Popular Astronomy

Share to Facebook Share to Twitter Stumble It More...

April and May 2015

 A period of settled weather over the UK at the start of April brought some better-than-average seeing conditions and started the reporting period off with a rush of observations. The whole period produced excellent observations of Jupiter, as it moved further past opposition, and more observations of the mutual events between the Galilean Moons around Jupiter. Venus was also well represented with a number of narrowband images showing ultra-violet (UV) cloud features and at least two exciting observation of clouds in the near infra-red (IR). The sheer volume of observations means I must list features seen, rather than listing individual images and sightings, and with this in mind I will start with Jupiter.

Now well past opposition, Jupiter has been settling into the western evening sky and reducing in visual size; nonetheless some periods of excellent seeing conditions have allowed section members to make detailed observations. The South Polar Region was a bland grey-brown throughout the period and bordered by a distinct dark northern edge merging into the South-South Temperate Belt (SSTB), within which a series of white-oval storms circled the pole.  Paul Crossland caught 4 of them on the 5th of April then added a fifth on the 12th of the month with Simon Kidd and Dave Finnigan bringing the total to 6 by the middle of the month.  Oval BA continued to be pale and hard to spot visually; the white oval storms circled past BA without interacting with it during the period.
.
The SSTB, South Temperate Belt (STB) and South Tropical Zones (STrZ) were particularly inactive with Paul Crosland’s sequences of images from the 9th and 12th of April showing the STB to start just north of Oval BA and to continue some 220 degrees prograde to just south of the Great Red Spot (GRS), where it fades for much of the rest of the circumference into a faint feature. This is also clear in images taken by Dave Scanlan on the 6th of April, using a vintage 5 inch Cooks refracting telescope and a Canon Eos 450D camera, and by Dave Finnigan, Simon Kidd and myself, showing the belts to be more obvious in infra-red (IR) images.
.
By the 22nd of April Simon Kidd caught a series of dark spots and disturbances starting to appear retrograde of Oval BA suggesting a darker outbreak of the STB was imminent but apart from some relative motion between the spots little changed by the end of the period, as shown in images by Martin Lewis as late as 21 May.
The GRS stayed relatively unchanged throughout the period with a darker outer annulus and a paler middle region plus a small dark concentration at the centre. This was shown in a number of images by most contributors including visual observations by Matthew Barrett.  All the received observations show the GRS to be very isolated with a pale area extending beyond it into the South Equatorial Belt (SEB) forming an obvious ‘socket’ containing the GRS itself. Simon Kidd’s image from the 6th of April shows a dark line of material entering the socket from the prograde southern edge of the SEB to form a hook or ‘eyebrow’ feature which persisted throughout the month, becoming less obvious in May but still visible by the 21st in images by Dave Finnigan.
.
By the 6th and 7th of April Mike Brown, Steve Norrie and Dave Finnigan started to pick up a whiter than usual region within the SEB just retrograde of the GRS. This region is usually turbulent, showing both dark and light eddies, but by May it had grown into a more than usually obvious feature extending perhaps 80 degrees around the planet. The northern edge of the SEB was generally linear showing a clear demarcation from the pale Equatorial Zone (EZ). The EZ itself was disturbed by a number of large festoons, particularly in IR-light images, giving a turbulent and ‘messy’ appearance. The faint equatorial band seen extending from the festoons last reporting period was absent this time but excellent detail was caught by Martin Lewis using an Hoya 839 Nm IR filter,  and by Mike Brown, the latter catching dark festoons with a one-shot-colour (OSC) camera.
.
The North Equatorial Belt (NEB) was very disturbed throughout the period, showing a distinctly ‘waved’ northern edge and a light-coloured snake like feature; this apparently being material diverted around the large northern storm  ‘White Spot Zulu’ (WSZ) and across the width from north to south of the NEB. Simon Kidd  first caught this developing on the 5th of April as increased disturbance around WSZ with Dave Finnigan, Paul Brierley and Mike Brown following-up on the 7th, 9th and 14th to show the snake of light-cloud crossing the dark belt. By mid-May this feature had merged with the background turbulence within the belt but even as late as the 21st, Martin Lewis’s images show the wave pattern on the northern edge persists with white-storm features within the North Tropical Zone (NTrZ) intruding into the belt.
.
From early in the period it was clear that the North Temperate Belt (NTB) was only visible as a faint ghost-feature. In contrast the North-North TB was complete over most of the circumference.  A distinct red-brown dark bar had been sat on the latitude of the NTB, directly above the longitude of the GRS; this short bar has persisted in images from before Christmas and was clearly photographed by Simon Kidd on the 6th of April and noted by Alan Heath on the 7th during his observation of a Ganymede transit. By the 23rd of April this feature had nearly faded out completely. To finish off, the North Polar Region of Jupiter remained undisturbed in observations sent in to the section being a reflection of the SPR in colour and visible detail.
.


