Neptune - 162 years later

Session name: 20080923.2100



On 23 September 2008, 162 years passed since Neptune was first discovered in the telescope by Johnann Galle relying on predictions of mathematician Urbain Le Verrier.

At an average distance of 4.5 billion km, Neptune takes 164.79 years to complete one orbit. For the Neptunian inhabitant, on July 12, 2011 Neptune will have completed one lap around the Sun since it was first observed by Galle.

Today, Neptune is the most distant of the 8 planets, by revision of astronomical definitions. From 1979 to 1999, Neptune was the most distant of 9 planets, prior to the revision of astronomical definitions and because Pluto's highly eccentric orbit brought the ninth planet inside of Neptune's nearly circular orbit. This will occur again between 2216 and 2236.

From the city, the early Autumn Sky rising in the east may appear barren of stars. With a little patience, observing effort, and a knowledge of where to look, you'll find Neptune in no time. Even at the darkest sites, magnitude 7.9 Neptune requires optical aid to observe. Binoculars or telescopes are suitable to easily locate and identify the planet in the tail of Capricornus, the Sea Goat. Once the two tail stars of Capricornus are located then we can find Neptune about 2° from gamma Cap, Nashira.

To get your bearings, locate Deneb Algedi (delta Cap) & Nashira, the trailing pair of stars that appear nearly horizontal when Capricornus rises in the East. If you don't see them immediately look closer to the horizon where a bold star, Fomalhaut, shines brilliantly in soliloquy. Scan your bins in a one o'clock direction across 21° of the sky, there you'll see the pair of stars outshining any others in the area. In the field of view, note the trail of three mag 5 stars in a southeast to northwest line just north of Deneb Algedi. To the west of that line, I identify the triangle of fainter stars.

At a distance of 4.4 billion km (2.7 billion mi) from Earth, Neptune's size only subtends an angle of 2.3 arc seconds, an angular diameter the same size as Jupiter's moon Io. In binoculars the reward is finding and observing the most distant planet of the Solar System. In a telescope at 100x magnification there is no appreciable features to observe other than it may appear as a tiny disk rather than a star to the keen observer.

In Peter Grego's Solar System Observer's Guide, there is a brief description of Jupiter occulting Neptune during Galileo's time and how the immersion took more than a half hour to complete. One can easily appreciate this fact and the apparent size of Neptune in a telescope by observing the ingress of an Io transit across Jupiter. Away from the planet, Io looks like another bright star, but as it approaches and ingresses the planet one can absolutely see the diameter of this moon.