The author of the Astronomica is not quoted or mentioned by any ancient writer. His identity is uncertain (in the earliest manuscripts the work is anonymous, and in the later ones it is given as Manilius, Manlius or Mallius), but is probably Marcus Manilius. Due to this uncertainty, Marcus Manilius has been confused with Manilus Antiochus (fl. c. 100 BC, mentioned by Pliny the Elder in his Naturalis Historia); Flavius Manlius Theodorus (fl. c. AD 376–409, a consul in AD 399) and Boëthius (the sixth-century Roman senator and author of De Consolatione Philosophiae, whose full name was Anicius Manlius Severinus Boëthius). Although the poem suggests that the writer was a citizen and resident of Rome, classicist Katharina Volk writes that Manilius was a non-Roman "due to the poet's supposedly inferior Latinity or on the wish to see Manilius as the member of a Greek intellectual milieu at Rome". Nineteenth-century classicist Fridericus Jacobs and historian Paul Monceaux have said that he was an African, based largely on his writing style (which resembles that of other African authors). Volk counters that view, saying that Manilius writes "from ... a conventional Roman perspective" and "takes recourse to Roman history to illustrate the astrological facts he discusses".
The work's date has also been controversial, and the only clear reference to an historical date is a mention of the Battle of the Teutoburg Forest – a decisive loss for Rome, forcing it to withdraw from Magna Germania – in AD 9. Three hypotheses have been proposed: that the poem was completed under Augustus, that parts were written under Augustus and Tiberius, and that the entire work was written under the reign of Tiberius. The first theory was favored primarily from the Renaissance to the 19th century. In 1815, Karl Lachmann wrote that references to the emperor in the poem were better understood as referring to Tiberius. It has been argued (notably by A. E. Housman) that the first two books were written under Augustus; the final two were completed under Tiberius, and the middle (or third) book is "undatable". A consensus has not yet been reached but, according to Volk, the poem can "be dated roughly to the second decade of the first century AD."
The text of the Astronomica as it is known today depends on three manuscripts, known as G, L, and M, which belong to two separate manuscript families. Manuscripts G and L are descendants of an earlier (now lost) manuscript known as "α" (after which the first family is named), and ms. M is derived from a lost manuscript known as "β" (after which the second family is named). G, dating from the late 10th to the 11th century, was found at the monastery of Gembloux in Brabant. Manuscript L, from the library of Leipzig, was probably written around the mid-11th century and has many corrections made by a scribe. M is a descendant of the manuscript (ms. β) rediscovered by Poggio Bracciolini near Constance during a break in the Council of Constance (which he attended) c. 1416–17. Bracciolini had the poem transcribed by a German amanuensis, but due to the scribe's incompetence he sarcastically remarked that the resulting copy had to be "divined rather than read" (divinare oportet non legere). Manuscript M has been singled out as possibly the most-important surviving manuscript, because it is apparently a direct copy of the original Astronomica and of better quality than the postulated manuscript α (which was probably derived from the original text and was corrupted during transcription).
The first edition of the Astronomica was prepared by the astronomer Regiomontanus from very-corrupted manuscripts and published in Nuremberg around 1473. The text was then critically edited by Joseph Justus Scaliger, whose first edition was published in Paris in 1579; a second edition, collated with much-better manuscripts, was published in Leiden in 1599–1600 and a third edition was published in 1655. A greatly-improved edition was published by Richard Bentley in 1739. Housman published, in five volumes between 1903 and 1930, what is considered by many the authoritative edition of the poem; according to Katharina Volk, "[Housman's] work is famous – some might say notorious – for its bold handling of the text, its incisive commentary, and its merciless [...] invective against other scholars." In 1977, G. Goold published a Loeb English translation of the poem. This version of the poem (with substantial introductory notes and diagrams) was called "masterly" by Volk and Steven Green, and "marked a significant development in the accessibility of Manilius to a larger audience". Goold's version was also the first time that the poem had been translated in English prose.
Manilius's Astronomica is the first extant work on astrology which is extensive, coherent, and (for the most part) intact. Volk writes that since he dedicates the poem to stellar phenomena, it is "indicative of the great fascination [...] that the stars held for the Romans of Manilius' period".
