What’s Out Tonight?

A general ASTRONOMY site to get you started exploring the night sky


Facts about Neptune 

• Neptune is the 8th planet from the Sun—the last planet in our solar system. 
• It is a Gas Giant. 
• It was discovered in 1841/1846. 

Diameter: 30,777 miles 
Mass: 17.2 Earth masses 
Density: 1.64 where water = 1 
Gravity: 1.14 times that of Earth 
Albedo (% of Sunlight reflected): 41% 
Rotation on Axis: 19.2 hours 
Inclination of Axis to Orbit: 28.3° 

Distance from Sun: 2,794,3500,000 miles 
Revolution about Sun: 163.73 years 
Inclination of Orbit to Earth’s Orbit: 1.77° 

Atmosphere: 74% hydrogen gas, 25% helium gas, 1% Methane gas Surface Temperature: –373° F just below cloudtops 

Moons: Neptune has about a dozen moons. Triton, its largest has a diameter of 1,678 miles and orbits the planet every 5.9 days. 

Observing in a small telescope: In a small telescope, Neptune appears as a small, pretty blue disk. You need magnifications of 200x or more just to see a hint of a disk. To find it, It is easiest to use a computerized GO TO telescope that can automatically move to any object chosen from a list in its hand-controller.

Neptune has spots like Jupiter’s Great Red Spot. However, its spots representing turbulence in the atmosphere, similar to hurricanes, form and dissipate. 

The Planet Neptune

Uranus was discovered accidentally; however, Neptune was discovered by mathematically calculating its position. In the years following the discovery of Uranus, astronomers noticed that Uranus’ path in the sky did not match the path computed by scientists. Uranus was straying from its computed course, but why? Some scientists thought the cause could be the gravitational attraction of an undiscovered planet. 

In 1841, John Couch Adams of England calculated where this new planet might be found. But Adams did not engage in observational astronomy, so his results were forwarded to the Royal Observatory in Greenwich, England. Unfortunately, the director did not use this information to look for a possible new planet. Four years later and in-dependent of Adams, Urbain Le Verrier of France made his own calculations. He then asked Johann Galle, a German astronomer, to search for the planet. Galle found the planet the first night that he looked for it on September 23, 1846. Neptune thus became the first of two planets discovered by using a scientific method of investigation. The other planet was Pluto, which has been demoted to a dwarf planet. Neptune was named after the Roman god of the sea. 

Twin Planets 
Neptune is the smallest of the four gas giants but could be Uranus’ twin because their diameters, colors, atmospheres and internal makeup are similar. Uranus and Neptune are the two most similar planets in our solar system. Voyager 2’s last call In 1989, twelve years after it was launched from Earth, Voyager 2 flew by Neptune. No one had ever seen this planet close up before. Neptune is colder than Uranus because it is farther away from the Sun. This fact led scientists to believe that the atmosphere on Neptune would be much simpler and even plainer than on Uranus. But astronomers were surprised to find that Neptune’s atmosphere is much more complex and beautiful than its twin. Voyager 2 brought surprise after surprise on its journey through our solar system, and Neptune was the last planet that Voyager visited. 

Observing Neptune 
Neptune is an 8th magnitude object and cannot be seen with the unaided eye. It is just visible with binoculars but more easily seen with a small telescope. Neptune has slightly more blue coloring than Uranus. Except for its color, it can appear indistinguishable from other stars since magnifications of about 200x are required to discern a hint of a disk. Neptune’s narrow rings, which contain areas of concentrated particles called ring arcs, and its atmospheric clouds cannot be seen in amateur telescopes. The easiest way to find Neptune is to use a computerized GO TO telescope that will automatically move to Neptune after this planet is chosen from a list in the hand controller. Otherwise, use a planetarium app or software program to find its location. During the years 2000 to 2050, Neptune moves east from the constellation Capricornus to just inside the boundary of Taurus.

Neptune’s Rings.
The top picture are two images of Neptune’s rings taken 90 minutes apart by Voyager 2 as it passed the last planet in 1989.
The bottom image is more recently from the James Webb telescope in 2022. It captures images in the infared showing “hot spots” on its surface. The three specs are moons of Neptune.

Triton, Neptune’s largest moon. Although Triton was discovered just 17 days after Neptune, it was not named until 60 years later. As can be seen in this photograph, part of its surface resembles the skin of a cantaloupe. Triton has a thin atmosphere, ices on its surface and gaseous eruptions from its interior.