DescriptionWhat does my star sign mean
You ask the app for your reading. It agrees, then gets distracted by the fact that your sign is a real object, and that none of it is joined together.
Part II
The back garden
Learning to Find It
The useful half of astrology, which is a map: how to point at your own sign without an app.
- 5The Road the Sun TakesWhat is the ecliptic
- 6Where It Is TonightHow to find constellations in the night sky
- 7Four Minutes Earlier Every NightWhy do constellations change with the seasons
- 8Bright Is Not the Same as CloseWhat is apparent magnitude
- 9The Light Is LateHow far is a light year
The app stops arguing and takes you outside. By the end of the part you can find your sign from a date and a direction, and you know why it wasn’t there last month.
Part III
Babylon
Where the Dates Came From
Twelve equal slices, cut by scribes who counted in sixties, pinned to the first day of spring and never moved again.
- 10Twelve Slices of Thirty DegreesHistory of the zodiac
- 11Why Twelve and Not ThirteenWhy are there 12 zodiac signs
- 12Pinned to Spring, Not to the StarsTropical vs sidereal zodiac
- 13The Sky SlippedPrecession of the equinoxes explained
- 14Your Birthday Against the SunIs my zodiac sign wrong
- 15The ThirteenthOphiuchus the 13th zodiac sign
- 16The Tropics Kept the Old NamesWhy is it called the Tropic of Capricorn
The dates turn out to be an administrative decision with a date on it. Then the sky moves out from under the decision, and nobody updates the paperwork.
Part IV
The delivery room
Why It Cannot Forecast
Four different kinds of nothing: no force, no mechanism, no measured effect, and predictions that fail when someone checks.
- 17The Pull, and the Part That MattersDo the planets affect us with gravity
- 18The Reading Everyone GotBarnum effect explained
- 19The Test Astrologers Helped DesignHas astrology been scientifically tested
- 20Two Thousand Babies, One MinuteTime twins astrology study
- 21The Birth-Month Effects That Are RealDoes birth month affect personality
The app finally does your reading. You rate it. Then it shows you that every sign got the same sentences, and walks you through the times somebody tested this properly.
Part V
The better story
What It Is Actually For
Why a thing with no mechanism works so well anyway, how to spot the same trick elsewhere, and what to do with a clear night.
- 22Why It Works AnywayWhy do people believe in astrology
- 23The Same Trick, Everywhere ElseCold reading techniques explained
- 24Your Turn: Go Find OneStargazing for beginners
Nobody is asked to give anything up. The reading gets handed back with its workings shown, and the sky is offered as the better story, because it is.
Module titles are drafts. The playable ones on this page are built. The rest are written from the same sources, and every number a lesson quotes is computed in the app rather than typed into the copy.
Five minutes. Something to play with. Something to go outside for.
01
Look
A thing you were told, or a thing you can see. Your sign, a date, a shape.
02
Play
You move it before anyone names it. Flip the constellation side on. Drag the latitude. Pick your birthday.
03
Name it
Then it gets its proper name: the ecliptic, precession, apparent magnitude, the Barnum effect.
04
Details
Two short cards, each one tied back to the thing you just moved.
05
Back to it
You answer the question from the top, and this time you can say why.
06
Say it
You leave with one sentence you could say to somebody else without sounding like a textbook.
Then a Sky Card
Every module ends with a one-page card for a clear night: what to look for, which direction, roughly when, and whether you need anything more than your eyes. Print it, take it outside, and the module stops being a screen.
One page for the next clear nightLearn it together
Every module also ends with a Dinner Table Challenge, one question to ask whoever is across the table. It works the same on a nine-year-old and a grandparent, and saying it out loud is the fastest way to find out whether you’ve got it.
Dinner Table Challenge · Module 3
Ask someone: if the stars in a constellation are hundreds of light years apart, who decided they were a picture, and would somebody on a different planet see the same one?
An app that agrees to read your chart, and keeps failing to
In module 1 you ask for your horoscope. The app says yes, then notices your sign is a real place and goes to look at it. Twenty-one modules later it has taken you outside, checked your birthday against the sun, found you a thirteenth sign nobody offered you, and weighed the midwife against Jupiter. In module 18 it finally gives you the reading. In module 19 it shows you what it just did. It’s a running gag with a curriculum attached, and the gag is why the curriculum lands.
