ABSTRACT
Neptune is the third most massive planet in the Solar System and, like Uranus, may be described as an ‘ice giant’. It is too faint to be seen with the naked eye, but binoculars show it easily, and a small telescope will reveal its pale bluish disk. Neptune is a slow mover; it takes almost 165 years to complete one journey round the Sun, so that it was discovered less than one ‘Neptunian year’ ago. Neptune was observed on several occasions before being identified as a planet. Neptune is a twin of Uranus–but it is a non-identical twin. It is very slightly smaller, but appreciably denser and more massive. The four major planets are often called ‘gas giants’, but this is strictly true only for Jupiter and Saturn; Uranus and Neptune are better called ‘ice giants’, since their total mass of hydrogen and helium is no more than about two Earth masses, as against 300 Earth masses for Jupiter.
