Human World

Neptune discovered on this date in 1846

Neptune from Voyager 2 showing dark and light ovals.
Voyager 2 image of Neptune. At the top is the Great Dark Spot, accompanied by bright, white clouds that undergo rapid changes in appearance. To the south of the Great Dark Spot is the bright feature that Voyager scientists nicknamed “Scooter”. Still farther south is the feature called Dark Spot 2, which has a bright core. Image via NASA/ JPL.

Today in science: Discovery of Neptune

Astronomers found the outermost major planet in our solar system – Neptune – on September 23, 1846. It was the first planet to be discovered using mathematics. Johann Gottfried Galle, Urbain Jean Joseph Le Verrier, and John Couch Adams all worked independently to help find this distant world, which isn’t visible to the eye. Their separate endeavors led to an international dispute as to who should get the credit for Neptune’s discovery.

A telescope is necessary to see Neptune. However, Its discovery didn’t come solely through the use of a telescope. It came from astronomers’ analysis of data related to the orbit of Uranus. Astronomers noticed discrepancies in Uranus’ observed position in contrast to its predicted position. So, the mathematically predicted location of the planet Uranus did not match its actual location.

Deviations in the orbit of Uranus led to discovering Neptune

Many popular notions of the time attempted to explain Uranus’ deviation from its predicted orbit. One thought was that perhaps Newton’s law of universal gravitation ceased to work or worked differently at such great distances from our sun. But assuming that gravity works the same throughout space, what could be causing the discrepancies in Uranus’ orbit? By the way, today gravity is considered constant and one of our universe’s four fundamental forces.

The problem with the orbit of Uranus caused astronomers to begin speaking of another possible planet beyond it. The French astronomer Urbain Le Verrier began using mathematics to try to locate the mystery planet’s position in June 1845. The British astronomer John Couch Adams was also working on this problem. Neither was aware of the other’s calculations.

On September 23, 1846, Galle used Le Verrier’s calculations to find Neptune only 1° off Le Verrier’s predicted position. The planet was then located 12° off Adams’ prediction.

Who really discovered Neptune?

After Neptune’s discovery, an international dispute arose concerning who was the ‘real’ discoverer of the new planet, Le Verrier or Adams. The existing political tensions between France and Great Britain amplified this conflict. Today, both of them – and Galle, who was the first to knowingly see the new planet through a telescope – share the credit for discovery.

Ironically, as it turns out, both Le Verrier and Adams had been very lucky. Their predictions indicated Neptune’s distance correctly around 1840-1850. If they had made their calculations for another time, both predicted positions would have been off. Their calculations would have predicted the planet’s position only 165 years later or earlier, since Neptune takes 165 years to orbit once around the sun.

Galileo recorded Neptune as a faint star

By the way, it was possible to discover Neptune simply by using a telescope. Like all planets in our solar system – because it’s closer to us than the stars – it can be seen from Earth to move relative to the starry background. For example, the great astronomer Galileo, using one of the first telescopes, apparently recorded Neptune as a faint star in 1612. If he had watched it over several weeks, he’d have noticed its unusual motion.

Blue gas planet with cloud features.
Another Voyager 2 image of Neptune, acquired on August 20, 1989, at a range of 4.4 million miles (7 million km) from the planet, 4 days before Voyager’s closest point to Neptune. You can see Neptune’s Great Dark Spot and its bright smudge companion. On the west limb, a fast-moving bright feature called Scooter by astronomers is visible, with another little dark spot. These clouds persisted for as long as Voyager’s cameras could resolve them. Image via NASA.

Bottom line: The fascinating story of the discovery of Neptune – outermost major planet in our solar system – found on September 23, 1846. It was the first planet to be discovered not solely by looking in the sky … but by using mathematics.

September 23, 2022
Human World

Like what you read?
Subscribe and receive daily news delivered to your inbox.

Your email address will only be used for EarthSky content. Privacy Policy
Thank you! Your submission has been received!
Oops! Something went wrong while submitting the form.

More from 

EarthSky Voices

View All