One page for the next clear night
These are the cards that end a module. They’re written to be printed, taken outside and used with cold hands. Three of the twenty-four, in full.
Find your own sign, without an app
- Check whether it is even up. A constellation is only in your evening sky for part of the year, and never when the sun is in it, which is the part people find funniest. Your sign is at its best about six months away from your birthday.
- Face the right way. Everything on the zodiac rises in the east, crosses the southern sky (northern sky, if you’re south of the equator) and sets in the west. If you’re facing north from anywhere in the northern hemisphere, you’re looking at the wrong half of the sky.
- Find the bright one first. Every zodiac figure has one star far brighter than the rest: Aldebaran, Regulus, Spica, Antares. Find that, and the rest of the shape arrives on its own a minute later.
- Give your eyes twenty minutes. Dark adaptation is most of stargazing. Twenty minutes away from a phone screen roughly doubles what you can see, and costs nothing.
- Use your hand as a ruler. An outstretched fist is about ten degrees of sky, a thumb about two. A sign is thirty degrees wide: three fists. This works because your arms and hands are in proportion, so it works for a child too.
If it isn’t there, you haven’t failed. Check the month: it’s probably up at four in the morning, or behind the sun.
From module 6
Check any horoscope in thirty seconds
- Read it as if it were somebody else’s. Take the sign off the top and hand it to a friend. If it still fits them, it was never about you, and the sign was carrying none of the weight.
- Count the escape hatches. “A certain amount of”, “at times”, “you tend to”. Vagueness isn’t sloppy writing. It’s the mechanism. It lets you supply the specific memory that makes it true.
- Look for the compliment. Almost every reading includes unused potential, an independent mind, or a self-criticism you can be proud of. Nobody’s horoscope tells them they are boring.
- Ask what would make it false. A forecast you can’t fail is not a forecast. If no Tuesday could disprove it, it predicted nothing.
None of this means you have to stop reading them. It means you know what you’re reading.
From module 18
Take a kid outside and make it stick
- Start with their sign, not yours. They already have a stake in it. That’s the hook astrology built for you, and you’re allowed to use it.
- Let them find it before you name anything. Point at the bright one. Wait. The moment where the shape appears out of the noise is the whole lesson, and it doesn’t survive being narrated.
- Tell them how late the light is. Work out the year it left: subtract the light years from this one. “That light started before you were born” is a good one. “Before your grandmother was born” is better, and for some of these it’s true by centuries.
- Then say the honest thing. It isn’t watching them and it doesn’t know their name. It’s just extremely old, extremely far away, and genuinely there. That’s a better story than the one it replaced, and children take it well.
Binoculars beat a cheap telescope for all of this. Anything you own already is fine.
From module 24
So why is it so good at what it does?
From module 22 of the course
Because it isn’t a forecast, and most people using it know that. A horoscope is a prompt. It hands you a vocabulary for saying “I’ve been avoiding a hard conversation” without having to announce it, and a socially acceptable reason to think about yourself for two minutes. Almost nothing else in an adult day does that. It’s also a genuinely good bit of design: twelve archetypes, a daily cadence, a language your friends already speak, and a compliment tucked into every profile. Products spend years failing to build what it manages in a paragraph.
None of that requires the stars to be doing anything, and this page isn’t asking anyone to stop. Enjoy the ritual. Read the column. Just don’t take a job, a diagnosis or a person on its recommendation, and when someone charges you for a chart, understand exactly what you’re buying, which is a well-written mirror.
The sky, meanwhile, is worth the attention astrology borrowed. Those are other suns. A few of them are older than the Earth, one in Leo is so far away that its light has been on its way for something like two thousand years and nobody can tell you the number closer than that, and you can find every one of them from your own back garden with nothing but a date and a direction. That part is real, and it’s the better story.
Questions about the zodiac, and about this page
What is the 13th zodiac sign?
