This week, the Guardian and the Observer are running a series of seven pamphlets on the Romantic poets. To coincide with it, I'm blogging daily on one of each day's selected works

A bibulous dinner party given by the artist and diarist Benjamin Haydon to celebrate the completion of the first stage of his vast painting, Christ's Entry into Jerusalem brought together the "Lakeland" and "Cockney" schools of poets, ie William Wordsworth, Charles Lamb and John Keats. In fact, among the crowd of dazzled spectators with which Haydon has surrounded the triumphant Christ, are portraits of Wordsworth and Keats, as well as Voltaire and Newton. Lamb humorously took the pious Haydon to task for including Newton, "a Fellow who believed nothing unless it was as clear as the three sides of a triangle". The poetic company concurred, rising to drink to "Newton's health and confusion to mathematics".

Keats was bantering, perhaps, when he asserted at Haydon's gathering that Newton had spoiled the rainbow by reducing it to a prism. But again in his poem Lamia he alludes to the power of "cold philosophy" to "unweave the rainbow". There's no doubt that his death-shadowed early life provided an additional impulse to his devotion to literature: he needed an escape route into enchantment. And yet, Keats's openness to experience and his powerful impulse towards self-education are hardly the qualities of an opponent of science. That famously stated willingness to remain "in uncertainties, Mysteries, doubts" is surely proof enough of an essentially scientific temperament. He was medically trained, and, as all biographical accounts make clear, he confronted his own mortality to the very end with courageous, pitiless realism.

It's in an early sonnet, "On First Looking into Chapman's Homer" (1816) that Keats displays the attitudes of a true scientist. With a superbly sustained metaphor of exploration, the sonnet describes an episode in the imaginative voyage that was of paramount importance to his development – that of reading. He had learned Latin, but Greek was not available to "Cockney" poets. It was thanks to his schoolmaster Charles Cowden Clarke that Keats had first discovered Edmund Spenser. Now Clarke introduced him to the work of another Elizabethan, George Chapman, whose translations of Homer the young men read together during an evening's get-together which the enthralled Keats would describe as "our first symposium". The celebratory sonnet was completed the same night, in time to be delivered to Clarke in the following morning's post.

Chapman was the first poet to try to render Homeric rhythms in English. Translation, for such a writer, is a voyage in pursuit of the truthfulness of linguistic beauty, and reading a translation aimed at fidelity to the original was perhaps the profoundest way in which Keats could appropriate scientific method to the literary art to which he had sworn "fealty".

Keats's metaphor would be less effective if he did not invoke two actual discoveries in the poem - one astronomical, the other terrestrial. It's well-known that the sighting of the Pacific Ocean, alluded to in the last four lines, should not have been attributed to Cortez but to another conquistador, Balboa. (Yes, Keats should have done more research – but he was in a forgivable hurry.) Less widely known is the fact that the "watcher of the skies" summoned in lines nine and 10 is the astronomer, William Hercshel, who had discovered a new planet, Uranus, in 1781.

Keats had learned about astronomy in boyhood. At school he had taken part in a learning-game devised by the marvellously imaginative educator John Rylands, in which the boys arranged themselves on the school playground in the form of an orrery. Later, for the self-imposed project of translating Virgil, Keats was awarded a copy of Bonnycastle's Introduction to Astronomy, an early work of popular science fully up-to-date on the latest developments, including Herschel's discovery of Uranus.

The rhythms of the Chapman sonnet convey a wide-sweeping sense of movement – of planets circling the heavens, and ships circumnavigating the earth. These patterns were perhaps already implicit in the Petrarchan sonnet. But the last object to move physically in the poem is the planet that "swims into" the watcher's ken at the start of the sestet. And then there is immobility: the stock-still immobility of wonderment.

Keats and his readers are truly in a new world – a rather cinematic one. The moment of revelation on Darien is viewed in lingering long-shot. Somehow, the comfortable closure which the sonnet-form invites is "translated" into an open question. We see the focused, interrogative stare of the expedition's leader, and, on the faces of his men, the "wild surmise" that generates further questions. We have not only reached the Pacific Ocean, but the emotional core of scientific discovery.

There is no need for overstatement, and the poem's quiet last line – literally quiet in that the men are silent with amazement, but also quiet in its perfectly-realised naturalism – is perhaps its greatest triumph. No other proof is needed of Keats's power to conquer territories in his short swift voyage to poetic mastery. "On First Looking into Chapman's Homer" is an early page in the log-book of a journey from lush fancifulness to telescopic clarity of observation.

On First Looking into Chapman's Homer

Much have I travelled in the realms of gold,

And many goodly states and kingdoms seen;

Round many western islands have I been

Which bards in fealty to Apollo hold.

Oft of one wide expanse had I been told

That deep-browed Homer ruled as his demesne;

Yet did I never breathe its pure serene

Till I heard Chapman speak out loud and bold:

Then felt I like some watcher of the skies

When a new planet swims into his ken;

Or like stout Cortes when with eagle eyes

He stared at the Pacific – and all his men

Looked at each other with a wild surmise –

Silent, upon a peak in Darien.

