Neptune the eighth planet from the Sun reaches opposition in Aquarius on September 2nd. On this day, the most distant planet in the Solar System will be 28.9454 AU (approx. 4330 million km or 2691 million miles) from Earth and visible all night long. It rises above the eastern horizon at sunset, reaches highest point in the sky during the middle of the night before setting in the west at sunrise.
With an apparent magnitude of +7.8, Neptune is the only planet that's not bright enough to be visible to the naked eye. However, provided you know exactly where to look it's relatively easy to find with binoculars and small scopes.
Location
The constellation Aquarius is a faint member of the zodiac with its brightest stars only of 3rd magnitude. Neptune is currently located about 30 degrees south of the centre of the Great Square of Pegasus and a degree southwest of star lambda Aqr (λ Aqr - mag. +3.7). Positioned about 20 degrees or so further south of Neptune is bright star Fomalhaut (α Psc - mag. +1.2).
The finder chart below shows the position of Neptune on September 2, 2016. Since the distant planet moves slowly with respect to the background stars the chart is also accurate enough for some time before and after opposition.
Neptune Opposition Finder Chart - September 2, 2016 - pdf format (credit:- freestarcharts)
Observing
Even though Neptune's apparent diameter is only 2.4 arc seconds a small scope at high magnification will reveal a small featureless blue disk. However, even with the largest amateur telescopes the view is not significantly improved. It's also possible to spot Neptune's largest moon Triton (mag. +14.0) but to achieve this a scope of the order of 300mm (12 inches) aperture is recommended, especially from suburban areas.
On September 15th, the almost full Moon passes 1.2 degrees north of Neptune with an occultation visible from UK, Europe and West Russia (20:30 UT).
Neptune Opposition Data Table
| Date | September 2, 2016 (17 UT) |
|---|---|
| Right Ascension | 22hr 48m 56s |
| Declination | -08d 29m 00s |
| Apparent Magnitude | +7.8 |
| Apparent Size | 2.4" |
| Illumination (%) | 100.0 |
| Distance from Earth (AU) | 28.9454 |
| Constellation | Aquarius |