Neptune, the eighth planet from the Sun, reaches opposition in Aquarius on September 5th. On this day, the most distant planet in the Solar System will be 28.939 AU (approx. 4,329 million km or 2,690 million miles) from the Earth and visible all night long. It rises above the eastern horizon at sunset, reaches its 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.
The constellation Aquarius is a faint member of the zodiac with its brightest stars shining at only third magnitude. Neptune is currently positioned about 30 degrees south of the centre of the Great Square of Pegasus and 2 degrees east of lambda Aqr (λ Aqr - mag. +3.7). First magnitude star, Fomalhaut (α Psc - mag. +1.2), is 22 degrees directly to the south. When Neptune reaches opposition the full Moon is also in Aquarius, and on September 6th it passes less than a degree south of the planet.
The finder chart below shows the position of Neptune on September 5th. 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.
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 the largest backyard instruments don't significantly improve the view, but it is possible to spot Neptune's largest moon Triton (mag. +14.0). For this task a scope of the order of 300mm (12 inches) aperture is recommended, especially from suburban areas.
Neptune Opposition Data Table
|Date||September 5, 2017|
|Right Ascension||22hr 57m 19s|
|Declination||-07d 41m 46s|
|Distance from Earth (AU)||28.939|