By the end of May Jupiter became a low altitude target by sunset but I’m hoping daytime observations of it in the next reporting period 
may reveal important changes before it moves too close to the Sun to observe safely.
There were a number of observations showing the four main moons of Jupiter including hi-resolution images with discernible surface contrast features on these moons. In images showing the transit of moon-shadows across Jupiter it was clear that these shadows no longer passed exactly through the centre of the EZ as Jupiter has moved in its orbit past the point where the Sun falls in line with orbital plane of the moons. From our point of view on Earth we are also moving past the point where Earth’s and Jupiter’s moons’orbital planes align such that visible mutual events between the moons (occultations and eclipses) are visible from Earth. Making the most of the remaining opportunity to see these events both Peter Grego and Martin Lewis observed and recorded one of the best of them.
.
On April 21st from 2135UT onwards Ganymede was first seen on its own close to Jupiter, then Io emerged from the deep shadow of the planet, visually close to Ganymede, then the two moons appeared to merge making a small bright figure ‘8’  beside Jupiter before  rapidly moving apart again. Peter’s drawing shows the moment of fullest occultation of Io by Ganymede while Martin’s stunning series of images shows the complete sequence from start to finish; lovely work.
.
Moving on to observations of Venus; there was a wide range of observations including ones taken in full daylight and after sunset, images taken with one-shot-colour (OSC) and with monochrome cameras and using a range of filters. Venus can be one object where the type of telescope you use can affect the results achieved. I had an e-mail conversation with Simon Kidd where he pointed out that the large commercially available Schmidt-Cassegrain telescopes (SCT), which are popular for planetary work, are optimised for the red end of the spectrum so might not perform as well as simple Newtonian telescopes or well-corrected refractors for taking ultra-violet images of Venus.  UV imaging reveals upper cloud detail on the planet and images taken perhaps 30 or 40 minutes apart will show significant rotation of that cloud.
.
I received excellent images of these clouds taken with specialist Baader or Astrodon UV filters but also by using a simple Wratten  W47 blue filter, a filter that leaks significant amount of UV light, matched with an IR-cut filter to avoid swamping the image with infra-red light, a frequency the W47 also leaks badly. Simon Kidd used this combination in daylight on the 14th of April to successfully catch UV cloud detail using an 80 mm ‘ED’ refractor; incidentally Alan Heath, an expert visual observer, also used a W47 filter to catch “vaguely mottled” detail and darker shading close to the limb on the 28th of May. Dave Finnigan managed a long sequence of images between the 5th and the 20th of April using a Baader UV filter and showing varying amounts of cloud detail according to his local atmospheric ‘seeing’ conditions; UV imaging definitely benefits from steady seeing to get the best results as exposure times need to be quite long given the relatively poor sensitivity of cameras at these frequencies. 
Mike Brown also achieved excellent UV results with his 10” Newtonian and added IR images as well. Camera sensitivity to IR light is generally very good and, for the first time ever, I saw images showing cloud details on Venus in IR light taken by section members. Simon Kidd achieved this on the 15th of April using his Celestron C14 SCT and so did Martin Lewis on the 21st of May using a large aperture (444mm) Dobsonian. I managed very faint IR detail with a 10” Newtonian and hope to improve on this during the excellent morning apparition of Venus due in September.  There are a number of IR-pass filters available that will show results but it is too early for me to say which might be ‘best’ for this work; perhaps I will be able to produce a result later in the year. 
.
In addition to these monochrome images, Paul Crossland, Carl Bowron and Dave Scanlan caught lovely Venus images using OSC cameras; Dave successfully used a DSLR, the Canon Eos 450D, which he had also used with very clear results on Jupiter, and Paul caught a lovely wide-field image of Venus close to the background stars of the Pleiades while Carl concentrated on close-ups. These colour images show the rapidly changing phase of the planet during the period and often show a sulphurous-yellow colour, quite appropriate given the poisonous nature of the atmosphere. Venus will still be well placed during the early part of the next reporting period so I hope to have more results to pass-on then.
.
To finish, I must mention observations of less well-placed planets. Thank you to Carl Bowron for his images of Saturn. Despite reaching opposition on the 23rd of May and being observable all night, the short spring nights and the low elevation of the planet from UK latitudes meant that his was the only observation received in the period, taken on the 20th of May, and showing the Cassini-Division and faint banding in the northern hemisphere; so close to opposition that no shadow of the planet could be seen cast on the wide-open ring system. Perhaps others might be able to observe in June and July when Saturn will still be on view. Thanks also too Steve Hubbard for his descriptions of two “very enjoyable twilight sessions” observing Mercury on the 3rd and the 5th of May, either side of dichotomy (50% visual phase), when he used an eyepiece fitted with reticule-marks to estimate the apparent size of the planet.
.
Overall excellent work was done by the section and, in no particular order, I would like to thank all the contributors during this period being:-
Steve Hubbard, Alan Heath, Steve Norrie, Paul Crossland, Mike Brown, Simon Kidd, Davis Scanlan, Dave Finnigan, Paul Brierley, Carl Bowron, Peter Grego, Matthew Barrett  and Martin Lewis.
.
Alan Clitherow.