So, for example, sunlight is approximately 900 times weaker on Neptune than on Earth because Neptune is approximately 30 times distant from the sun (30 x 30 = 900). It would be roughly half the size of what it seems to be in the Earth's sky if it could be seen. At most, Neptune can be seen as a dim star by an extremely sensitive observer from somewhere within its orbit.
Neptune was originally discovered by the Italian astronomer Giovanni Domenico Cassini on 15 September 1655. He called it "New Planet" because at that time no one knew whether it was a moon or a new planet beyond Saturn. Later studies showed it to be a gas giant just like Jupiter and Saturn. The name Neptune has been used since 1769 when Johann Gottfried von Gapless named it after the Greek god of the sea.
From our point of view, Neptune is invisible because it travels around the Sun every 12 years. When it crosses our line of sight from the Earth to the Sun, we cannot see it anymore because it goes behind the Sun. However, when it comes out from behind the Sun again, we can see it with our telescopes once more.
According to some sources, if humans were able to travel at near light speed (approximately 0.999999999 c), they could see Neptune before it disappears behind the Sun.
Neptune's magnitude ranges from 7.8 to 8.0, making it around two magnitudes fainter than Uranus. It's visible using firmly supported binoculars, but only if you look closely. While Uranus is frequently brighter than any other star visible in the same binocular or finderscope area, the sky is densely packed with stars as luminous as Neptune. Most observers cannot see past its brightness and confusion with so many others.
Neptune was originally discovered by William Lassell in 1846. He called it "the Planet X" because astronomers at the time theorized that there might be an undiscovered planet beyond Pluto. Lassell later learned of Uranus' existence and declared himself satisfied with just two planets. Today we know there are at least nine other members of our solar system more massive than Neptune.
Uranus is the seventh-brightest object in the night sky after the Sun, Moon, Venus, Mars, Jupiter, and Saturn. It is a giant gas planet that takes 12 years to orbit the Sun. At an average distance of 6,400 miles (10,200 km), Uranus is almost twice as far from the Sun as Earth. But because it is on the opposite side of the Solar System from Earth, one can observe it during a new moon. Its atmosphere is made up of hydrogen and helium with some methane present near the surface. There are five known moons: Miranda, Ariel, Umbriel, Caliban, and Merope.
According to the National Aeronautics and Space Administration (NASA), Neptune is roughly four times the size of Earth. Neptune's equatorial diameter is 30,775 miles, while Earth's is just 8,000 miles. The planet has a mass about 17% that of Earth but with only 1/10th our density.
Neptune is composed mainly of hydrogen (79%) with small amounts of helium and oxygen. It has a thin atmosphere of methane clouds and nitrogen ice particles. It has several large moons: Triton, Uranus's largest moon; Miranda, one of Jupiter's moons; and Nereid, one of Saturn's moons.
Neptune was first observed by astronomer William Herschel in 1777. He used a telescope made from a water tank and glass lenses which allowed him to see objects beyond the Moon.
Neptune was originally called "Haumea" after Herschel's wife. In 1846, Johann Galle discovered another moon of Neptune named Glaucus. This new moon was not seen again until 1955 when it was rediscovered by American astronomer Carl Wirtanen. It will never be able to be seen directly due to its position outside of Neptune's orbit but instead we get a glimpse of it every time Neptune passes behind something far away, such as a star.
Neptune's optical magnitude is at 7.74. Because of its brightness, Neptune should be viewable with a binocular with a 30-40mm aperture or a small telescope. It is located about 4.5 billion miles from Earth.
The planet was first observed by the Italian astronomer Giovanni Domenico Cassini in 1670. At that time, people knew nothing about planets other than Earth and Sun, so when they saw this new star-like object moving across the night sky, they thought it must be some kind of comet!
It wasn't until 1846 that British astronomer William Lassell correctly identified this as another planet. The reason for this long delay was that around 1750 astronomers began to believe that our own Moon may be inhabited by creatures who could drive vehicles like carts and carriages across its surface, which is why they were unable to identify Mars as being different to the Moon. In fact, these lunar vehicles were actually comets!
Comets are objects composed of ice and dust that orbit the Sun outside of Pluto. They can be very large, with one called Comet Shoemaker-Levy 9 being made up of more than 10 million particles that extend for hundreds of miles across the comet's nucleus.
Neptune. The average distance between Neptune and the Sun is 4.495 billion kilometers (2.8 billion miles), or 30.1 AU. It has a perihelion of 29.8 AU and an aphelion of 30.4 AU. The sun seems 30 times smaller at that distance than it does here on Earth.
At its closest, Neptune is about 5.2 AU away from the Sun, and at its farthest, it's about 3.3 AU away. That's quite a range! A reasonable estimate would be that Neptune is about 5,000 kilometers (3,100 miles) across.
Since it takes light 8 minutes to travel around the earth, at Neptune it would take 9 minutes for the sun to disappear below the horizon. But since there are only 12 hours of daylight there, it would only seem like an hour had passed here on Earth.
The closest approach comes on February 15th, 1989 when Neptune is about 3.5 AU away from the Sun. At this distance, the planet takes 14 years to orbit the Sun.
From Neptune you could see out to Pluto which is still farther away at 2160 AU or 1.46 billion miles. So even though Neptune is the furthest planet from the Sun, it doesn't mean that it can't be seen in the night sky. It's just that you have to go outside to do it!
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