A century ago today, Russian forces were beginning the 133-day siege of Przemyśl and the German army took Péronne. Meanwhile, in a Nottinghamshire farmhouse, a young man wrote a poem about a mariner who sails off the earth into the sky. The Voyage of Éarendel the Evening Star deserves its day in the spotlight alongside war commemorations. It was the founding moment of Middle-earth.

Neither elves nor hobbits were yet in JRR Tolkien’s mind. But the star mariner is remembered in The Lord of the Rings, as Eärendil, forefather of kings, whose light in a phial wards off Mordor’s darkness. In the vast backstory of The Silmarillion, he carries the last Silmaril, a jewel preserving unsullied Edenic light, seeking aid against the primal Dark Lord.

None of this is in Tolkien’s poem from 24 September 1914. As an invented origin myth for the evening star, it is all energy and enigma:

Éarendel sprang up from the Ocean’s cup

In the gloom of the mid-world’s rim;

From the door of Night as a ray of light

Leapt over the twilight brim,

And launching his bark like a silver spark

From the golden-fading sand;

Down the sunlit breath of Day’s fiery Death

He sped from Westerland.

The poem hurtles on for a further five stanzas, but reveals nothing of background or motives. Yet Éarendel’s maverick course has much to do with Tolkien’s predicament as war broke out.

His taste for poetry had been formed at school when he translated poetry into Latin and Greek, and by his interest in the epics of Virgil and Homer. Until 1913, he had pursued a classical education at school and Oxford. But then he switched to English, an upstart discipline with a stiff backbone of Germanic philology. This was the study of the history of English and its cousin languages, German, Norse and Gothic, and it used rigorous comparison to reconstruct the common ancestral language spoken in pre-literate times. Philologists also reconstructed the unrecorded legends that could be glimpsed in fragments scattered through medieval literature.

JRR Tolkien at 24 in 1916 in his military uniform

By the time the 22-year-old Tolkien visited his aunt’s farm in Gedling in September 1914, he was laying the groundwork for an academic career. War threatened everything; relatives urged him to enlist. He was determined instead to complete his degree. Éarendel’s oblique voyage across the paths of the fixed stars accords with the visible movements of Venus but also, I suggest in Tolkien and the Great War, matches Tolkien’s journey off the well-trodden path of enlistment.

An insight from the historian Hugh Brogan got me looking at the poem again for the journal Tolkien Studies. What I found reveals the poem as a daring piece of cultural reappropriation, and one of two steps in late 1914 which set Tolkien on the threshold of Middle-earth. It is almost too perfect that this should be a myth of origin and transformation.

Brogan told me he thought Tolkien had plagiarised Shelley:

Arethusa arose

From her couch of snows

In the Acroceraunian mountains,

From cloud and from crag,

With many a jag,

Shepherding her bright fountains.

He certainly appears to have borrowed the rhyme scheme and much of the rhythm, plus some of the spirit. Shelley describes how a nymph flees a river god by diving under the earth – a transformation myth to explain the origin of natural phenomena. This is just the kind of thing Tolkien was trying to emulate for the evening star.

His opening also closely echoes a phrase in the Aeneid, translated by one contemporary as “Aurora arose/ and left the ocean’s rim.” However, the “Ocean’s cup” is translated from Beowulf. Éarendel is Old English, too, and has kindred names in other Germanic legends that also relate to stars.

Yet others refer instead to the sea; a German philological book Tolkien had just read argues that all these names hark back to a lost tale of the ultimate seafarer, an Odysseus of the northern oceans. But that didn’t explain the star references. He abandoned philological rigour to imagine a myth in which the seafarer became the evening star.

This is not plagiarism. Tolkien consciously uses Shelley’s classical template as a vessel for a Germanic-style myth, as if to say: enough with the Mediterranean, it’s time English literature looked north. The big step, however, was the realisation that he could turn philological reconstruction into creative narrative. That is one of two principles underpinning Middle-earth.

The other principle followed within weeks, when he was immersed in a literature and language entirely unrelated to Latin, Greek, Germanic or any other Indo-European language. In a talk to fellow students about the Finnish epic Kalevala, he put its distinct flavour down to the fact that Finnish comes from Finno-Ugric stock. If the language is different in descent, its legends will be too, he said.

Meanwhile, he was privately reworking the Kalevala story of Kullervo. As if he could not help himself, he illogically began replacing some of the names with ones in an entirely invented language. It looks and sounds a lot like Finnish, yet it is recognisably a prototype of Quenya or “high-elven”. Tolkien had been inventing languages for years, often inspired by real languages, but now for the first time one of them set up home in story.

It was a breakthrough that promised to breathe life into his linguistic creation. He abandoned The Story of Kullervo for something far more original. If languages and legends were interdependent, as he now saw, he would create legends with their own nomenclature – the hallmark of Middle-earth. And with his 24 September 1914 poem, he had found a way to invent those legends. He could go beyond philology, in imagining the half-forgotten origins of recorded legends.

While finishing his degree and after enlisting in June 1915, Tolkien worked on this formula. On his return from the Somme in 1916, he used it as the basis for The Book of Lost Tales, the first version of The Silmarillion. That seminal opening line “In a hole in the ground there lived a hobbit” is usually seen as Tolkien’s breakthrough moment. The real honour should go to “Éarendel sprang up from the Ocean’s cup”.

• The Voyage of Éarendel the Evening Star appears in The Book of Lost Tales, part two, edited by Christopher Tolkien

• Mordor, he wrote: how the Black Country inspired Tolkien’s badlands