Written in hexameter, the Astronomica opens with Manilius's claims that he is the first to sing about astrology and Mercury spurred his interest in celestial bodies. In the first book he ponders the origin and nature of the universe, considering theories by Xenophanes, Hesiod, Leucippus, Heraclitus, Thales, and Empedocles before arguing that the universe was created from the four elements and is governed by a divine spirit (ll. 1.118–254). Manilius holds the view that the universe is made up of two spheres: one solid (the Earth) and one hollow (the "sphere of stars", often known as the firmament). The constellations are fixed in the firmament; the Earth is stationary and the firmament revolves around it, accounting for the movement of the stars at night. The planets, the moon, and the Sun revolve around the Earth in the space between its surface and the edge of the firmament. Because the Earth is in the center of the universe, it is equidistant on all sides from the firmament; this is why it remains in place, instead of falling. According to Manilius, the universe is ruled by a god (conspirat deus) and governed by reason (ratione gubernat); however, he is vague about the exact relationship between the deity, the universe, and reason. After Manilius finishes describing the universe, he discusses the constellations and stars (ll. 1.255–560), the celestial circles (with particular emphasis on the Milky Way) (ll. 1.561–804), and comets – which Manilius sees as harbingers of disasters, such as plagues.
According to Volk, books two and three primarily deal with "laying out the elements of a birth chart". Book two opens with a preface in which Manilius presents a brief history of hexameter poetry, singling out Homer and Hesiod. His purpose is not to insert himself into this poetic tradition, but to emphasise the uniqueness of his poem in comparison to others. According to Manilius, "Every path that leads to Helicon has been trodden" (omnis ad accessus Heliconos semita trita est; all other topics have been covered) and he must find "untouched meadows and water" (integra [...] prata [...] undamque) for his poetry: astrology. Manilius ends the book's preface with the assertion "that the divine cosmos is voluntarily revealing itself both to mankind as a whole and to the poet in particular" and because his poetic mission has been sanctioned by fate, he is set apart from the crowd. An explanation of the zodiac follows. Manilius considers the signs of the zodiac (ll. 2.150–269), aspects and relationships between the signs and other objects (ll. 2.270–692), and dodecatemoria. (ll. 2.693–749) Near the end of book two, Manilius digresses about the didactic method (ll. 2.750–87) before concluding with a section on the fixed circle of the observer (ll. 2.788–970). The third book opens with Manilius's reiteration that his work is original; since his topic is difficult, the audience can "expect truth but not beauty". Subsequent verses discuss lots (ll. 3.43–202), calculating the ascendant and the horoscope (ll. 3.203–509), chronocrators (ll 3.510–59), determining the length of one's life (ll. 3.560–617), and the tropic signs (ll. 3.618–82). Due to its technical nature, classicist Steven Green writes that in this book "the disjuncture between instruction and medium is most obviously felt [since] complex mathematical calculations are confined to hexameter and obscured behind poetic periphrasis".
According to Volk, books four and five revolve around "the effects of particular celestial phenomena on the native". Book four covers many topics which originated in Egypt, leading classicist G. P. Goold to suggest that Manilius based his work on an Egyptian source. The topics include discussions of character (ll. 4.122–293); decans (ll. 4.294–386); the partes damnandae (ll. 4.308–501); the degrees of the zodiac and zodiacal geography (ll. 4.502–817), and ecliptic signs (ll. 4.818–65). At lines 4.387–407 and 4.866–935 are "exhortation[s] of the frustrated student", where complaints that astrology is difficult and nature is hidden are countered by assertions that "the object of study is nothing less than (union with) god" and "the universe (microcosm) wishes to reveal itself to man". Most of the fifth (and final) book is a discussion of paranatellonta via the myth of Andromeda and Perseus; this discussion is by far the longest digression in the poem. According to Steven Green, "The digression is very well chosen, in as much as no other mythological episode involves so many future constellations interacting at the same time: Andromeda (e.g. 5.544), Perseus (e.g. 5.67), the Sea Monster – strictly, Cetus (cf. 5.600), but often referred to in more generic terms during this story as belua (5.544, 608) and monstrum (5.581) – Medusa's head (e.g. 5.567), and Andromeda’s parents, Cepheus and Cassiopeia." Green further contends that the story is perfect for Manilius, as he is able to use it to justify the constellations' proximity to one another and their eternal arrangement (as he had previously argued in 1.354–360). The last few lines of book five concern stars and other stellar phenomena, and the book ends with a simile about the "res publica of stars".
According to Volk, "the basic tenet of what we might call Manilius' natural philosophy is the idea that the universe is divine". However, there is some discrepancy in the Astronomica as to the presence of the divine in the universe. In his first book, for instance, Manilius, while discussing the argument from design, claims that the perfectly regular movement of the heavenly bodies is proof enough that the universe is not just the product of a god, but rather some sort of god itself (mundum ... ipsum esse deum). However, later in the same book, Manilius argues that the universe is the "work of a great divinity" (magni ... numinis ordo). Concerning this inconsistency, Volk writes: "It is clear that there is a certain elasticity to Manilius' concept of the divinity of the universe ... Is the world simply ruled by a diuinum numen (cf. 1.484) or is it a deus (cf 1.485) itself?" Volk answers that in the cosmology of the Astronomica, "god can be understood as the soul or breath ... present within the world [and] since this divine entity completely pervades the cosmos, it makes equally much sense to call the cosmos itself a god." This interpretation of the universe – which states that it has a sense of intellect and that it operates in an orderly way – thus allows Manilius to contend both that there is an unbroken chain of cause and effect impacting everything within the cosmos, and that fate rules all.
