Neptune in 2016–2017

2023 June 7

John Sussenbach

A report of the Saturn, Uranus & Neptune Section. Director: Mike Foulkes.

In 2016 and 2017, Neptune was observed by amateurs with medium to large telescopes. During both apparitions, bright storms were detected at different latitudes. Unfortunately, the number of observations in 2016 was too small to derive the daily drift of the spots. In 2017, during the months October and November, a major bright spot was observed, positioned in the equatorial zone of the planet. For this spot, a daily drift of 47.5° could be established. On 2017 Oct 5, a rare occultation of a star by the large Neptunian satellite Triton took place and was recorded by amateurs. Several observers performed a photometric analysis of the event.

Introduction

Neptune is the outermost planet in the solar system. Its small diameter of only 2.4 arcseconds makes it a difficult object for amateurs to study. Telescopes with apertures of 20cm and larger are required to detect any details that may be occasionally present on its tiny bluish disc. The planet has a large satellite, Triton (magnitude 13.5), which moves in a retrograde way and is relatively easy to observe in medium-sized telescopes. In general, the Neptunian disc appears blank, except that the southern hemisphere is seen to be slightly brighter than the northern hemisphere (Figure 1). This seems to be a long-term feature.

In 1989, Neptune was visited by Voyager 2 and this spacecraft discovered dark and bright storms on the planet. In 2014, amateurs became involved in the observation of the bright storms, when the Spanish professional astronomer Ricardo Hueso invited observers to investigate how well these spots could be detected with amateur telescopes. The results of this project have been presented and indicate that amateurs can make a valuable contribution to studies of the distant planets.^1–3

Neptune in 2016

In 2016, Neptune was located in the constellation of Aquarius and was at opposition on Sep 2. The number of observers and the observations submitted were rather limited. A list of the observers, their locations and their instruments is presented in Table 1. Most observers used 610nm (red) or 685nm (IR) long-pass filters, which give the best resolution.

Most of the time, no details could be detected on the planet (Figure 1). However, in the period August–October, several observers recorded bright spots. M. Delcroix and colleagues detected bright spots on 2016 Aug 8 & 9, using the 1,060mm Cassegrain telescope at Pic du Midi (Figure 2). In the southern hemisphere, the spots formed a smear from latitudes B –63.4 to –15.8°, with the brightest spot at B –50° and longitude L 220°. A minor spot was detected at B +34°, L 271°. It should be realised that measurements on a tiny disc are rather difficult and errors to ±5° and more occur easily.

On 2016 Sep 21, D. Milika and P. Nicholas detected several bright spots at B –38°, L 200–270°. They followed the planet for several hours and their images nicely illustrate the rotation of these spots (Figure 3). On 2016 Aug 30, J. Sussenbach also recorded several bright spots, the brightest being at B –52°, L 124° with a fainter one at B +0°, L 100° (Figure 4). Also, M. Lewis was active that night and imaged Neptune at 23:09 UT. Comparison of this image with the one by Sussenbach demonstrates the rotation of the spot in about one hour.

P. Miles recorded Neptune on 2016 Sep 7, showing a bright spot at B –44°, L 240°. A. Wesley did so on 2016 Sep 16 at 11:03 UT; he detected a bright spot at B –51°, L 315° and a fainter one at B –36°, L 31°. These results demonstrate that the bright storms on Neptune may last for several weeks and sometimes even months. Unfortunately, the number of observations was too small to calculate the daily drift of the spot.

Interestingly, some bright spots have also been detected visually (Figure 5).

Neptune in 2017

In 2017, Neptune was still found in the constellation of Aquarius and was at opposition on Sep 5. Fortunately, for this apparition the number of observers submitting images was higher than in 2016 (Table 2). Most images were obtained with digital cameras using an IR filter, but S. Maksymowicz and D. Gray submitted drawn images (Figure 6).

One reason for the increased number of observers was the occultation of the star UCAC4 410-143659 by Triton on 2017 Oct 5. In the course of the 2017 apparition, several distinct bright spots were again detected, also boosting the interest in this most distant planet.

Bright spots on Neptune in 2017

Also in 2017, Neptune showed a southern hemisphere that was brighter than the northern (Figure 7). Several observers detected bright spots on the planet. On 2017 Oct 10, Delcroix reported a bright spot at coordinates B +4°, L 34° (Figure 8). The images by D. Milika and P. Nicholas illustrate the rotation of the spot (Figure 9).

In the months October and November, several reports of a bright equatorial spot were received from Delcroix, Kardasis, Lewis, Miles, Wesley, Milika & Nicholas, and Sussenbach (Figure 10). In general, bright storms around the equator show large daily drifts due to the high wind speeds in the Neptunian atmosphere.^1,2 The drift of the bright equatorial spot was analysed by measuring its latitude and longitude over a period of two months.

The coordinates of the bright spot were determined using WinJUPOS.⁴ It should be realised that positional measurements on Neptune are rather inaccurate, for several reasons. During the processing of the images, the precise position of the disc’s rim might easily be lost by contrast enhancement. Furthermore, the orientation of the disc is difficult to establish from the image of Neptune alone. The accuracy can be improved significantly by establishing the relative position of the brighter satellite Triton and comparing that with the predictions of WinJUPOS. The positions of the brightest spot presented in Figures 8–10 were established using WinJUPOS and the results are shown in Table 3.

The values of the spot’s coordinates from 2017 Oct 10 till Nov 14 are presented in a graphic presentation (Figure 11). This analysis illustrates that bright storms might live six weeks or longer, and also shows that the drift is rather constant. For the bright equatorial spot, a daily drift of 47.5° was established. Besides the major bright spot, in some images fainter spots are also detectable (Figure 10A, E, F & H). These observations illustrate again that Neptune, just like Jupiter and Saturn, is an interesting planet from a meteorological point of view and worthwhile for amateurs to observe in a systematic way.

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