This is the easiest month in several years to track Neptune’s glacial movement. The distant world slides just north of a reasonably bright star — 5th-magnitude Sigma (σ) Aquarii — which makes the planet easy to locate through binoculars or a telescope. This sun ranks among the brightest in central Aquarius, a region not known for easily visible stars. Neptune and Sigma stay in the same low-power telescopic field all month.
Neptune passes a Full Moon’s diameter (0.5°) north of Sigma on September 11, and it remains near this position for several days. The planet’s slow westward motion keeps it within 41' of the star throughout September. At magnitude 7.8, Neptune appears brighter than any of the faint suns to Sigma’s north and west. When viewed through a telescope, the planet displays a deep blue-gray color. Switch to high magnification (200x or more) to resolve its 2.4"-diameter disk.
Uranus resides one constellation east of Neptune, floating among the background stars of southern Pisces the Fish, and trails about two hours behind its sibling. Glowing at magnitude 5.7, Uranus is an easy object to find through binoculars and even shows up to naked eyes from under a dark sky. Although Uranus won’t reach opposition until early October, its appearance this month suffers little in comparison.
The major disadvantage for Uranus watchers is that it doesn’t lie close to any bright star. It begins the month 2.6° south-southwest of 4th-magnitude Epsilon (ε) Piscium and drifts along a southwesterly path throughout September. It ends the month 3.5° from this star. Point a telescope at Uranus, and you’ll see its pretty blue-green hue. Its disk spans 3.7" and shows up at magnifications of 100x or more.
If you find yourself craving some sweets as the night winds down, two tempting planets await.
Jupiter climbs above the eastern horizon around 4 a.m. local daylight time in early September, nearly an hour before twilight begins. It then lies 3° east-southeast of the Beehive star cluster (M44) in Cancer the Crab, and the two will make a nice pair through binoculars. By month’s end, the dazzling planet has moved 8° from M44 and rises before 3 a.m. Jupiter shines at magnitude –1.8 (on the 15th) and spices up the rather dim region of sky between Gemini and Leo.