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Observer's Challenge – opposition of Neptune

On 10th September 2019, Neptune will come to opposition. The planet will be nearly 29 astronomical units (AU) from Earth, and the other ice giant, Uranus, 19.2 AU from Earth, compared to 9.5 AU for Saturn and 5.2 AU for Jupiter on the same evening; this really puts into context just how far away the ice giants are from Earth. Light from Neptune takes over 4 hours to reach the Earth; the round trip for the light from the Sun to Neptune and back to Earth is well over 8 hours!

Whilst Uranus, at magnitude 5.7, is theoretically visible to the naked eye, Neptune being magnitude 7.8 can only be seen through binoculars or a telescope.

For many amateurs with binoculars or a smaller telescopes, just finding Neptune in the constellation of Aquarius can be a challenge. Some may be able to identify the planet as having a green-blue hue and having a disc-like [non-stellar] appearance.

With larger telescopes (typically over 30 cm in aperture) the hue of the planetary disc will be more evident and the largest of Neptune’s moons, Triton, may be observed if viewing from a dark location with good seeing conditions, yet it is highly unlikely you’ll see any surface detail on the planet.

Amateur astronomers can image Neptune, and this technique can be used to detect surface detail, but again requires good seeing and a lot of patience. Images on the BAA Member Pages show the range of results amateurs can achieve using a range of telescopes, cameras and filters. The recently posted observation by Terry Evans clearly shows Neptune and Triton passing close to the magnitude 4.2 star, Phi Aqr.

Whether you want to hunt down Neptune with binoculars or a small telescope, or set yourself a goal to observe or image its brightest moon, the next few weeks provide a great opportunity to observe the planet. More details and a finder chart can be accessed here.

Please do submit any observations to your BAA Member Page and to the Saturn, Uranus and Neptune Section.

[Thumb nail image by Martin Lewis] 

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Xilman's picture

For imagers, Triton is just too easy.  Nereid is fairly bright and can be picked up without too much trouble.  The other outer satellites of Neptune are almost certainly too faint for amateurs as they are below mag 25.  (That said, the TNO "2018 VG18" at magnitude 24.6 appears in the 2020 BAA Handbook, so perhaps someone is more optimistic than I.)

Of the innermost satellites, only Proteus (Neptune VIII) stands even a remote chance of being detected by an amateur.  It is bright enough, at mag 20.3, and moving slowly (< 1arcsec/hour) so tracking and stacking should be straightforward, but the separation is under 6 arcsecs and the scattered light from Neptune is very likely to drown it out.  Nonetheless, I may give it a go and see whether clever image processing can pull it out of the glow.

An image of Neptune, Triton and Nereid appears on my member page but the exposure wasn't really long enough for Nereid and too long for Triton.  I hope to do better this season.