Neptune the eighth planet from the Sun and most distant in the Solar System reaches opposition in Aquarius on September 1, 2015. On this day it will be 28.9533 AU (approx. 4331.4 million km or 2690.6 million miles) from Earth and visible all night, rising above the eastern horizon at sunset and then setting in the west as the Sun reappears.
Shining at magnitude +7.8, Neptune is the only planet that's never visible to the naked eye - although Uranus requires dark skies to be glimpsed. However provided you know exactly where to look, Neptune is a relatively easy to find with binoculars or small telescopes.
Locating Neptune
The constellation Aquarius is a faint member of the zodiac; its brightest stars are only of 3rd magnitude. To locate Neptune look towards the centre of Aquarius about halfway along an imaginary line connecting lambda (λ) Aqr (mag. +3.7) with Sigma Aqr (σ Aqr - mag. +4.8). Slightly brighter star HD 214686 (mag. + 7.0) is positioned 7.5 arc minutes southwest the planet on September 1st.
Located thirty degrees to the northeast of Neptune is the "Great Square of Pegasus" with the brightest star in the surrounding region of sky being Fomalhaut (α Psc - mag. +1.2) to the south. Currently Neptune is better situated for observation from tropical or southern latitudes where it appears higher in the sky.
The finder chart below shows the position of Neptune on September 1st, 2015. Since the distant planet moves relatively little with respect to the background stars the chart is also valid before and after opposition.
Neptune Opposition Finder Chart - September 1, 2015 - pdf format
Observing
Even though the planet's apparent diameter is only 2.4 arc seconds a small telescope at high magnification will reveal the disk, appearing small and featureless with a blue tinge. Sadly the view is not significantly improved even in the largest amateur scopes. It's also possible to spot Neptune's largest moon Triton (mag. +14.0) but a scope of the order of 300mm (12 inches) aperture is recommended for the task.
Neptune Opposition Data Table
| Date | September 1, 2015 (4 UT) |
|---|---|
| Right Ascension | 22hr 40m 29s |
| Declination | -9d 15m 56s |
| Apparent Magnitude | +7.8 |
| Apparent Size | 2.4" |
| Illumination (%) | 100.0 |
| Distance from Earth (AU) | 28.9533 |
| Constellation | Aquarius |