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Highlands Astronomical Society

Stargazey Pie February 2006

January was a long month! It always is, and it seems to take forever for payday to come round. But the other thing that people seem to look forward to as January ends is the February meeting of the Highlands Astronomical Society, certainly if the number of people at the meeting was anything to go by. It could have been that the talk was on the amazing Cassini-Huygens mission and was being given by Pauline Macrae, or perhaps it was the chance to get together with other observers and discuss what had been seen in the relatively good nights of January. Maybe it was simply the lure of tea, biscuits, and the snippets of astronomical information that that get freely handed around at the meetings. It all started off with the notices:

  • AGM Notice. Advance warning was given that the Annual General Meeting will take place at the April meeting, on Tuesday 4th at the usual time of 7.30pm. The Secretary post is up for re-election but Pat Williams has agreed to stay on for another term if re-elected.
  • Observing Sessions. Come on everybody, let’s enjoy February’s great dark nights together at the HAS Observatory at Culloden! The schedule is as follows, but the sessions do depend on the weather. If it is raining or 100% cloudy it will not be on; if merely doubtful then give the contact number a call before 7.30pm and use the mobile number after that.

Friday 17th Feb .......Pauline Macrae
Saturday 18th Feb.......Trina Shaddick
Friday 24th Feb .......Maarten de Vries
Saturday 25th Feb .......Antony McEwan

  • Mercury Observing Session. The most elusive inner planet, Mercury, will be at greatest elongation from the Sun on the evening of Friday 24th February. This will be an ideal time to have a go at spotting the planet, as it will be bright (but low) in the southwestern sky after about 5.20pm when the Sun sets. Everybody is invited to come along to the observatory at Culloden Visitor Centre to observe the speedy little planet, as there are many observers who have never seen Mercury, and many more who have not seen it very often. There is talk of the Visitor Centre café staying open late as well if the skies stay clear and the event goes ahead, but this is still to be confirmed. We need as many people as possible to send out good weather vibes for this event, as there is also an observing session later on that evening. It would be good if the skies allowed us to just have one great big session starting off with Mercury!
  • Volunteer please… You will no doubt be aware of the astronomy course being run by Maarten de Vries in Cromarty, starting on Thursday 16th February (if not, see here for details), and that there will be observing sessions for the students after each classroom session if the weather permits. These will take place at Resolis on the Black Isle, from about 9pm onwards. If there is anyone with some knowledge of telescopes and finding celestial objects who would like to assist with these sessions, please contact Maarten or myself.
  • Moonwatch. Moonwatch is a mass-participation astronomy experiment that anybody can get involved in. It is run by the Institute of Physics and intends to find out just when the new crescent Moon becomes visible to people. You can find out more at the Moonwatch website, but basically it involves going out in the evenings following the time of new Moon and making a note of when the crescent is first detected, your location, weather conditions etc, and then you can submit your data to the project organisers. They hope that data received this way will help to increase the accuracy of the lunar calendar.
  • A Timely Reminder. If you are lucky enough to be able to receive BBC4 (Don’t even get me started on digital reception…) then you will be able to watch a four-part series hosted by string theory pioneer Michio Kaku. The subject matter will be time, and further information about the series can be found here. The first part is on Sunday 26th February at 8pm. Maybe one day we’ll all be able to receive digital transmissions and certain parts of the population will no longer be discriminated against due to their choice of location, or lack of it. All in good time, eh?

Eyes On The Skies

February is a short month, but it has a lot going for it astronomically. The Milky Way should still be visible nearly overhead and the gigantic constellation of Orion, both a repository of superb deep sky objects and a superb pointer to many other items of interest in the sky, is riding high in the South by mid-evening.

February is traditionally a good time to look for the Zodiacal Light. This is a subtle triangular glow caused by sunlight (from below the horizon) reflecting off meteoric dust that lies on the plane of the solar system. No optical aid is needed, and it is best looked for a couple of hours after sunset, on the ecliptic in the west. Light pollution and moonlight will both reduce your chances of spotting it, so try from a dark site if you can. I shall be looking- I’ve never seen it! Perhaps this could be something else to look for after hunting Mercury on the 24th?

