Happy First Birthday, Neptune!

Neptune is currently low in the southeastern sky around midnight in the constellation Aquarius. Created with Stellarium

When I first saw Neptune through my 6-inch reflector in 1967, it was in the constellation Libra and the second most distant planet. In the ensuing 34 years, the aqua planet has crept four constellations eastward into Aquarius. Then in 2006 it became the most distant planet, when Pluto was demoted to dwarf planet status.

Talk about a slow ride. At 2.8 billion miles, Neptune is so far from the sun that it takes 165 years to complete one orbit. No wonder it hasn’t gone very far since my teenage days. Yesterday July 12, Neptune reached a major milestone,� arriving at the same location in the sky where it was first discovered 165 years ago on September 23, 1846. Six generations of humanity have come – and some have gone – in that lengthy interval.

Neptune is unusual in that its existence was inferred from unexplained motions of the planet Uranus, discovered in 1781. By the early 1800s astronomers noticed that Uranus was slightly ahead of its predicted position. After 1830 however, it lagged behind. French astronomer Alexis Bouvard suspected that gravitational effects from an unseen outer planet might be responsible for the irregularities. But how to prove it?

John Adams (left) and Urbain LeVerrie (center) both successfully predicted the approximate location of the planet Neptune in the sky. In 1846, German astronomer John Galle, using LeVerrier's positions for the object, became the first person to see Neptune.

In 1845, the director of the Paris Observatory assigned the problem to the brilliant French mathematician and astronomer Urbain LeVerrier. Meanwhile, British mathematician and astronomer John Couch Adams learned of the discrepancies after a chance reading of an 1832 astronomy progress report. The two soon set upon figuring out where the “mystery object” tugging on Uranus might be found in the sky.

After lengthy calculations involving the sun, Uranus and the possible new planet, LeVerrier published his results in July 1846. Since no one began searching straightaway, LeVerrier took matters into his own hands and penned a note to German astronomer Johann Galle at the Berlin Observatory, providing him with the planet’s predicted location along the border of the constellations Aquarius and Capricornus.

1838 painting by Carl Daniel Freydanck of the Berlin Observatory where Neptune was discovered in 1846

Galle was enthusiastic and ready to get to get down to business. He convinced the director to let him search for the putative planet with the observatory’s fine 9-inch refracting telescope. With the help of his graduate student assistant Heinrich d’Arrest and charts from a brand new star atlas, Galle aimed the scope at the position given by LeVerrier and began calling out the stars he saw to d’Arrest. D’Arrest followed them one after another until Galle called out an 8th magnitude “star” that didn’t match the atlas. D’arrest immediately shouted out: “That star is NOT on the map!”Careful scrutiny revealed that the “star” showed a small bluish disk and was indeed the sought-after planet. Neptune was found about a degree from LeVerrier’s calculated position.

Adams meanwhile had calculated a position within 2.5 degrees of where Neptune was found. For a variety of reasons, ranging from a slow initial search start to poor charts, the British astronomy establishment missed the chance to claim discovery for their camp.

If you’d like to see Neptune, I’ll be making finder charts for it in August, when the moon is out of the sky and the planet becomes visible at a more convenient hour. At magnitude 8, it’s quite easy to spot in binoculars. Matter of fact, Galileo was the first person to see Neptune through his tiny telescope back in December 1612. He was observing Jupiter at the time and made a sketch that night and again the following month, during which time the planet had moved in relation to a background star. Unfortunately he never followed up on his observations and missed the chance to add yet another entry to his long list of astronomical firsts.

To commemorate Neptune's first observed circuit around the sun, the Hubble Space Telescope took these photos of the planet during a full 16-hour rotation on June 25/26. The pink wisps are high altitude clouds composed of methane ice crystals. Credit: NASA/ESA

Through a telescope magnifying around 100x and higher, Neptune shows a pale blue disk, a color caused by absorption of sunlight by methane gas in its massive atmosphere. Measuring about 31,000 miles across, Neptune is the fourth largest planet. Since the planet’s axis is tipped 29 degrees, it’s similar to Earth in having seasons, but instead of several months, Neptune’s each last about 40 years, due to it much longer orbital period.

While birthdays may be a rarity on Neptune, seeing the planet with binoculars and telescopes will be easy later this late summer and fall, as it plods its way westward through the sparse star fields of Aquarius.