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.

Neptune as imaged by the Hubble Space Telescope in 1998 (NASA, L. Sromovsky, P. Fry (credit:- University of Wisconsin-Madison))

Location

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.

Neptune Opposition Finder Chart - September 5, 2017 (credit:- freestarcharts)

Neptune Opposition Finder Chart - September 5, 2017 - 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 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

DateSeptember 5, 2017
Right Ascension22hr 57m 19s
Declination-07d 41m 46s
Apparent Magnitude+7.8
Apparent Size2.4"
Illumination (%)100.0
Distance from Earth (AU)28.939
ConstellationAquarius

See also
The Planets this Month - September 2017


Sky Highlights - September 2017

Opposition
Neptune reaches opposition on September 5th

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for September

Northern Hemisphere
Evening
West:- Jupiter (mag. -1.7)
Southwest:- Saturn (mag. +0.5)
East:- Neptune (mag. +7.8)
Midnight
South:- Neptune
East:- Uranus (mag. +5.7)
Morning
West:- Neptune
South:- Uranus
East:- Venus (mag. -3.9), Mars (mag. +1.8) (from second week), Mercury (mag. +0.5 to -1.3) (from second week)

Southern Hemisphere
Evening
West:- Jupiter
Northwest:- Saturn
East:- Neptune
Midnight
West:- Saturn
North:- Neptune
Northeast:- Uranus
Morning
West:- Neptune
Northwest:- Uranus
Northeast:- Venus
East:- Mars (end of month)

Deep Sky

Small telescopes:-
Messier 13 - M13 - Great Hercules Globular Cluster
Messier 92 - M92 - Globular Cluster
Messier 11 - M11 - The Wild Duck Cluster (Open Cluster)
Messier 7 - M7 - The Ptolemy Cluster (Open Cluster)
Messier 6 - M6 - The Butterfly Cluster (Open Cluster)
Messier 4 - M4 - Globular Cluster
Messier 8 - M8 - Lagoon Nebula (Emission Nebula)
Messier 16 - M16 - Eagle Nebula (Emission Nebula with Open Cluster)
Messier 20 - M20 - Trifid Nebula (Emission and Reflection Nebula)

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