Neptune cannot be seen using the eyes alone. Neptune was the first planet whose existence was predicted mathematically in 1846 independently by J.C. Adams and Le Verrier (the planet Uranus's orbit was perturbed by an unknown object which turned our to be another gas giant, Neptune). Neptune was then observed by J.G. Galle and d'Arrest later that year. Neptune was named for the mythical Roman god of the seas.

Neptune is about 30,775 miles (49,528 km) in diameter. This is 3.88 times the diameter of the Earth. If Neptune were hollow, it could hold almost 60 Earths.

Neptune's mass is about 1.02 x 10²⁶ kg. This is over 17 times the mass of the Earth, but the gravity on Neptune is only 1.19 times of the gravity on Earth. This is because it is such a large planet (and the gravitational force a planet exerts upon an object at the planet's surface is proportional to its mass and to the inverse of its radius squared). A 100-pound person would weigh 119 pounds on Neptune.

Each day on Neptune takes 19.1 Earth hours. A year on Neptune takes 164.8 Earth years; it takes almost 165 Earth years for Neptune to orbit the Sun once. Since Neptune was discovered in 1846, it has not yet completed a single revolution around the Sun.

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