As its title suggests, the principal theme of the book is fatalistic astrology. This is the belief that human character and action are predetermined by fate. In his reaction to The Nero Prediction the eminent historian Michael Grant confirms that astrology was "so predominant and indeed universal in the Mediterranean world that it exceeded every religion in power and influence."

We know something about three of the astrologers who feature prominently in the story. Thrasyllus, close friend of the emperor Tiberius, was the towering intellect of his time. His son Tiberius Claudius Balbillus, born in Alexandria in about 3 AD, was equally eminent. Like his father he was a polymath, astrology was his principal interest, but not his only one. He served as Director of Alexandria's Museum and administrator of imperial buildings in Egypt. He also held other important administrative posts in the empire.

Ptolemy Seleucus is a shadier figure. He was Otho's astrologer (see Suetonius, The Twelve Caesars, Otho), so there is every reason to believe that he was also consulted by Otho's wife Poppaea. He successfully predicted that Vespasian would become emperor and afterwards became a fixture at Vespasian's court (Tacitus, Histories 2).

Only fragments of the astrological methods of Thrasyllus and his son survive in the commentaries of other writers, and nothing of Ptolemy Seleucus. However the substantial agreement between the two astrology manuals that have come down to us intact, the second century Tetrabiblos and the fourth century Matheseos, suggest that the basic tenets of astrological dogma were widely accepted and remain so up to the present day. One prominent exception is the familiar twelve house system where each of the houses is dedicated to a particular area of life: Character, Money, Misfortune, Power etc. Although these houses are featured prominent in the enormously popular Matheseos, they are hardly mentioned at all in Claudius Ptolemy's much earlier Tetrabiblos, although Ptolemy obviously has the equal house system in mind when describes the four cardinal points of the chart: the ascendant, mid heaven, descendant and the lower heaven or nadir of the chart.

Since the solar system is a vast chronometer with gigantic planets circling the sun with far greater predictability than any pendulum, then as now the current and future positions of the planets could be calculated with the use of astronomical tables. But so can the positions of the planets in the past, even the remote past. With the help of modern computer programs we can pin point the ever-changing positions of the planets during Nero's time with a higher degree of accuracy than the Neronian astrologers themselves.

The confluence of these two facts, our knowledge of ancient astrological dogma and our ability to recreate the positions of the ancient planets, gives us a key to unlock the secrets of the past. Where we know the date and time of birth of someone like Nero, we can re-create, with a fairly high level of confidence, what his astrologer would have told him lay in wait at any time in the future.

Fortunately Nero's date and time of birth have survived. So has the correct date of his death - two days later than commonly believed. In the notebooks of the astrologer Vettius Valens, who was active about fifty years after Nero's death, charts for an unnamed person's birth and death are positioned next to each other. The birth chart is for December 15, 37 AD at about sunrise, precisely the time specified for Nero by the historian Suetonius.

The death chart is cast for June 11, 68 AD, the day Nero died according to the ancient historian Cassius Dio who was a stickler for accurate chronology. What Vettius is trying to demonstrate is how a person's death can be predicted on the basis of the return of the planets to the same basic pattern of angular distances (aspects) from each other. Vettius' cryptic commentaries on these two horoscopes detail how Neronian astrologers could have predicted the day of Nero's death from his birth chart or perhaps how they actually did so.

It is no mystery why Vettius does not say that these charts are Nero's. Dabbling in imperial horoscopes (particularly concerning an emperor's death) had been a capital crime since Augustus's day.

The future is predictable and the key that unlocks its secrets is the movement of the planets. This was the dominant philosophical idea of Nero's time.

It was grounded on the assumption, popularized by the Stoics, that heaven and earth are mutually dependent parts of the same universal flux which is ordered by a predetermined sequence of cause and effect called fate or destiny. The belief was that since it is possible to make accurate predictions about the future movements of the seven planets as they circle the earth, one can make equally accurate predictions about the future of the men and women around whom they circle.
From earliest times these seven planets were identified with seven gods who interacted with each other rather like people do. The explanation of their changing inter-relationship was to be found in the constantly changing angular distance between them. Extrapolating from music, where certain chords are harmonious and others disharmonious, a system of "aspects" expressed in degrees was worked out, each of which was thought to indicate harmony or disharmony between two particular planets.

As it is in heaven so it is on earth, this was the belief. When the planets at an infant's birth were placed at disharmonious angular distances from each other (i.e. were unfortunately aspected), the infant's future could be expected to be unfortunate. The opposite was true if the infant was fortunate enough to be born under harmonious stars.
This is the bare essence of astrology. What follows is a summary of the way the ancients expanded this idea.

