Watcher of the sky William Herschel

'On First Looking into Chapman's Homer' is a sonnet written by Keats in October 1816, describing the overwhelming excitement he felt when he first encountered the works of Homer.

On First Looking into Chapman's Homer

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-brow’d 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 Cortez when with eagle eyes
He star’d at the Pacific—and all his men
Look’d at each other with a wild surmise—
Silent, upon a peak in Darien.

Keats writes that the surreal feel of experiencing Homer's world was like an astronomer discovering a new planet. The underlined part refers to the discovery of Uranus by William Herschel in 1781, 35 years prior to the writing of the sonnet. The discovery of the new planet in the solar system must have been a news that rocked the whole world back in the 18th century.

William Herschel, a musician and an astronomer, observed Uranus on 13^th March 1781 from the garden of his house in Bath. Although his profession was a musician, he became deeply interested in astronomy, first as a relaxation from his long hours of teaching music. And his growing interest to the stars made him move to a house with a cellar and a garden where he could construct his telescope and devices for his star observations. William's sister kept the house and looked after him but she was constantly displeased with his brother turning almost every room of the house into a workshop (His brother used horse manure to make a mold for his telescopic mirror... No wonder she was displeased.). 

With many trials and errors, he made his own telescope and observed the night sky. He studied the stars on the parallax of the fixed stars, calculating the distance of the stars with a device he also constructed. After nights of observation, he came to the conclusion that one certain star he discovered which was showing a fuzzy movement, was not a fixed one but a new planet.

I went to the house where William Herschel lived (The Herschel Museum of Astronomy) and saw his workshop and the telescopes and devices he constructed. With no electricity, no digital devices, all the construction, experiments, and nightly observation would have brought more disappointment and failure than joy and success. The chances of a discovery would have seemed a miniscule! However, Herschel's obstinate trial lead him to a triumph. 

Imagine the excitement, the joy he experienced - a discovery of a new planet, farthest from the Sun. It changes the knowledge of the world.

Keats alluding to Herschel's discovery describes reasonably and brilliantly his excitement to seeing a new world.