It wasn’t long after the planet Uranus accidentally was discovered by William Herschel in 1781 that astronomers realized the new planet was not moving around the sun in the manner prescribed by Newton’s laws of motion and gravity. Even when taking into account all the small perturbations caused by the other six known planets tugging on it this way and that, Uranus was straying from its predicted orbit.
Rather than questioning the validity of Isaac Newton’s law of gravity, two astronomers independently concluded that the strange behavior of Uranus could be explained by the presence of a hitherto-unknown planet even farther from the sun. Each man made careful calculations to determine the whereabouts of this hidden world.
Englishman John Couch Adams, the young, unknown upstart of an astronomer, sent his predicted position to the director of the Cambridge Observatory, James Challis, in summer 1846, but an inadequate star atlas hampered Challis’ search for Adams’ planet such that he did not recognize it when he saw it Aug. 8 and 12.
Meanwhile, Urbain Leverrier, the well-known veteran French astronomer, sent his predicted position to the director of the Berlin Observatory, Johann Galle, on Sept. 23, 1846. Using a much better set of star charts, Galle found the new planet within 1 degree of Leverrier’s predicted position after only one hour of searching. The amazing discovery was a triumph not only for Leverrier and Galle but for Isaac Newton and his law of gravity, as well.
History now credits both Adams and Leverrier with independently making the calculations that led to the discovery of our solar system’s eighth planet. It is not a stretch to say that Neptune was discovered with a pen rather than a telescope.
Neptune orbits the sun at a distance 30 times greater than the distance of Earth from the sun, and it requires 165 years to complete one orbit. That means that this year, Neptune completes its first full orbit of the sun since its discovery in 1846. The faint and distant planet has returned to the same region of the sky that it occupied when Galle first recognized it amongst the stars of the constellation Aquarius, 165 years ago this week.
Since Neptune’s discovery one Neptune year ago, just think of what has happened here on Earth. We’ve endured a Civil War and two world wars as well as many smaller battles and military actions. Albert Einstein was born and died, rewriting the physics books in his brief time here. We split the atom and mapped the human genome. The Titanic sank and rockets carried humans into Earth’s orbit. Astronauts left their footprints on the moon, and our robotic emissaries have explored all of the major planets and their moons at close range, including Neptune. Pluto, an even more distant planet, was discovered and later demoted, leaving Neptune as the most distant planet in our solar system. All of this human triumph and tragedy occurred in 165 years, while Neptune moved silently through the cold darkness to close the circle.
Happy anniversary, Neptune.
Jimmy Westlake teaches astronomy and physics at Colorado Mountain College’s Alpine Campus. He is an avid astronomer whose photographs and articles have been published all around the world. Check out Jimmy’s astrophotography website at www.jwestlake.com.