Neptune is the furthest out of the “Gas Giant” planets and is right out at the limits of our Solar System, 2800 million miles from the Sun.

The story of its discovery is fascinating…

Neptune

Neptune’s existence was suspected long before it was actually discovered.

Astronomers in the early 1800′s found that they could not reconcile the observed position of the newly discovered planet Uranus, with the position they expected through calculations.

So some astronomers began to suggest that perhaps there was an undiscovered planet in the same area of the Solar System, that was affecting the orbit of Uranus.

Various people became involved in the search for the new planet. Notably, these included a young French mathematician called Urbain Le Verrier and a young English mathematician called John Couch Adams.

Both of these men calculated where in the sky, they would expect Neptune to be found. Adams did not publish his work, but Le Verrier did by means of two notes, in 1845 and 1846.

The search to find Neptune observationally began.

John Challis, Professor of Astronomy at Cambridge, used the Northumberland 11.6 inch refractor but did not find it.

(Incidentally, this telescope can be visited and is still used at the present time, in Cambridge. I have seen it and it is a wonderful piece of astronomy history)

Reviewing the episode later, after the new planet had been discovered, Challis found that he had actually seen and recorded Neptune almost immediately in the predicted position, but he had not realised it. I bet he was cross!

Meanwhile, Le Verrier had sent his calculations and predictions to Johann Galle at the Berlin Observatory.

With this valuable information, Galle looked and found very quickly. Neptune was discovered on his first night’s observation in 1846.

Later, it was discovered that Adams had also been correct with his predictions of Neptune’s position, but unfortunately, it had just taken too long for the English astronomers to follow it through to the observation stage.

Neptune is not actually that difficult to see with a telescope, if you know where to look and what you are looking for.

Through a telescope, the Neptune’s disk looks blue. Like Uranus, this is due to light absorption by methane.

Neptune is also very similar in size to Uranus.

The largest of Neptune’s moons, Triton was discovered soon after the planet itself in 1849, but the second moon, Nereid, took much longer (1949). It is close to the planet and hard to see.

The most significant recent event was the Voyager space mission. It flew past Neptune in 1989.

Voyager found six additional satellite moons, together with a faint ring system – another similarity with Uranus.

However Neptune does not share Uranus’ extreme axial tilt. Neptune’s axis is inclined at just 29 degrees to the plane of the orbit. Only a few degrees more tilted than Earth.

Voyager also succeeded in measuring the period of rotation. Neptune’s “day” was found to be 16 hours 7 minutes long.