In Portugal, astrology and seafaring were the same profession.

Pilots of the 15th and 16th centuries relied on tables of star positions to know where they were, and the people who produced those tables were the same people who used them for astrological purposes. Pedro Nunes, a 16th-century mathematician and cosmographer, is the name that best represents this period when reading the sky was a matter of State.

It is worth grasping the size of the problem they were solving. A ship in the middle of the Atlantic has no landmark: the only way to know where it is is to measure the height of a star above the horizon and check it against a table. If the table is off by half a degree, the error on the map runs to dozens of kilometres. Portuguese astronomical precision was not born of curiosity. It was born of risk to life.

In European universities of the same period, Coimbra among them, astrology was part of the teaching of MEDICINE. Choosing the moment for a procedure by the Moon's position was standard practice, not roadside superstition: it was in the manuals, and a doctor who ignored it would be considered poorly trained.

What broke up that arrangement was pressure on judicial astrology, the kind that predicts a person's life, seen as dangerous because it put free will at risk. The distinction was fine and mattered a great deal in practice: the astrology that calculated tides, seasons and planting times went on being taught; the kind that read an individual's fate came to be practised with caution, and sometimes in private.

It is a split that has never fully gone away. Even today, whoever defends astrology tends to fall back on safe ground, "it's just self-knowledge", and whoever attacks it takes aim precisely at the part that speaks of a person's future.

Portugal gave astrology what seafaring demanded of it: precision. A wrong table could lose a ship, and that demand shaped a tradition of careful calculation.

Abraham Zacuto is the name missing from most versions of this story. Astronomer, astrologer and rabbi, trained in Salamanca, he reached Portugal after the expulsion of the Jews from Castile in 1492 and was taken in at the court of John II. The Almanach Perpetuum, with his tables, became a bedside book for Portuguese sailors: it gave the Sun's declination day by day, the number a pilot needed to work out latitude from the Sun's height at noon.

Five years later, the forced conversion decreed in Portugal made him leave again. The tables stayed. The author did not. Vasco da Gama carried that almanac to India, and Zacuto died far from home. It is not a picturesque detail: it is the same hand that wrote horoscopes writing the figure that kept ships alive.

And the problem those tables solved deserves to be told in full, because it is the most astrological thing that seafaring ever did. South of the equator, the Pole Star disappears below the horizon. The whole method of finding latitude, measuring the height of the Pole Star and reading it straight off, stops working overnight, in the middle of the ocean.

The way out was to swap the fixed star for the Sun, which is always there but changes height through the year. For that you need to know, for every day, how far the Sun stands from the celestial equator, and that is an ephemeris, a table of positions, exactly what was calculated to draw up a birth chart. The regimentos that taught the calculation were shipboard pamphlets, learned by heart by men who often could barely read. Portugal taught sailors to use an ephemeris.

The first European to map the southern sky was an astrologer, and he wrote from Brazil. Mestre João Faras, physician and astrologer to Manuel I, sailed with Cabral's fleet, and in 1500 sent the king a letter from Porto Seguro with a sketch of the stars he saw, among them the group that would come to be called the Southern Cross. He warns in the letter that a measurement taken on a rocking ship cannot be trusted, and asks that it be confirmed on dry land. It is the caution of someone who knows what the calculation depends on.

And there is a case that surprises anyone who knows him only by his verse: Fernando Pessoa was a practising astrologer. He left behind hundreds of hand-drawn birth charts, advertised the service, and even wrote promotional copy for an astrology business. He drew up the chart of Portugal, of public figures, and of his own heteronyms, with an invented date, time and place of birth for each one.

This last point is the strangest and the most telling. To give Álvaro de Campos or Ricardo Reis a chart, Pessoa had to choose the moment that would produce the character he already had in mind, which is the operation of electional astrology run backwards. He used the chart as a tool for building a character, not for finding out who someone was.

It is worth putting the two threads together. In Portugal astrology served two things that look opposite and are not: getting a position at sea right, where a mistake cost lives, and giving consistency to an imagined person. In both cases what was asked of the sky was the same thing: a fixed reference point, outside the one looking. The rest of this page is about the reference point you can actually measure.

The latitude problem was treated as State policy, and handled with a committee. John II gathered a board of mathematicians, Zacuto among them, to work out the Sun calculation, and sent José Vizinho to the coast of Guinea, in 1485, to measure the Sun's height in practice and check the tables against the real world. Checking a calculation by going to the place and measuring is an attitude popular astrology has almost never had, and those people had it because the price of error was a ship.

And there is a Portuguese chapter almost nobody links to this subject: because of the padroado, the Jesuits headed for Asia sailed out of Lisbon and passed through Goa and Macau. In the seventeenth century, European Jesuits went on to run the Beijing Astronomy Bureau, a body whose job was to produce the imperial calendar, which carried, alongside the eclipses, the days proper for each kind of act. European priests calculating the astrological almanac of the Chinese emperor, along a route Portugal had opened.

And there was a place in Lisbon to study all this. The Colégio de Santo Antão opened, at the end of the sixteenth century, the Aula da Esfera, a public course in mathematics, astronomy and seafaring, taught by Jesuits and attended by pilots, engineers and noblemen. It ran for more than a hundred and fifty years. It was there that reckoning the sky turned into open teaching, instead of a trade secret.

The part of astrology that depends on PLACE, not just date, is the Ascendant: the degree of the zodiac rising on the horizon at the moment of birth. It changes every two hours and depends on the city's latitude.

And the twelve signs do not rise at the same speed. Some take more than two hours to cross the horizon and others less than one, and this difference grows the further you are from the equator. I drew 200,000 instants at random between 1950 and 2010 and calculated the Ascendant for Lisbon, at 39° north:

In Lisbon, the most common Ascendant is Scorpio, at 10.5%, and the least common is Pisces, at 5.1%; one is 2.1 times more likely than the other, where a fair draw would give 8.3% to each sign.

Notice what this means: two people born at exactly the same time, one in Lisbon and one in a city at another latitude, can have different Ascendants, and the frequency of each Ascendant changes with the place. The city of birth is not an administrative detail on a form: it enters into the count.

The pattern across this series is pure geometry, and it fits in one sentence: the further from the equator, the more uneven the draw. In Copenhagen, at 56 degrees north, the gap between the most common Ascendant and the least common one is 4.3 times; in São Paulo, at 23 degrees south, it drops to 1.5.

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