Then felt I like some watcher of the skies
When a new planet swims into his ken,
Or like stout Cortez when, with eagle eyes,
He stared at the Pacific, and all his men
Look’d at each other with a wild surmise,
Silent, upon a peak in Darien.
Keats, of course. From On First Looking into Chapman’s Homer. A Petrarchan sonnet written in iambic pentameters.
The planet was Uranus, identified by Hershel in 1781, the year of the Iron Bridge, before Keats was born, and the first to be added to the list since antiquity. It is visible to the naked eye, but had been thought to be a star.
The discovery of four moons of Jupiter by Galileo, and of five of Saturn, one by Huygens, four by Cassini, preceded Hershel’s discovery. Jupiter’s Ganymede and Callisto may just be visible with the naked eye.
Herschel went on, after a few years, to discover two Uranian moons, followed by two more of Saturn.
The first four asteroids were discovered during Keats’s boyhood. Three of them are at the extreme margin of visibility with the naked eye. The first was observed on the first day of the nineteenth century.
The first Europeans to see the east coast of the Pacific were members of Vasco Núñez de Balboa’s expedition. Wikipedia: “Keats had been reading William Robertson’s History of America and […] conflated two scenes there described: Balboa’s finding of the Pacific  and Cortés’s first view of the Valley of Mexico . The Balboa passage: ‘At length the Indians assured them, that from the top of the next mountain they should discover the ocean which was the object of their wishes. When, with infinite toil, they had climbed up the greater part of the steep ascent, Balboa commanded his men to halt, and advanced alone to the summit, that he might be the first who should enjoy a spectacle which he had so long desired. As soon as he beheld the South Sea [Mar del Sur] stretching in endless prospect below him, he fell on his knees, and lifting up his hands to Heaven, returned thanks to God, who had conducted him to a discovery so beneficial to his country, and so honourable to himself. His followers, observing his transports of joy, rushed forward to join in his wonder, exultation, and gratitude’ (Vol. III).”
“Keats’ generation was familiar enough with the polished literary translations of John Dryden and Alexander Pope, which gave Homer an urbane gloss similar to Virgil, but expressed in blank verse or heroic couplets. Chapman’s vigorous and earthy paraphrase (1616) was put before Keats by Charles Cowden Clarke, a friend from his days as a pupil at a boarding school in Enfield Town. They sat up together till daylight to read it: ‘Keats shouting with delight as some passage of especial energy struck his imagination. At ten o’clock the next morning, Mr. Clarke found the sonnet on his breakfast-table.’” No source given for the quotation.
The earlier lines:
“Much have I travell’d 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:”
Landscapes (old post).
A Study of History, Vol X, OUP, 1954