Neptune, the eighth and farthest planet from the Sun in our Solar System, is at opposition on 10 September 2019 and will be visible for the entire night. The mag. +7.8 planet requires at least binoculars to see, as there is not the faintest chance of seeing it without.
Currently, the blue ice giant is in the constellation of Aquarius located in close proximity to the star Phi (φ) Aquarii.
As darkness falls on the evening of 5 September, Neptune is just over 1 arcminute from mag. +4.2 Phi Aquarii on the eastern side of the star.
On 6 September, Neptune swaps sides to appear southwest of the star.
If you can catch it just after darkness has descended, the planet will appear separated from the star by around 30 arcseconds, an extremely close pass.
Neptune tracks through Aquarius, and passes close to the star Phi Aquarii on 5–6 Sep. Credit: Pete Lawrence
The planet then moves west, but never really leaves the vicinity of Phi Aquarii all month long.
Binoculars show the planet as little more than an eighth magnitude star. However, if you make the switch to a telescope the story is different.
A 75mm telescope at 75x magnification will hint at Neptune’s planetary nature.
Upping the telescope’s power to 150x will start to reveal Neptune’s beautiful blue disc.
How to see Neptune throughout September 2019:
Best time to see: 10 September, 01:00 BST (00:00 UT)
Features: Small disc, colour, Triton
Recommended equipment: Binoculars, 75mm telescope and larger
If you fancy a challenge, have a go at using a 250mm or larger instrument and a magnification of 250x and see if you can spot Neptune’s largest moon, Triton (pictured left). At mag. +13.5, it requires good dark skies and properly dark adapted eyes. In total the ice giant has 13 moons.
Neptune is so remote that it takes almost 165 years to complete an orbit of the Sun. Its distinctive blue appearance is due to the red-light absorbing methane in its atmosphere.
The phase and relative sizes of the planets in September 2019. Each planet is shown with south at the top, to show its orientation through a telescope. Credit: Pete Lawrence
How to see the rest of the planets this month
Best time to see: 29 September, 15 minutes after sunset
Altitude: 1˚ (extremely low)
Venus is an evening object rather close to the Sun at the start of the month but pulling away to an apparent separation of 12˚ by 30 September.
Despite this, the shallow angle the ecliptic appears to make with the western horizon at this time of year means that Venus sets around 30 minutes after the Sun and for this reason it’s tricky to see.
On 13 September Venus appears 0.3˚ from mag. –0.9 Mercury (see page 46).
Your best chance to spot Venus will be on 29 September when it appears 3.3˚ below a thin, 1%-lit waxing crescent Moon, just after sunset with Mercury visible nearby.
Best time to see: 30 September, 40 minutes before sunrise
Altitude: 3˚ (very low)
Mars is in conjunction with the Sun on 2 September and is unlikely to be very visible this month.
But clear skies and a low eastern horizon, you may catch a glimpse of its mag. +1.8 dot after 06:30 BST (centre of UK) in the morning twilight.
Best time to see: 1 September, 21:00 BST (20:00 UT)
Jupiter remains a bright beacon low in the region west of south as darkness falls, but the planet’s low southerly aspect does it no favours.
The first quarter Moon lies 6˚ to the right of the mag. –2.0 planet as seen from the UK on the evening of 5 September, and 7˚ above and left of Jupiter on the evening of 6 September.
The return to darker evenings helps Jupiter stay in dark skies for longer, but its low altitude makes observing it rather challenging.
Best time to see: 1 September, 21:30 BST (20:30 UT)
Saturn is moving west among the stars of Sagittarius for most of the month, heading below the Teaspoon asterism.
Shining at mag. +0.7 on 1 September the planet dims to mag. +0.8 by the month’s end.
Its retrograde apparent westward motion also does it no favours as it’s slightly to the west of south as darkness falls by the end of September.
On the evening of 8 September, Saturn is joined by a 76%-lit waxing gibbous Moon, 3˚ to the east-southeast.
Best time to see: 30 September, 02.45 BST (01:45 UT)
The longer nights, and the fact that Uranus is at opposition towards the end of next month, means that it’s well placed for observation.
At mag. +5.7 it may just be possible to see Uranus with the naked eye from a dark-sky site.
The planet is currently in Aries and manages to attain a maximum altitude of 50˚ when due south, making it the best-placed planet for observation from the UK.
Mercury is not visible this month
This article originally appeared in the September 2019 issue of BBC Sky at Night Magazine. Pete Lawrence is an experienced astronomer and a presenter on The Sky at Night.