The planets:

Only seven planets (this number includes the sun and the moon) were known to the ancients. Each one had a distinct personality that originated in the god or goddess they represented. These, in order of their supposed distance from the earth, were:

	Moon: Benevolent or malevolent depending on whether she is waxing or waning. A waning moon is generally malicious, while a waxing moon is more inclined to be benevolent. The moon represent the mother in a child's chart and the wife or mistress in an adult's.
	Mercury: Neither benevolent or malevolent but easily influenced both by other planets and by its position in the chart. His gift is intelligence or cunning.
	Venus: Benevolent. Her gifts are artistic skills, entertainment, romance and children.
	The Sun: Generally neither benevolent nor malevolent.
	Mars: Malevolent.
	Jupiter: The most benevolent of all the planets. Our word "jovial" comes from Jupiter's astrological nature. In fact Jupiter is so benevolent that even normally unfortunate aspects with Jupiter are often pronounced fortunate.
	Saturn: Generally malevolent.
	The Lot or Part of Fortune: This point in the chart is found by counting anti-clockwise from the ascendant the same number of degrees as the sun is from the moon also when counting anti-clockwise. If the birth takes place at sunrise, the Lot of Fortune coincides with the moon. It was thought to predict good or bad material fortune depending on its position in the chart and whether it was influenced by benevolent or malevolent planets.

The aspects:

Even in Nero's day the earth was known to be an orb. The planets (Greek for "wanderers") were believed to circle this orb from west to east and therefore in the opposite direction to the apparent diurnal motion of the heavenly sphere to which the rest of the stars were fixed. Because the planets' orbital speeds were different, the angular distances between them, measured in degrees of longitude, constantly changed. Whenever the angular distance between two planets came within more or less ten degrees of any of five specific angular relationships, they were said to be in aspect. In order of declining power these aspects are:

	1. Conjunct. The planets (A and B) share the same longitude as seen from earth (center) and therefore their qualities were believed to mingle. This aspect can be either fortunate or unfortunate depending on which planets are involved.
	.2. Opposition. The planets are on opposite sides of the celestial circle and are therefore separated by 180 degrees of longitude. Unfortunate since it sets the one planet against the other.
	3. Square. Also known as quartile. The planets are separated by 90 degrees of longitude. Unfortunate because this is half of an opposition.
	4. Trine. The planets are separated by 120 degrees of longitude. Fortunate.
	5. Sextile. The planets are separated by 60 degrees of longitude. Fortunate because it is one half of a trine.

The arrows indicate the direction of apparent planetary movement.

The signs.

These are the twelve signs of the zodiac familiar to anyone who has turned to the astrological section of the daily paper. Each represents 30 degrees (one twelfth) of the zodiacal circle. Like the planets the signs have natures. In Nero's time the signs of the zodiac corresponded roughly to the constellations of the same name. They are ordered in the same sense as the motion of the planets: anti clockwise around the chart, beginning with Aries the ram and ending with Pisces the fishes. Each sign was believed to be ruled by a planet which was thought of as that planet's zodiacal house or domicile.

	Aries (ruler Mars)		Libra (ruler Venus)
	Taurus (ruler Venus)		Scorpio (ruler Mars)
	Gemini (ruler Mercury)		Sagittarius (ruler Jupiter)
	Cancer (ruler Moon)		Capricorn (ruler Saturn)
	Leo (ruler Sun)		Aquarius (ruler Saturn)
	Virgo (ruler Mercury)		Pisces (ruler Jupiter)

The angles.

These are the four cardinal points of the horoscope. The most important of these is the ascendant (ASC) which corresponds to the eastern horizon at the moment of birth where stars, whether visible or invisible, are rising. Next in importance is the mid heaven (MC) which is represented as being at right angles to the ascendant. The descendant (DES) represents the western horizon where stars are setting. Finally the imum caelum (IC) is the nadir of the chart.
In a world where nothing is believed to happen by chance, when a planet falls on one of these angles its importance is magnified. (The midheaven arrived at in this way does not always correspond with the astronomical midheaven.)

The houses.

In addition to dividing the zodiacal circle into twelve signs, it was further divided up into twelve houses. Each house was thought to yield predictions about a particular area of life. The first house begins with the ascendant which represents the eastern horizon at the moment of birth. The houses then proceed around the circle of the horoscope anti-clockwise, just as the signs do. It is presently unknown when this twelve house system came into widespread use.

Transits:

While the stars at the moment of birth were believed to paint the fundamental portrait of a person's future, their influence was thought to be constantly, though ephemerally, modified by the subsequent movement of the planets. Therefore if the astrologer wanted to discover whether some specific time in the future was favorable or unfavorable, he would look up his star tables for the positions of the planets at that time and then draw them into the birth chart so that they formed a second ring outside the first. These secondary planets were called the transits. They were thought to influence the birth planets in precisely the same way as the birth planets influenced each other.
Just as the planets of any day subsequent to birth (the transits) were thought to influence the birth planets, the planets of another person, when drawn into the birth chart, predicted how these two people were destined to interact.
This is a broad overview of Neronian astrology - there were other computational techniques then in use that introduced so many variables that it was possible for the unscrupulous (or perhaps self-deluded) astrologer to justify telling his client whatever he or she wanted to hear. This is why astrology was so seductive, and so dangerous.