Mars passes very close to the Pleiades mid-February, and is at its closest on Saturday 18th, when it appears to be just 2.3 degrees from the cluster. Interestingly, this is the second photo opportunity combining a planet and an open cluster within two months. Last month it was Saturn’s turn to come within the celestial boundaries of M44, the Beehive cluster.

Mercury is at its greatest elongation, 18 degrees east of the Sun, on Friday 24th so this should be a perfect opportunity to try and spot the elusive and fleet-footed little planet. Many observers have never seen Mercury, and some have only seen it very few times, myself included. The planet should show its phase when viewed through a telescope, but will not show any surface detail. Regardless, it is quite an achievement to be able to track the planet down, so is definitely worth a look if the evening skies are clear. Don’t forget the Mercury Observation Session (or MOBS) at Culloden on Friday 24th. We will not start looking for it with optical aids until after the Sun has set, which will be about 5.20pm, so feel free to come along and see for yourself. Observing will go on until later as well, but to see Mercury you will need to be at the Visitor Centre car park between 5.20pm and about 6.30pm to be in with a chance. Remember- Do Not search for Mercury using binoculars or telescopes while the Sun is still above the horizon as you could severely damage your eyes or blind yourself if you stray across the Sun! More information on Mercury can be found in the February Seeing Stars article by Pauline Macrae.

Getting back to Orion, some of the lesser-observed phenomena within its borders include the long swirls and patches of nebulosity that make up Barnard’s Loop. This is a very large arc of nebulous material blown away by the stellar winds of stars that were formed in the nearby Orion Cloud. It is situated to the east of Orion’s Belt, covers about 12 degrees north to south, and is extremely subtle and quite tricky to observe visually. Ultra High Contrast filters may help, and some areas respond well to Hydrogen Beta filters. Low power and a very wide field are required, as is a dark site and little or no moon-glow. Hmm… I think I just found myself an observing challenge for this month!

I know I keep going on about Saturn, but guess what- Saturn is nice and high now for observing! It is past opposition so will be getting smaller as it moves further away from us again, and is actually quite a small target even at opposition, so does require fairly high magnifications to see the subtle cloud details and ring structure, but is always worth the effort when seeing allows. If you have a non-astronomer friend and they show even the slightest interest in what you look at in the sky, you just HAVE to entice them to look at Saturn on a good night through your telescope! They should be hooked…

The Main Event.

‘Cassini-Huygens’ by Pauline Macrae

Pauline’s recent presentations have included various solar system bodies and a talk about what the space industry has done for us. It therefore seemed appropriate that this latest talk focused on Saturn, one of the highlights of the solar system, and the Cassini-Huygens mission that set out to learn more about the ringed planet and its largest moon, Titan.

The Cassini-Huygens mission was a very complex undertaking. It followed in the footsteps of the Pioneer 11 and Voyagers I and II spacecraft, and took with it a whole new array of imaging and data gathering equipment, as well as the Huygens probe- the first spacecraft (that we know of) to set down on the surface of Titan.

Pauline’s introduction to Saturn itself was quite extensive, and included a very detailed look at the structure of the planet. Although Saturn could contain 764 Earth-sized planets, its mass is only 95 times that of Earth and its density is only about two thirds that of water. This gives rise to one of the most fascinating things about Saturn- it would float on water! The composition of its atmosphere is mostly (97%) hydrogen, with some helium (<3%) and traces of a few other gases, notably ammonia and methane. It is thought that Saturn has some form of hard rocky core (although this is not known for certain) surrounded by a rather interesting layer of fluid metallic hydrogen! Pauline suggested the closest analogue to that would be mercury, which exists as a flowing metal on Earth. Saturn has a huge magnetic field too, which surrounds many of its 49 moons, and electrical activity such as aurora and lightning storms have been reported and imaged by the Cassini orbiter.

25 Years passed between the Voyager’s flybys of Saturn and the arrival of Cassini-Huygens, and in that time some things have changed. The Voyager spacecraft measured Saturn’s rotation period to be 10h 39m 24s, but Cassini has found it to be 6 minutes slower! Scientists are convinced the data from the Voyager missions were accurate, so that poses an interesting question- why has Saturn slowed down by so much in so short a time? In actual fact, they don’t believe that whole planet has slowed down that much, but instead think it is some effect being caused by Saturn’s unusual magnetic field. It is the only planet in the solar system to have its magnetic field aligned almost perfectly with its axis of rotation- does this mean magnetic field variations affect the rotational speed? That’s something for the scientists from the 17 nations who participated in Cassini to ponder.

