See prime-time Neptune’s close encounter with star φ Aquarii during September

By Ade Ashford

This wide-field finder chart for Neptune depicts a quadrant of the sky centred on the south-southeast as seen from the heart of the British Isles around 1am BST at the beginning of September 2019, or by 11pm BST at the end of the month. The outermost planet lies close to the magnitude +4.2 naked-eye star phi (φ) Aquarii throughout the month (passing very close to it on 6 September). You can locate φ Aquarii near the mid-point of a line drawn between the western side of the ‘Square of Pegasus’ and first-magnitude star Fomalhaut low in the south. See below for a detailed finder chart for Neptune. AN graphic by Ade Ashford.
When Uranus was found to be drifting from its predicted orbit in the early 19th century, astronomers reasoned that the gravitational influence of a distant undiscovered planet was pulling it out of line. Hence Neptune was the first planet to be located mathematically from the calculations of John Couch Adams in England and Urbain Jean Joseph Le Verrier in France.

This composite picture shows the relative sizes of Earth and Neptune. The Voyager 2 spacecraft imaged the planet’s Great Dark Spot in 1989, an anticyclonic storm with winds speeds measured up to 2,400 kilometres (1,500 miles) an hour — the fastest in the Solar System. Neptune completes a revolution in 16.11 hours. Image credit: NASA/JPL.
Neptune was eventually found just a degree away from its predicted position in the constellation of Aquarius on 24 September 1846 by Johann Gottfried Galle with the assistance of Heinrich Louis d’Arrest at the Berlin Observatory. Interestingly, Galileo was actually the first person to observe Neptune, but he mistook it for a background star in observations of Jupiter from December 1612 and January 1613.

We now know that Neptune is a gas giant almost four times the diameter of Earth that lies 30 times further away from the Sun than we are. At such a vast distance, the outermost known planet takes almost 165 years to orbit the Sun.

Neptune reaches opposition on 10 September this year, having returned to its discovery constellation of Aquarius. On this date Neptune is highest in the southern sky of the UK around 1am BST, some 30 degrees (or three spans of a fist held at arm’s length) above the horizon.

While Neptune lies opposite the Sun on 10 September, it is closest to Earth at 11h UT the previous day when the least distance between our two worlds for 2019 is 28.928 astronomical units or 4,328 million kilometres (2,689 million miles).

Neptune’s vast distance means that it only attains magnitude +7.8 this year. Hence it’s too faint to see with the naked eye, but well within the reach of a pair of binoculars from a dark sky site.

This graphic shows a zoomed-in view of the Neptune finder chart at the top of the page, focused on the region of phi (φ), lambda (λ) and the three psi (ψ) Aquarii stars. It depicts field stars to magnitude +9, about the limit of a 10×50 binocular under dark, moonless skies. The red scale bar shows the width of a 10×50 binocular field of view. Neptune’s retrograde motion through the constellation of Aquarius over the course of two months at five-day intervals is shown. Note that the outer planet passes just 13¼ arcseconds – about a third of Jupiter’s apparent size at opposition – southeast of φ Aquarii on 6 September. AN graphic by Ade Ashford.
Neptune is not difficult to find with a star chart like the one at the top of the page and its zoomed-in binocular view above, particularly when the planet’s retrograde (east-to-west) motion carries it very close to a convenient naked-eye guide star called phi (φ) Aquarii on 6 and 7 September.

Phi Aquarii shines at magnitude +4.2 and lies just 13¼ arcseconds northwest of Neptune at 12:36 UTC on 6 September, which means that antipodean observers will get the best views of the pair at their closest.

For observers in Western Europe, Neptune lies almost 48 arcseconds – about the angular size of Jupiter at opposition – east of φ Aquarii at 1:30am BST on 6 September. Twenty-four hours later, Neptune has moved to within 55 arcseconds southwest of the star. By 1 October, the planet still lies less than two-thirds of a degree west of φ Aquarii, further aiding Neptune’s identification.