Observing Uranus and its satellites, 2006−2016

2017 July 21

John Sussenbach

Uranus is one of the remote gas giants of our Solar System. The planet was discovered by William Herschel in 1781 using a 7-foot reflector with an aperture of 6.2 inches [157mm]. Observed from the Earth the planet has a very small angular size of about 3.6 arcseconds and is a real challenge for small telescopes. Visually it is a pale tiny greenish-blue disk. Occasionally some bands have been recorded visually by amateurs.
A concise report of the discovery and history of observation of this planet is the classic book The Planet Uranus by A.F.O’D Alexander. Uranus is unique in the extreme tilt of its axis of rotation of 97.8°. The planet has more than 25 satellites of which only the five largest can be detected with amateur telescopes (Figure 1). With larger telescopes faint banding has been reported visually.
Atmospheric features on Uranus
In contrast to Jupiter and Saturn, Uranus is rather featureless. NASA’s Voyager 2 spacecraft flew close to the planet in 1986 and revealed some banding and white clouds in the Uranian atmosphere. The Hubble Space Telescope has imaged bands and spots on the distant planet (Figure 2).
This image also illustrates one of the consequences of the peculiar large tilt of Uranus’ axis. During its 84-years’ revolution around the Sun, in certain phases only the northern hemisphere is illuminated by the Sun and 42 years later the southern hemisphere. The appearance of Uranus in the period 2006-2016 is shown in Figure 3. In this paper all Uranus images are presented for the appropriate date with north at the top. In 2007 the Earth passed the equatorial plane of Uranus (on 2007 February 20, May 3 and August 16, respectively) leading to a view of the planet very similar to the 2006 image.
At the start of this century amateur astronomers in increasing numbers began digital imaging of the planets using webcams and other cameras. In the beginning Jupiter, Saturn and Mars were the favourite objects. However in 2006 August & September members of the Dutch Working Group on the Moon and Planets initiated a campaign to image Uranus. The objective was to investigate what amateurs can detect and observe with the help of the latest digital cameras and other technical equipment.
It should be noted that circumstances for observing Uranus from The Netherlands were quite unfavourable during this apparition. In 2006 the altitude of the planet was not more than 30°. Despite these conditions several acceptable images were obtained (see http://maanenplaneten.nl/documenten/report_uranus_campaign_2006.pdf). The general conclusion was that with amateur instruments no details could be imaged on the tiny blueish disk of Uranus in RGB or IR (Figures 4 and 5). No distinct spots or other features could be detected in the RGB, R or IR images. Limb darkening was very clear and in some images the southern hemisphere was a bit brighter than the northern.
I imaged Uranus in the years that followed, but since it travelled through the summer ecliptic, conditions for high resolution imaging of the planet were not very favourable. In 2008 I noticed in the images that the northern hemisphere was a bit brighter than the southern, and this difference became more distinct in the years that followed. In 2012 I also noticed faint banding on the southern hemisphere and this became clearer in the following years (Figures 6 & 7).
In particular the northern hemisphere is brighter than the southern in 2015 and 2016. In addition a distinct banding pattern is present, with a darkening of the north polar region and two darker bands in the northern temperate and tropical zone. Subsequently, towards the south pole a brighter zone can be distinguished and a relatively dark band in the most southern region (Figure 7)….

(Login or click above to view the full illustrated article in PDF format)