Neptune was discovered one year ago today, so I guess that you can call today a "birthday"!

At an average distance of 2.8 billion miles from the Sun (just over 30 AU - 30x as far from the Sun as the Earth), Neptune takes about 165 Earth years to finish a single orbit. Having been discovered on September 23, 1846, today it completes its first whole orbit of the Sun since that date.

Neptune was predicted mathematically before any actual observation. The motion of the orbit of Uranus didn't quite follow predictions, and that led Alexis Bouvard (a French astronomer) to calculate that its orbit was subject to gravitational influence by an unknown planet.

Neptune was subsequently observed by Johann Galle (a German astronomer) within a degree of the position predicted by Urbain Le Verrier (a French mathematician) - the three men are all now credited with its discovery.

Neptune has been visited by only one spacecraft, Voyager 2, which flew by the planet on August 25, 1989.

Named for the Roman god of the sea, Neptune is the fourth largest planet. It has 13 known moons. Its largest, Triton, was discovered shortly after Neptune itself, and unusually has a retrograde orbit, which means that it orbits backwards when compared to the rotation of the planet itself - the most likely and widely held explanation for this is that Triton is a captured object. Its composition is similar to that of Pluto, which leads most astronomers to propose that it came from the Kuiper belt; quite possibly, gravitational resonance between it and another body caused its orbit to change slowly until it came under the influence of Neptune.

I've never seen Neptune in a telescope - something to fix at some point.