What is a birth chart?

A chart lists the positions of the planets against the twelve signs of the zodiac, then divides the sky using the horizon and meridian at the place of birth. That is why an exact time and place matter. Greek writers called the point rising in the east the horoskopos, and it is known today as the Ascendant.

The chart is split into twelve houses, each linked to areas of life. The first, fourth, seventh and tenth are the angles. Geometric relations between planets, such as the trine and the square, are called aspects.

Western charts use the tropical zodiac, which is tied to the equinoxes and solstices. The Internet Encyclopedia of Philosophy credits Ptolemy as the first astrologer to use it. Because the equinox point drifts slowly through the stars, the signs no longer line up with the constellations of the same names.

An astrologer looks at each planet's sign and house, the angles between planets, and the sign on the Ascendant. Ancient practitioners also timed periods of a life and chose favorable moments to act.

The methods have never been uniform. Ptolemy rejected the division of the chart into twelve places and most of the mathematical points called lots, both of which other Greek astrologers used. House systems and the size of allowed aspects still vary from one astrologer to another.

About thirty cuneiform horoscopes survive from Mesopotamia. The two oldest, from Babylon and Nippur, date to 410 BCE, and the latest to 69 BCE. The earliest draw on the older practice of reading omens in the sky. Later ones list the Sun, Moon and five planets at birth whether or not anything ominous was happening.

Greek manuals for casting birth charts appear by the late second century BCE, and Alexandria was probably a center of this work. The oldest known Greek horoscope is usually taken to be the lion relief at Nemrud Dagh in Commagene. It is most often dated 62 BCE, but 109 and 49 BCE have also been argued, and the event it marks is disputed. Claudius Ptolemy wrote the Tetrabiblos in the second century CE, and Vettius Valens compiled his Anthologies, full of sample charts, between about 152 and 162 CE.

Abu Ma'shar (787 to 886), born near Balkh and active in Baghdad, became the leading astrologer of the Muslim world. His Great Introduction was put into Latin by John of Seville in 1133 and by Hermann of Carinthia in 1140, and his work carried this learning into western Europe.

Astrologers then held places in universities, courts and medicine. Pico della Mirandola's attack on astrological prediction appeared in 1496, and Copernicus, Kepler, Galileo and Newton changed the picture of the heavens the art relied on. Popular demand lasted longer: William Lilly's English almanacs sold nearly 30,000 copies a year by 1659. By the eighteenth century astrologers had been largely pushed out of learned institutions.

Alan Leo, born William Frederick Allan (1860 to 1917), was a Theosophist who built karma and reincarnation into astrology and argued that a chart shows tendencies. That change helped turn the subject toward character description.

The newspaper horoscope is younger still. In August 1930 the London Sunday Express ran R. H. Naylor's forecast for the newborn Princess Margaret. Its success led to a regular column, and Naylor's forecasts by date of birth later grouped readers into twelve 30-degree sections of the Sun's path, by 1937 at the latest.

How horoscopes traveled from southern Iraq to the Mediterranean remains an open question. Berossus, a Babylonian priest said to have taught on the island of Cos around 280 BCE, is the traditional link, and accounts differ even on his date.

Many founding texts are pseudonymous. Works circulated under the names of Hermes, Nechepso and Petosiris, so it is impossible to say how much these writers invented and how much they collected. Scholars also doubt that the Tetrabiblos had much effect in its own century, and some question whether Ptolemy practiced astrology at all.

The reasons for astrology's fall from learned life are disputed. Historians once credited the scientific revolution alone, and more recent work points to social, political, religious and institutional causes as well.

A controlled test did not support the chart. In a double-blind study published in Nature in 1985, Shawn Carlson asked astrologers recommended by advisers from the National Council for Geocosmic Research to match charts to personality test results. He reported no better than chance. A 2009 reanalysis by Suitbert Ertel reported a marginal result above chance.