Ophiuchus, the serpent bearer. When the International Astronomical Union drew exact edges around all 88 constellations in 1930, the sun’s path crossed thirteen of them, not twelve. The sun spends about eighteen days a year in Ophiuchus, between Scorpius and Sagittarius. Horoscopes leave it out because the zodiac was designed as twelve equal slices, and a thirteenth breaks the arithmetic.
What stars are in my zodiac sign?
Pick your sign at the top of the page. The map shows the constellation’s brightest stars, and the table under it gives each one’s brightness, its distance, and the year its light left. The stars in one sign are usually hundreds of light years apart and have nothing to do with each other.
Why don’t zodiac dates match the constellations anymore?
Precession of the equinoxes. Earth wobbles like a slow top, one turn every 25,772 years, so the point where spring starts slides backwards along the sun’s path by about a degree every 72 years. The dates were fixed about two thousand years ago and never moved, so your horoscope now runs roughly one sign behind the sky.
Is astrology real?
As a tradition and a vocabulary, yes. As a way to forecast anything, no. It has been tested under conditions astrologers agreed to, and the results came out at chance. The evidence section above has the two biggest studies and the gravity numbers.
Is this making fun of me?
No. It’s making fun of a claim, and being fairly cheerful about it. The only thing the page insists on is that your sign is a direction in space rather than a description of you. It’ll happily show you that direction, tonight, from wherever you’re standing.
Don’t astrologers already know about precession?
Most do, and their answer is that the tropical zodiac is symbolic rather than astronomical: a seasonal frame, not a claim about where the stars are. That’s a fair answer to the dates. It isn’t an answer to the Carlson trial or the time twins, where the charts were cast the way the astrologers asked for them and the predictions still came out at chance. Symbolic systems can be meaningful. They can’t also be forecasts.
What about sidereal or Vedic astrology, which uses the real constellations?
Sidereal astrology fixes the same objection this page raises about the dates, and none of the rest. It has the sun in the right constellation and still has no mechanism and no measured effect. Being right about where the sun is only makes the second half of the problem clearer.
The moon moves the tides. Why not people?
Because a tide needs a body wide enough for the pull to differ measurably from one side of it to the other. The ocean is thousands of kilometres across, which is why it works there. You’re about half a metre across, and the moon’s tide on you is hundreds of thousands of times weaker than the tide from the person sitting next to you. Both numbers are in the table above.
Where did the star positions come from?
Published J2000 coordinates, visual magnitudes and parallax distances for the brightest stars in each figure, rounded. Everything the page calculates on top of those numbers runs in your browser: the sun’s real constellation, the drift in days, the altitude tonight, the force comparisons. All of it is covered by unit tests, which is a strange sentence to write about a horoscope app and the reason this one is worth reading.
Can I use this with my kids?
That’s who it was built for. Pick their sign, flip it side on so the shape falls apart, then take the Sky Card from module 24 outside and find the real thing. Email pettigrewlabllc@gmail.com if something in it is wrong. Corrections are the point.
Is this a course or one clever page?
Both, for now. Eight of the twenty-four modules are playable on this page: the sky map covers two of them, and then there’s tonight’s position, the year you can drag through, the brightness comparison, the birth-year star finder, the birthday check and the reading. The outline below is the rest of the course they belong to. It’s being written the same way the other Learn courses were: outline first, then the lessons, then the app.
Is the joke going to get old by module 24?
The joke is the delivery, not the content. Once you’re outside finding Antares with your fist as a ruler, the gag has done its job and quietly stops. Part V puts the reading back in your hands with the workings shown, which is a better ending than a punchline.
Where are you starting from?
I am here for the joke
Start at the top: pick your sign, then flip it side on and watch the lion fall apart.
Try the lessonI am a parent or a teacher
The Sky Cards work as a standalone activity, no app required. Get one email when the rest is out.
Email me when it’s outI actually like astrology
Then read module 22 first. Nobody is being asked to give anything up, and that part of the course is written for you rather than at you.
Read the honest bit
Check this page too.
A page that spends this long on other people’s unchecked claims should be easy to check itself. If a coordinate, a date or a study is off, send it over and it gets fixed.
Send a correction