Today the planet Neptune completes its first revolution around the Sun since it was discovered in 1846. So it’s a birthday of sorts for this icy blue world. Here’s a brief account of the amazing discovery of Neptune, and a few words on how to look for this most distant of major planets.
The tale of Neptune’s discovery is a classic piece of astronomical lore that features the extremes of human brilliance and pig-headedness, along with a little good luck.
In the early 19th century, astronomers noticed the position of Uranus was a little “off”. Sometimes it seemed to unexpectedly speed up in its orbit, and sometimes it slowed down. This strange motion, they realized, might be the gravitational effect of a more distant and undiscovered planet.
Newton’s law of gravitation gave astronomers the mathematical tools to determine the position of this undiscovered planet, but the number crunching required was formidable in this era before computers, when the only help was a good brain and a lot of pencils and paper.
Two brains were equal to the task. In England, the young mathematician John Couch Adams took on the problem in 1843 when he was still an undergraduate. Adams took his solution to the Astronomer Royal, George Airy, with the suggestion to have an English astronomer search for the planet based on Adams’ calculations. Adams was quiet and unpersuasive young man, and the powerful and formidable Airy never took Adams seriously.
The second man to calculate the position of the putative planet was the Frenchman Urban-Jean-Joseph Leverrier, who published his results in 1845 predicting the planet to be very close to the position predicted by Adams. Airy read Leverrier’s report, still ignoring Adams, and assigned the task of searching for the planet to the methodical and plodding James Challis, who commenced a glacial search for the planet (amazingly) without a decent star map.
Leverrier republished his results in late August 1846 and implored French astronomers to search for the planet. But Leverrier was a famously abrasive, and had few friends among French academics. So he, too, was ignored. So Leverrier wrote to German astronomers in Berlin to ask them for help. The Germans were game and gave the job to the astronomer Johann Galle and graduate student Heinrich Louis d’Arrest. The two used a 9-inch refractor and newly published star maps to find the planet on the first night, and confirm its existence the next night. Just like that.
The discovery by the Germans was headline news all over the world, and set in motion decades of finger pointing in France and England about the lost opportunity for a major discovery. The French were humiliated. And George Airy, upon his death, was denied a place of interment in Westminster Abbey for this lost opportunity for England.
Adams held no hard feelings towards Airy or Challis, and later wrote, “I could not expect … that practical astronomers, who were already fully occupied with important labours, would feel as much confidence in the results of my investigations, as I myself did.”
Today, despite the first sighting of the planet by the German duo, the quiet Adams and the irascible Leverrier are credited as the discoverers of Neptune.
Now, as when it as discovered, Neptune lies in the constellation Aquarius. While today marks its first trip around the Sun since it was discovered on September 24, 1846, Neptune is closest to its discovery position this year on October 27th and November 22nd as it retrogrades later this year. But it’s within a degree or so of this position right now, so if you want to stay up late and take a peek, here’s an image to help you locate this most distant planet.
Look for Neptune about 2.2 degrees north of iota Aquarii, about 1/3 the way from that star to Ancha (theta Aquarii). The magnitude 5.4 star HIP 109472 will lie in the same medium-power field of view.
At low magnification, Neptune will appear star-like. To be sure you’re looking at the planet, switch in a shorter focal-length eyepiece to get higher magnification to reveal the planet’s tiny blue-gray disk.
So if you’re up late, say, past 3 a.m. when Aquarius is high enough in the sky, take a look at Neptune on its “birthday”. A word of warning… if you are a total beginner, unless you have a go-to telescope, finding Neptune is not an easy task.