According to Volk, Manilius's Astronomica is the earliest work on astrology that is extensive, comprehensible, and mostly extant. Volk wrote 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".
The Astronomica, which is written in hexameters, opens with Manilius contending that he is the "first to sing of astrology". He also claims that the god Mercury engendered his interest in celestial bodies.

In the first book he ponders the origin of the universe, considering the theories of 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. According to Manilius, the universe is composed of two spheres: one (the Earth) is solid and the other (the "sphere of stars", often called the firmament) is hollow. The constellations are fixed in the firmament; the Earth is stationary and the firmament revolves around it, explaining the movements of the stars. The planets, the Moon, and the Sun also revolve around the Earth in the vast space between its surface and the edge of the firmament. Because the Earth is in the center of the universe, it is equidistant from the firmament and is thus not compelled to "fall" in any specific direction. According to Manilius, the universe is ruled by a god (conspirat deus) and is governed by reason (ratione gubernat). Manilius next discusses the constellations and stars and the celestial circles. In this section, the poet spends considerable time contemplating the Milky Way band, which, after exploring several hypotheses as to its existence, he concludes is likely to be the celestial abode for dead heroes. The first book ends with an exploration of comets, which Manilius sees as harbingers of calamity or great disaster.

Books two and three deal mainly with the finer details of the zodiac. Book two opens with a preface in which Manilius presents a brief history of hexameter poetry, singling out Homer and Hesiod. The purpose, Volk argues, is to emphasize the uniqueness of his poem in comparison to others rather than to insert himself into this poetic tradition. 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 by saying "that the divine cosmos is voluntarily revealing itself both to mankind as a whole and to the poet in particular", and that he is set apart from the crowd because his poetic mission has been sanctioned by fate. The poet then begins his explanation of the first astrologically significant circle: the zodiac itself. He first c…

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Manilius frequently imitates Lucretius, who wrote the didactic poem De rerum natura. Some classicists have suggested that Manilius may have sought to emulate Lucretius by writing six books, but evidence for this hypothesis is scarce, and it remains mostly speculative. While Lucretius's work espouses Epicureanism (a philosophy that emphasizes materialism and skepticism of superstition and divine intervention), Manilius's work is largely Stoic, and promotes a Greco-Roman understanding of creationism as well as fatalistic determinism. Both Volk and the Lucretian scholar David Butterfield have argued that Manilius is in many ways, an "anti-Lucretius", with the former arguing that "his presentation in the Astronomica of an orderly cosmos ruled by fate is a direct attack on the random universe depicted by his predecessor". Manilius sometimes conveys his philosophical stance 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 quae 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, 2.64), a "divine power" (divina potentia, 3.90), a "creator" (auctor, 3.681), and a "god" (deus, 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. Aratus's influence s especially noticeable, and it seems likely that Manilius based much of his first book on portions of Aratus's Phaenomena. Despite his debt to Aratus, Manilius diverges from his understanding of the cosmos; Aratus focuses on mythology and "graphic description", whereas 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. The latter position is favored by several 21st-century scholars, such as Dora Liuzzi and Emma Gee. In regards to the poet's relationship with Germanicus, Wolfgang Hübner writes: "The few echoes of Germanicus' translation of Aratus are insufficient for us to establish which of the two drew on the other, or whether the two were composed independently of each other."

The Astronomica directly refers to Homer (as the "greatest poet", maximus vates) as well as Hesiod (calling him "nearest to [Homer]", proximus illi), and alludes to numerous other Greek poets and writers such as Apollonius Rhodius, Choerilus of Iasus, Choerilus of Samos, and Aeschylus. The poem also contain…

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Although there are over thirty existing manuscript copies of the Astronomica, the text as it is known today is derived from three key manuscripts: Codex Gemblacensis (G), Codex Lipsiensis (L), and Codex Matritensis (M). These in turn belong to two separate manuscript families: "α" (which includes G and L), and "β" (which includes M). Of the two families, Robinson Ellis wrote: "[α] represents a text more correct, but worse interpolated; [β], a text which is fuller of copyists' errors, but less interpolated."

The first family, "α", takes its name from a now-lost source and includes manuscripts G and L. G, dating from the late 10th to the 11th century, was found at the monastery of Gembloux in Brabant, in modern-day Belgium; L, from the library of Leipzig, was probably written around the mid-11th century and has many corrections made by a scribe. Housman argues that L is the superior of the two, as it was probably copied straight from α, whereas G was probably derived from a copy of a copy.

The second family, "β", takes its name from the now-lost archetype that Poggio Bracciolini rediscovered near Konstanz during a break in the Council of Constance c. 1416–17. Included in this family is manuscript M, which was copied from the archetype by a German amanuensis on Bracciolini's request. However, due to the scribe's incompetence, manuscript M was riddled with mistakes, prompting Bracciolini to sarcastically remark that the new copy had to be "divined rather than read" (divinare oportet non legere). Although poorly written, M has been singled out as possibly the most important surviving manuscript, because it was a direct copy of the archetype (β), whereas G and L are derived from a less "faithful" copy (that is, α) of that archetype.

Following the invention of the printing press, the editio princeps of the Astronomica was published in Nuremberg around 1473 by the astronomer Regiomontanus from an error-riddled Italian copy. The text was critically edited by Joseph Justus Scaliger, whose first edition was published in Paris in 1579; a second and improved edition, collated at least partially from the Codex Gemblacensis, was published in Leiden in 1599–1600, and a third edition was published in 1655 after Scaliger's death by Johann Heinrich Boeckler. An edition with many corrections was published by Richard Bentley in 1739. Then, in five volumes between 1903 and 1930, Housman published what is considered by many scholars the authoritative edition of the poem (with a single-volume editio minor published in 1932). According to 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. P. Goold published a Loeb English trans…

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