On July 12, 2011, Neptune arrived at exactly the same location in space where it had been when it was discovered nearly 165 years ago, completing one full orbit. To commemorate this event, NASA’s Hubble Space Telescope has taken a set of “anniversary pictures” of the giant blue-green planet.
Neptune is the most distant major planet in our solar system. German astronomer Johann Galle discovered it on September 23, 1846. The discovery doubled the physical size of the known solar system. The planet is 2.8 billion miles from the Sun, 30 times farther than Earth. Under the Sun’s weak pull at that distance, Neptune plods along in its huge orbit, slowly completing one revolution every 165 years.
Four Hubble images of Neptune were taken with the Wide Field Camera 3 on June 25-26, during the planet’s 16-hour rotation. The snapshots were taken at roughly four-hour intervals, offering a full view of the planet. The images reveal high-altitude clouds in the northern and southern hemispheres. The clouds are composed of methane ice crystals.
The giant planet experiences seasons just as Earth does, because it is tilted 29 degrees—similar to Earth’s 23-degree tilt. Instead of lasting a few months, each of Neptune’s seasons continues for about 40 years.
The snapshots show that Neptune has more clouds than a few years ago, when most of the clouds were in the southern hemisphere. These Hubble views reveal that the cloud activity is shifting to the northern hemisphere. It is early summer in the planet’s southern hemisphere and winter in its northern hemisphere.
In the Hubble images, absorption of red light by methane in Neptune’s atmosphere gives the planet its distinctive aqua color.
Neptune has an intriguing history. It was Uranus that led astronomers to Neptune. Uranus, the seventh planet from the Sun, is Neptune’s inner neighbor. British astronomer Sir William Herschel and his sister Caroline discovered Uranus in 1781. Shortly after the discovery, Herschel noticed that the orbit of Uranus did not match the predictions of Newton’s theory of gravity. Studying Uranus in 1821, French astronomer Alexis Bouvard speculated that another planet was tugging on the giant planet, altering its motion.
Twenty years later, Urbain Le Verrier of France and John Couch Adams of England, who were mathematicians and astronomers, independently predicted the location of the mystery planet by measuring how the gravity of a hypothetical unseen object could affect Uranus’s path. Le Verrier sent a note describing his predicted location of the new planet to the German astronomer Johann Gottfried Galle at the Berlin Observatory. Over the course of two nights in 1846, Galle found and identified Neptune as a planet, less than one degree from Le Verrier’s predicted position.
Actually, Galle was not the first to see Neptune. In December 1612, while observing Jupiter and its moons with his handmade telescope, Galileo Galilei recorded Neptune in his notebook, but he thought it was a star. More than a month later, in January 1613, he noted that the “star” appeared to have moved relative to other stars. But Galileo never identified Neptune as a planet, and apparently did not follow up those observations—so he failed to be credited with his discovery.
Neptune is too faint to be seen with the naked eye, it but can be seen in binoculars
or a small telescope. Currently it is located in the constellation Aquarius, close to the boundary with Capricorn.
You can contact Bob Eklund at [email protected], or visit his websites at www.bobeklund.com and http://firststarbook.com.