It is the prevailing belief of critics that the worldview espoused by Manilius in the Astronomica is Stoic. A comparison between Manilius' beliefs and those of other Stoics reveals parallels that, according to Volk, "are immediately obvious". For instance, Stoics and Manilius agree on: the divinity of the universe, the argument from design, the assumption that the supreme god is both the creator of as well as the active force within the universe, the interconnectedness of everything, the understanding that humans are intimately connected to the cosmos, the importance of considering the heavens, and the belief in an inescapable, ruling fate. The agreement on this latter point is of special importance, as, according to Volk, belief in fate is "one of the most notorious aspects of the Stoic system". With this being said, there are dissenting minority opinions. Gustave Lanson contested the idea that the poem is Stoic, and, more recently, Alexander MacGregor argued that while contemporary scholars, such as Goold and Volk, read Manilius as a Stoic, the Astronomica actually "contradicts or ignores the central tenets and prejudices peculiar to Stoicism".
The Astronomica is considered a work of erudition, elegance, and passion; according to a Harvard University Press summary, Manilius "exhibit[s] great virtuosity in rendering mathematical tables and diagrams in verse form" and "the poet writes with some passion about his Stoic beliefs and shows much wit and humour in his character sketches of persons born under particular stars". The poem has been noted for its peculiar (albeit metrically-correct) style, largely due to its unusual content and lack of a stylistic antecedent. Jacobs, Monceaux, and others have attributed the Astronomica's idiosyncrasies to Manilius's reported African origin; he wrote and spoke a form of Africitas, a putative African dialect of Latin "with strongly marked peculiarities of vocabulary, syntax, sentence-structure, and style". However, according to Brock, there is very little evidence (other than the hypothetical presence of Africitas in the poem) that Manilius was from Africa.
In addition to its stylistic oddities, the Astronomica includes some contradictions. Steven Green calls the poem "riddled with confusion and contradiction", citing its "presentation of incompatible systems of astrological calculation, information overload, deferral of meaning and contradictory instruction". However, Green also notes that similar issues exist in other first-through-third century astrological works. According to Caroline Stark, Manilius paradoxically claims that astrological knowledge may be acquired by individuals and is only granted by divine favor. T. Barton suggests that Manilius may have included these contradictions and complexities so as to be seen as "a figure of unreachable knowledge for the novice student-reader". Green, while not ruling this hypothesis out, argues that Manilius was probably not motivated by a "desire to carve out for himself a position of power in the new imperial world of experts" as Barton suggests. Rather, Green contends that Manilius – due to his "pride in poetic innovation" and his "deference [...] to the Emperor" – sought to present "himself as a compliant imperial agent, intent on producing a creative poetic enterprise that plots its own way through the levels of acceptable stellar discourse in the early empire". David Pingree similarly concludes that the poem's "principal purpose seems to have been to delight its audience with poetry and to arouse admiration for the poet by its cleverness".
It is unknown if the Astronomica is a finished work. According to Housman, it is impossible to cast a full horoscope based on information in the extant poem. Despite Manilius's repeated claims that it would examine the zodiacal nature of the planets, no such treatment is found. The fifth book contains a large lacuna between lines 5.709 and 5.710, and Katharina Volk writes: "The text jumps abruptly from the topic of the extrazodiacal risings to that of stellar magnitudes". She proposes several hypotheses: perhaps the lacuna in book five originally contained a description of the planets, perhaps the lacuna is relatively small and the work is unfinished, or entire books may have originally existed between the two lines and were lost in the "hazardous process of textual transmission".
