How did astronomers discover the planet Neptune and When?

It was not the searching eye of an astronomer but the enormously complex calculations of mathematicians that made Neptune known to the world.

If skeptical at first, people were dazzled when the new planet was finally seen and the computations thus confirmed. This was the first planet to be discovered mathematically, and the method seemed miraculous.

Two brilliant young men, independent of and unknown to each other, addressed the problem in a similar fashion, with similar results. What sparked their undertakings was an attempt to explain mysterious perturbations in the orbit of Uranus.

Ever since this planet’s discovery by William Herschel in 1781, astronomers had been puzzled by its irregular orbit—it did not act as it should if Newton’s principle of universal gravitation was to be believed. Some suggested the presence of a mysterious cloud or gas that was detaining the planet. Others believed a comet had collided with Uranus and thrown it off kilter.

Still others decided that Kepler and Newton must simply be wrong. But many be-lived there might be a distant planet whose gravitational pull was responsible for Uranus’s course. No one, however, believed the position of the planet and its size could be computed, and anyway the task was for years prohibitively complex.

Then in 1843 a phenomenally talented Cambridge mathematician, John Couch Adams, having just received his degree, began work on the problem. By September 1845, his intensive work paid off—he had figured the orbit and size of the new planet.

Now he needed confirmation from the astronomers. Adams sent his findings first to the astronomer royal, George Airy, who cast an arrogant eye on the work of the 26-year-old, then to James Challis, director of the university observatory, who likewise showed little interest. The two simply could not rely on theoretical deductions, and the mathematics was undoubtedly too much for them. Airy, furthermore, thought Adams had made a good guess at the orbit, then considered whether the perturbations of Uranus, as observed, would follow. But Adams, of course, had approached the problem the other way around—from the perturbations to the orbit.

In France, a respected mathematician, Urbain J. J. Leverrier, 34, also tackled the complex problem and came up with a nearly identical solution in June 1846. He published his results, giving the same position of the planet, to within 1°, as that reported by Adams.

Challis and Airy noted this with some astonishment, but even then they were slow to act. Airy asked Challis to begin the search with the refractor at the Cambridge observatory, one of the largest telescopes existing at the time, with its 25-inch-aperture object glass. Challis’s tragic error was that he would not focus solely on that position designated by Adams, but insisted on searching an area of 30° wide and 10° deep. After two months he had measured some 3,000 star positions, but neglected to chart them and so failed to note the movement of a planet in their midst.

On the Continent, Leverrier was eager for recognition. Finding the. French astronomers as hopelessly stodgy as the English, he sent his computations to Johann Galle, director of the Berlin Observatory. On the very night he received Leverrier’s material, September 23, 1846, Galle turned his refractor (with a nine-inch aperture) to the specified position in the sky—only to find a clutter of stars.

The search might have quietly ended there, but Heinrich d’Arrest, an assistant, recalled a chart of that portion of the heavens that had been prepared by the observatory two years previously but never published. When they compared the stars before them with those on the chart, they found something new. With much excitement they watched all night, yet couldn’t observe enough movement of the body to be sure it was the undiscovered planet.

But the next night Galle saw all that he needed to: the dim irregular disk had moved the predicted amount and in the proper direction, and lay within a degree of the position that Leverrier had specified. This, then, was the eighth planet, Neptune, orbiting the sun at a mean distance of 2,796,700,000 miles.

As might be expected, a heated debate raged across the English Channel about who should take the credit. After some initial injustices to the more reserved John Adams, the world ultimately decided that he and Leverrier might share the honor equally.