First anniversary of Neptune's discovery
It's exactly one year since Neptune was discovered - one Neptunian year, or 165 of our own rather shorter ones.
The most distant major planet in our solar system, Neptune was discovered by German astronomer Johann Galle on September 23, 1846. While Galileo had seen it in 1612, he recorded it as a star.
At the time Galle correctly identified Neptune as a planet, it was twice as distant from the sun as anything that had been observed in the solar system at the time - 2.8 billion miles from the sun, 30 times farther than Earth.
NASA's marking the occasion by releasing four Hubble images taken on June 25-26, at roughly four-hour intervals. They reveal high-altitude clouds in both hemispheres, composed of methane ice crystals.
With the seasons slowly changing, Neptune has more clouds than a few years ago, when most were in the southern hemisphere. Now, though, it's winter in the northern hemisphere, and that's where most cloud can now be seen.
Neptune was discovered after British astronomer Sir William Herschel and his sister Caroline found Uranus in 1781, and soon after noticed that its orbit didn't match the predictions of Newton's theory of gravity.
In 1841, Urbain Le Verrier of France and John Couch Adams of England independently predicted Neptune's location by measuring how the gravity of a hypothetical unseen object would affect Uranus's path. Le Verrier shared this information with Galle, who then found and identified Neptune as a planet, less than a degree from Le Verrier's predicted position.
Neptune can't be seen with the naked eye, but is visible with binoculars or a small telescope, in the constellation Aquarius, close to the boundary with Capricorn.