Manilius frequently imitates Lucretius, who wrote the didactic poem De rerum natura, and some classicists have said that Manilius intended to write six books in imitation of Lucretius's work. Evidence for this hypothesis is scarce, and it remains speculative (attractive, according to Volk). Lucretius approached the world from an Epicurean standpoint (a philosophy that emphasizes materialism and skepticism of superstition and divine intervention), but Manilius’s work is largely Stoic in outlook, espousing creationism (in the Greco-Roman sense) and emphasizing the deterministic nature of fate. According to Volk, "Manilius is a veritable anti-Lucretius and his presentation in the Astronomica of an orderly cosmos ruled by fate is a direct attack on the random universe depicted by his predecessor". Part of this philosophical difference is conveyed by Manilius via grammatical voice: unlike Lucretius, who often uses a passive construction to convey his understanding of nature, Manilius uses active grammatical constructions to convey the intentionality he sees in creation (e.g. "God and reason, which rules all things, guide earthly animals by heavenly signs", deus et ratio qaue cuncta gubernat ducit ab aeternis terrena animalia signis). Furthermore, while Lucretius used De rerum natura to present a non-theistic account of creation, Manilius "was a creationist rather than a materialistic evolutionist", and he consequently refers to "one spirit" (unus spiritus, l. 2.64), a "divine power" (divina potentia, l. 3.90), a "creator" (auctor, l. 3.681), and a "god" (deus, l. 2.475) throughout his poem.
The Astronomica is influenced by Ovid’s Metamorphoses, Virgil’s Aeneid, Ennius's Annales, and the Greek didactic poet Aratus. The latter's influence on Manilius is especially noticeable; according to Volk, Manilius bases much of his first book on portions of Aratus's Phaenomena. Despite his debt to the poet, Manilius diverges from his understanding of the cosmos; although Aratus focuses on description and mythology, Manilius emphasizes the scientific aspects of his work. It is uncertain if Manilius had direct knowledge of Aratus's poem or if he used a translation by Cicero, Ovid, or Germanicus. Present scholars, such as D. Liuzzi and Emma Gee, favor the latter position. Manilius refers to Homer (as the "greatest poet", maximus vates) and Hesiod (calling him "nearest to [Homer]", proximus illi).
Manilius is not quoted by any extant Roman author. He is alluded to by many, including Lucan, Petronius, Titus Calpurnius Siculus, Tertullian and Claudian, suggesting that he was widely read. The work of Julius Firmicus Maternus, who wrote in the time of Constantine about astrology and other subjects, exhibits so many points of resemblance to the work of Manilius that he almost certainly used him (or someone inspired by him) as a guide. In Matheseos libri octo (composed c. 334–37), Firmicus follows Manilius's method of instruction and analyzes the poet's astrological fundamentals. Given the similarity between Firmicus's work and Manilius's, it is odd that Firmicus does not mention Manilius by name and writes that few Romans (except Germanicus, Cicero, and Fronto) wrote about astrology.
Although few copies of the Astronomica survived into the medieval period, a 988 letter from Gerbertus Aureliacensis (who would become Pope Sylvester II) to the abbey at Bobbio includes a request for a work "by M. Manilius (or possibly Manlius) about astrology" (M. Manilius (v.l. Manlius) de astrologica) and a copy of the Astronomica was probably kept in the library at Bobbio. Despite the lack of attention in antiquity and the Middle Ages paid to the Astronomica, the poem and its author engendered scholarly interest with its 15th-century rediscovery. Italian humanist Lorenzo Bonincontri delivered lectures on it to large audiences, and he compiled his lecture notes into its first commentary. Bonincontri was apparently interested in Manilius's treatment of the nature of comets in the first book of the Astronomica; according to Stephan Heilen, portions of Bonincontri's De rebus naturalibus et divinis are based on Manilius's work.
Despite the attention received after its rediscovery, the Astronomica was never as widely studied as other classical Latin poems; according to Katharina Volk, it was long "neglected by modern scholarship". Interest in the poem developed in the second half of the 20th century, with classicists and others beginning to study Manilius's philosophical and scientific ideas. The first full-length monograph in English on Manilius and the Astronomica, Volk's Manilius and His Intellectual Background, was published in 2009. Two years later, she and Steven Green edited Forgotten Stars: Rediscovering Manilius' Astronomica with essays from scholars worldwide. The book's purpose was to "encourage readers to discover Manilius" and expand scholarly interest in the Astronomica, since previous research of the work's poetic, scientific, and philosophical themes had been primarily limited to Germany, France, and Italy.
While Manilius and his poem have been analyzed by scholars, many lay readers find the Astronomica confusing and at times byzantine. According to Kristine Louise Haugen, "The ambiguous phrases and extravagent circumlocutions necessitated by Manilius's hexameter verse must often have made the Astronomica seem, as it does today, rather like a trigonometry texbook rendered as a Saturday New York Times crossword."
Scholars have noted the irony of Manilius's relative obscurity, because he wrote the Astronomica in the hope of attaining literary immortality. Housman voiced this sentiment in a dedicatory Latin poem written for his critical edition of the poem, "constrast[ing] the regular eternal movement of the stars [both] with his and [his friend Moses J.] Jackson's mortality, [as well as] the sad fate of Manilius himself". Comparing the poem to a shipwreck ([carmina] naufraga), he called it an example of how "no man ever ought to trust the gods".