A birth chart is the drawing of the sky at the exact moment and place where you were born. Where each planet stood, which degree of the zodiac was rising on the eastern horizon in that minute, and how the twelve houses cut that sky as seen from that city.

Before being interpretation, it is arithmetic from astronomy. Two people calculating the same birth arrive at the same chart, because planetary positions for any date and time have been tabulated data since the nineteenth century. What astrology debates is what that drawing means; the drawing itself is not up for debate.

That is why it asks for three things, and not one. The date says where the planets stood in the band of the zodiac, the time says which part of that sky was overhead, and the city says from which point on Earth you were looking. Remove the time and the city and what is left is a chart of the day, the same one for everybody born that day anywhere on the planet.

Birth chart, astrological chart and natal chart are three names for the same thing. The third is the most exact of them, because it says what it is: the chart of a birth.

Inside a birth chart there are four layers, and each answers a different question. Whoever learns the four stops memorising meanings and starts reading the drawing.

The planets answer what, which function of life is at stake, and require the date; the signs, how that function behaves, and require the date, and the time for the Moon; the houses, in which area it shows up, and require the date, the time and the city; and the aspects, how the pieces talk to one another, and require the date.

The planets are the functions. Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto are the ten classical pieces, and the reading adds calculated points that are not bodies, the Ascendant, the Midheaven, the lunar nodes. Each one answers for an area of how a person works: Mercury for the head and for speech, Venus for taste and affection, Mars for will and for the fight.

The signs are the manner. A planet in Aries behaves one way and in Capricorn another, and it is the same function in both cases. Your sign, the one you give when somebody asks, is the sign of a single planet: the Sun.

The houses are the areas. They divide the sky starting from the horizon of your city, and that is why they depend on the time, the twelve houses go all the way around once a day, while the zodiac takes a month to change sign.

The aspects are the conversation. When two planets sit at a certain distance in degrees, the reading takes them together. An aspect is a measured distance, and not an opinion: either the two are ninety degrees apart or they are not.

The birth chart is about 2,400 years old, and it was born in Babylon. The oldest one known is a clay tablet in cuneiform writing with the positions calculated for a child born on 29 April 410 before Christ. It records the Moon below the pincer of Scorpius, Jupiter in Pisces, Venus in Taurus, Saturn in Cancer, Mars in Gemini, and Mercury invisible that night.

Look at what that list already is: the same pieces, the same signs, the same question. What changed in twenty-four centuries was the precision of the arithmetic, and not the shape of the document.

It was also in Babylon, in the fifth century before Christ, that the band of sky the planets travel through was given twelve equal slices of thirty degrees each, named after the constellations that occupied each stretch at the time. Before that, astrology looked at the State, the harvest, the king, the war. The tablet of 410 marks the turn where it began to look at one person.

In the second century after Christ, in Alexandria, Claudius Ptolemy wrote both books at the same desk: the Almagest, which was the astronomy manual of the West for fourteen hundred years, and the Tetrabiblos, which organised astrology and fixed the convention the birth chart still uses, starting the zodiac at the point of the March equinox, and calling that point degree zero of Aries.

Signs take weeks to change; houses change in minutes. It is the difference in speed between the two layers that makes the time matter so much. Measured over ten years of sky above São Paulo, getting the recorded time wrong costs this:

With a 4-minute error in the time, 28.3% of charts have at least one planet in another house; with 15 minutes, 72.0%; with 30 minutes, 93.8%; and with 1 hour, 99.9%.

Four minutes is the distance between the clock in the delivery room and the time somebody wrote down afterwards, and it already moves almost a third of all charts. With one hour of error, practically no chart stands as it was.

The city comes in through the same door. The Ascendant is the degree that was rising on the horizon, and a horizon belongs to a place: in the same minute, someone born in Belém and someone born in Porto Alegre saw different degrees come up. Whoever does not have the time still has an honest reading ahead, as long as it says out loud which half of the chart it is talking about.

The Sun sign is one piece among eleven, and it is the only one that dinner-table conversation knows. This is the part almost no introductory text puts in numbers, and it is the reason the chart exists.

We measured 124,185 charts, every six hours between 1940 and 2025, counting the same eleven points the site's screen shows. What comes out of that takes apart the idea that two people of the same sign resemble each other:

• Mercury is in the same sign as the Sun in 51.0% of charts. Which means almost half the people of your sign think and speak from another one.
• Venus, in 23.7%. Three out of every four people of your sign like things from a sign that is not yours.
• In 17.3% of charts an entire element comes out at zero across the eleven points, and, looking only at the six personal points, that rises to 68.2%. The line “I have nothing in fire” is uncommon in the full chart and common in the part that is most yours.
• 68.1% of charts have three or more planets in the same sign. The stellium that sounds like a privilege of the few is the case for two thirds of all people.

And there is the layer the Sun sign does not even touch. The Ascendant changes sign every two hours, so twelve people born on the same day, in the same city, at different hours, arrive in the world through twelve distinct doors, the Ascendant is the piece that most changes the first impression somebody makes. The Moon changes sign every two and a half days, and it is the one that says what a person needs in order to feel at home.

Adding it up: the day of your birth defines one piece, and the minute and the place define the rest. A text describing your sign is describing one twelfth of the world. The birth chart exists to describe you.

It is not a calendar of what is going to happen. Astrology describes tendency and gives vocabulary for what a person already lives; it does not determine the future, and whoever sells that is selling something else. A tendency is material you can work with, and a verdict is not.

It does not diagnose illness, it does not speak of death, and it replaces no doctor, therapist or lawyer. Saying so does not weaken the reading: it is what separates someone who reads the sky from someone who takes advantage of another person's fear.

And half the drawing is convention, and not a fact of the sky. The division into houses depends on the system chosen, between Placidus and whole sign, more than half the planets land in a different house. Planetary positions are the same in any school; where the houses are cut is a choice of method, and an honest reading says which one it used.

The chart does not change either. It is the photograph of a moment that has already passed, and nothing you do moves it. What changes is today's sky passing over that drawing, and astrology calls that a transit, it is the material of forecasts, and it is another calculation.

And the questions that come up most on this subject, with the short answer: