See outermost planet Neptune at its best

When Pluto was reclassified as a dwarf planet in 2006 (albeit the largest known body of its type), Neptune regained the title of the solar system’s outermost planet. Neptune is a gas giant almost four times the diameter of Earth that lies 30 times further away from the Sun than our planet. Consequently, Neptune takes almost 165 years to orbit the Sun. As the planet reached opposition on 1 September, now is a good time to seek it out.

Discovery history
The first planet to be located by mathematical prediction from the calculations of John Couch Adams in England and Urbain Jean Joseph Le Verrier in France, Neptune was identified by Johann Gottfried Galle with the assistance of Heinrich Louis d’Arrest at the Berlin Observatory on 24 September 1846, just after midnight. Neptune was in the constellation Aquarius, just 1.3° away from Saturn, and within a degree of the position predicted by Le Verrier.

Of further historical interest, Galileo was actually the first person to observe Neptune — but he mistook it for a magnitude +7.7 background star in observations of Jupiter from December 1612 and January 1613, when the two planets were almost in the same line of sight in the constellation Virgo.

Locating Neptune
Back to the present day, Neptune has returned to the constellation Aquarius having completed a little over one orbit of the Sun since its discovery. Currently glimmering at magnitude +7.6, Neptune is too faint to be seen with the naked eye, but it is a relatively easy binocular object — if you know where to look.

If you wish to track down Neptune with binoculars, first familiarise yourself with the wide-field chart above. Around midnight in mid-September is an ideal time to look, since the Moon will be out of the sky. Since the planet lies in a region of the constellation Aquarius that is devoid of conspicuous stars, you will need to do some star-hopping to reach your target.

First, get your bearings by locating first-magnitude star Fomalhaut in Piscis Austrinus very low due south, then Altair in Aquila higher in the southwestern sky. Next, locate the conspicuously-shaped constellation Capricornus low to the south-southwest. The distance from delta (δ) to alpha (α) Capricorni is about the span of an outstretched hand at arm’s length.

I always find that the trio of stars surrounding magnitude +3.6 double-star zeta^1,2 (ζ^1,2) Aquarii makes for a conspicuous naked eye and binocular asterism; lambda (λ) Aquarii is just two binocular fields to the lower left of zeta^1,2.

Once you are confident that you have located λ Aquarii, you need to to move 3.5° — or 70 percent of a 10×50 binocular field — to the lower right, in the direction of sigma (σ) Aquarii, to find Neptune. For users of low-magnification binoculars of the 7×35, 7×50 or 8×40 variety, you may just get λ Aquarii, Neptune and σ Aquarii in the same field of view.

On the night of 5/6 September 2015, Neptune passes just 3.3 arcminutes — or one tenth of the apparent diameter of the full Moon — north of a magnitude +6.9 star known as HIP 111910 (α=22^h 40.1^m, δ=-09° 22′ J2000.0). HIP 111910 is about a magnitude brighter than Neptune, but the changing configuration of the two objects on or around this date will offer a clear demonstration of the planet’s retrograde motion.

Finding Neptune the easy way
While binoculars will not show Neptune’s tiny 2.4-arcsecond-wide disc, the planet’s pale blue colour is evident in telescopes of modest aperture. Users of telescopes on computerised GoTo mounts can select Neptune directly from the planetary menu, but for those instruments equipped with digital setting circles, here are the J2000.0 coordinates of Neptune at midnight British Summer Time in mid-September 2015: α=22^h 39.1^m, δ=-09° 24′.

Tracking down Triton
Observers with 10-inch aperture telescopes and larger may wish to track down the largest of Neptune’s 14 known moons — Triton. Discovered by English astronomer William Lassell just 16 days after its parent planet, Triton orbits Neptune at a distance of 220,440 miles (354,760 kilometres) in an almost circular, synchronous orbit every 5 days 21 hours.

Of a similar composition to Pluto and most likely a dwarf planet captured from the Kuiper Belt, Triton has a diameter of 1,700 miles (2,700 kilometres). Unusually, it is the only large moon to have a retrograde orbit — one that is in the opposite direction to its parent planet’s rotation.

Given that Neptune lies about 2,694 million miles (4,336 million kilometres) from Earth in September, Triton never strays further than 17 arcseconds — or seven Neptune diameters — away from its home planet. Currently glimmering around magnitude +13.6, it is therefore more than 200 times fainter than Neptune.

Clearly, it makes sense attempting to spot Triton at those times when it is close to either an easterly or westerly elongation from Neptune and, preferably, our Moon is absent from the sky or at a crescent phase. Triton is at western elongation near 2, 8, 14, 19 and 25 September, while eastern elongations occur near 5, 11, 17, 22 and 28 September. Clear skies!

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Inside the magazine

You can find out more about observing Neptune in the September edition of Astronomy Now in addition to a full guide to the night sky.

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