To commemorate Neptune’s first complete orbit around the sun since its discovery in 1846, NASA has released “anniversary images” that reveal all the faces that the planet displays during each of its 16-hour rotations. The images were taken using the Hubble Space Telescope on June 25-26.Sean Walker, Sky & Telescope’s Imaging Editor, mapped the Hubble images onto a sphere and used software to rotate it. The resulting video shows what the planet would look like if we could see it close up with infrared-sensitive eyes.
The images are close to natural color, but use blue, red, and near-infrared data to reveal clouds in the gas giant's upper-atmosphere. Neptune’s methane-rich atmosphere, which absorbs the red wavelengths, gives the planet its blue-green hue. The clouds, however, are tinted pink because of the reflection of near-infrared light. For comparison, a natural color image taken by Hubble last August can be seen here.
Neptune has an axial tilt of 28½°, so it experiences seasons much like Earth's. But each season on Neptune lasts about 40 years. These images show the planet in a phase of transformation: clouds, which were brooding over the southern skies a few years ago, are now beginning to gather in the north. This shows that spring is approaching north of Neptune’s equator and summer is waning to its south.
If you have good binoculars or a small telescope, you can spot the blue planet late at night using our Neptune finder chart.
A recent study suggests that scientists may have gotten Neptune's rotational rate wrong for decades. Click here for the latest news on Neptune's spin.