In 1613, Galileo first observed Neptune, when it came very close to Jupiter. But, he thought, it might be just a star nothing else. After those couple of days, he just could not locate it. When Uranus was discovered and scientists were observing Uranus’ behaviour, they found, its orbital behaviour was not working in accordance with Newton’s laws. This kept the research continued, and then it was predicted that another distant planet might be present at the Uranus’s orbit. Finally, on 23 September 1846, Neptune was discovered by Galle and d’Arrest. Its position was almost near the location which, Adams and Le Verrier predicted by their mathematical calculations which was based on the position of Jupiter, Saturn and Uranus.
Neptune is at the extreme distance from the earth and hence is invisible to the naked eye. As it is nearly 4.5 billion km from the sun, it orbits the sun about once every 165 years. In comparison to the planet’s rotation axis, the main axis of Neptune’s magnetic field is tipped over by about 47°. Similar to Uranus, Neptune undergoes wild variations during each rotation because of its misalignment, that is, its magnetic axis is tilted about 60° from the axis of rotation. However, its magnetic field is about 27 times powerful than the earth. Neptune's orbit is about 4,504,000,000 km from the Sun. With a diameter of 49,528 km (equatorial) and mass of 1.02x1026 kg, Neptune’s core is almost the same as the size of earth. It has great blue spot just as Jupiter has big red spot.
Approximately an earth-sized solid core, Neptune’s atmosphere extends to great depths, gradually merging into water and other ‘melted ices’. The atmosphere is almost similar to the planet Uranus. Methane is the main reason for its blue colour. Its atmosphere has high altitude cirrus, gigantic storms and swirls of clouds. This ocean slowly fades out into the atmosphere. Its surface pressure is about 1000 bars. Neptune’s atmospheric temperature at the top of its clouds is approximately -221.4°C, which makes this planet the coldest one in the solar system. Because of these cold temperatures and the flow of fluid gases in the planet, Neptune’s atmosphere might reduce friction that makes it easy to generate strong winds.
Neptune has an atmosphere composed of Helium (19%), Hydrogen (80%), Methane (1.5%) and Ammonia. By mass, the overall structure of Neptune involves 25% rock, 60%–70% ice, and about 5%–15% Hydrogen and Helium. Its internal structure is also similar to that of Uranus. Its core is mainly composed of iron and magnesium-silicate. Neptune’s core is probably made up of liquid rock, which farther slowly converts into an ocean. This ocean mainly contains water, helium and hydrogen but it also contain methane and ammonia. The temperature of its core is around 7000°C.
Thirteen moons have been discovered around Neptune. As Neptune is too far from earth, it becomes more difficult to locate its moons. These moons were named after the characters from Roman and Greek mythology. After the technology improved, now we know the existence of these thirteen moons orbiting Neptune. They are discussed as below:
- Naiad: It was discovered by Voyager 2, in 1989. Its diameter is about 58 km. It takes 0.29 earth days to complete the orbit.
- Thalassa: It was discovered by Voyager 2, in 1989. Its diameter is 80 km. It takes 0.3 earth days to complete the orbit.
- Despina: It was discovered by Voyager 2, in 1989. Its diameter is 148 km. It takes 0.33 earth days to complete the orbit.
- Galatea: It was discovered by Voyager 2, in 1989. Its diameter is 158 km. It takes 0.43 earth days to complete the orbit.
- Larissa: It was discovered by Voyager 2, in 1989. Its diameter is 104x89 km. It takes 0.5 earth days to complete the orbit.
- Proteus: It was discovered by Voyager 2, in 1989. Its diameter is 218x208x201 km. It takes 1.1 earth days to complete the orbit. It is a second largest moon. It has a circular orbit just over the cloud-tops of Neptune, also, it is a dark moon.
- Triton: It was discovered by William Lassell, in 1846. Its diameter is 2704 km. It takes synchronous earth days to complete the orbit. It is the largest out of the eight moons of Neptune. Triton rotates opposite to Neptune's rotation which means it retrogrades. In our solar system, it is the coldest moon with a temperature of -235°C. Triton is a rocky moon and it has a polar ice cap at its south pole.
- Nereid: It was discovered by Gerard P Kuiper, in 1949. Its diameter is 340 km. It takes 360 days, 3 hours to complete the orbit. Nereid is the third largest and Neptune's outermost moon. It has an irregular shape. Its orbit is very elliptical with 6 million miles of longer distance and 867,000 miles of closer distance. To orbit Neptune it takes almost one Earth year.
Fives moons out of thirteen have been discovered in recent times namely Halimede, Sao, Laomedeia, Psamathe and Neso.
In Roman mythology, Neptune is believed to be the god of fresh water and the sea. Neptune is a son of Saturn (Chronus) and Rhea. His two brothers are Jupiter (Zeus) and Pluto (Hades). In Roman culture, Neptune was mentioned as a god of violence and known for its unpredictable movements. The biggest, most unpredictable and most dangerous thing around the world one can imagine is obviously the Ocean, which led the Romans to associate it metaphorically as the god of Ocean. Romans were sailing on the ocean so they paid a lot of attention to Neptune. It is also the god of earthquake, which is very common in Greece. Its Greek name is Poseidon. It is believed that, it hits the earth with its trident or stamps its foot to make an earthquake. Neptune is also known as the Horse-God.