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Yesterday, Neptune was in approximately the same point in its orbit as when it was officially discovered on 23rd September 1846. In other words, 1 Neptunian year (165 terrestrial years) have passed.
To celebrate the event, NASA has released the 4 images that we see above, taken with the Hubble Space Telescope on 25-26 June, and separated by 4 hours from each other so as to photograph the entire atmospheric surface of the planet, which completes one rotation every 16 hours.
Methane is abundant in Neptune's atmosphere, and the fact that this preferentially absorbs red light gives Neptune its blue colour. The light patches are high altitude clouds of methane ice crystals. These clouds are more numerous and widespread than a few years ago, highlighting the significant variability that characterises the atmosphere, something that is likely to be driven by the huge temperature difference between Neptune's hot interior and frozen upper atmosphere (at around -160°C).
To a lesser extent, the seasons on Neptune, that last for about 40 years but are otherwise similar to those on Earth insofar as they are the result of the planet's inclination to the orbital plane (29 degrees compared to 23 degrees for the Earth), also have their role to play in atmospheric instability. Summer has begun in the southern hemisphere, and winter in the north.
The discovery of Neptune is a remarkable success story for both astronomy and mathematics. A few years after the discovery of Uranus in 1781 by Herschel, it became clear that the motion of the planet was being perturbed by another body, even further from the Sun. Amongst those that tried to apply Newton's universal laws of gravity in the complex task of finding the location of this unknown body, two succeeded; the Frenchman Le Verrier and the Englishman John Couch Adams.
Adams was actually the first to correctly determine the location of Neptune, but the astronomer to whom he gave the coordinates did not make observations soon enough, and in the meantime Le Verrier sent his coordinates to the German astronomer, Galle, who found Neptune within 1° of the predicted position. In fact, Neptune was actually observed for the first time 234 years earlier by Galileo, who, however, did not recognise it as a planet.
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