Another subject Pauline spent a lot of effort on, and showed some beautiful and intriguing images of, was Saturn’s ring structure. Although the diameter of the visible rings that we may see through a telescope is almost the distance from the Earth to the Moon, the rings actually extend much further than that. The other rings are so sparse though that they don’t reflect much light and so are almost invisible. Although scientists thought that perhaps the moon Enceladus was contributing somehow to the brighter part of the faint E ring, they were astonished when Cassini returned images of this little moon actually venting (Pauline used the technical term ‘splurting’) ice particles from its surface!

The ‘thickness’ of the rings varies, but in some places is only about 10metres! They are made up of particles varying in size from dust-sized grains to boulders the size of a garage. There are many mysteries associated with the rings. It is thought they may have formed when a small moon-sized object was captured by Saturn’s gravity and pulled in to within the Roche limit distance. Once there it would disintegrate and the material could be spread around the planet in almost the same way as an accretion disc might be formed during the formation of a planet. This seems logical, as the mass of all the ring material combined adds up to about the same amount required to construct a body the size of the moon Mimas. It is also thought that the rings were formed recently compared with Saturn’s age, and so might actually be a temporary feature which we are lucky enough to be able to see. Eventually the ring material may be pulled ever inwards towards Saturn and be incorporated into the planet itself, leaving a ring-free planet where once was ringed Saturn. The whole process could, theoretically, be repeated when another moon-sized object gets caught in Saturn’s gravity!

There are various gaps and amazing structure in the rings too, such as the Cassini division, Encke and Keeler gaps, and bands of differing widths that exist within the rings. These seem to be caused by gravitational influences caused by ‘resonances’ between Saturn’s moons and the material in the rings. Another mystery mentioned by Pauline was the strange ‘spokes’ that appear across the rings. They appear as dark areas spread out across the brighter background of the ring material. Are they particles that have become separated from the ring and are carried around in rotation by gravitational or magnetic interactions? Is there some other explanation for this phenomenon first observed by revered astronomer Stephen O’Meara and confirmed first by Voyager II and then by Cassini? And what of the bright arcs that change location within the rings from time to time? What causes them? The more probes we send out the more questions we find to answer…

As well as its on-board data-gathering equipment, Cassini also carried with it Huygens, a detachable probe that separated from Cassini, and actually landed on the surface of Titan on 14th January 2005. It descended through the murky nitrogen-rich atmosphere scanning all the time, analysing its surroundings and passing the data back to Cassini for transmission to Earth. Titan has a very thick atmosphere, abounding with organic compounds- the possible building blocks of life. It is so thick and smog-laden that it is impossible to see through the atmosphere to the moon’s surface by normal means. Infrared imaging revealed differences in the terrain, but even so it was impossible to accurately predict what Huygens would find on touchdown. When it did finally make landfall it came to rest on a material that could be described as ‘sandy’, except that the granules it rested on were not made of quartz grains but of ice. The intense cold (about –180’c) gave the ice the hardness of terrestrial rock. As it came to rest, puffs of methane gas were released, which suggests that the ‘sandy’ surface was soaked in liquid methane. Truly an alien world, but one which further analysis of the data provided by Huygens will gradually explain to us.

I can briefly summarise the talk here, but I can’t really summarise the huge range of images that Pauline showed in her presentation. Images of the spacecraft involved, the Planet Saturn and its family of moons, images of the rings taken from various angles, and even of weather patterns revealed by infrared cameras on Cassini that resembled those that we commonly see on Jupiter. There were also excellent diagrams showing the composition of Saturn, the magnetosphere, and comparisons of some of the surfaces off several of the more noteworthy moons. Add to this wonderful talk the fact that Saturn is high in the sky and easy to observe at the moment, and it really is Saturn’s hour in the spotlight this month.
Thank you Pauline for a great tour of Saturn and its family, and a thorough exploration of the scientific mission that is Cassini-Huygens.

Next Time.

The next meeting takes place on Tuesday 7th March at 7.30pm in the Green House. The subject of the main talk is ‘Exoplanets’, and it will be given by Jane Greaves. There will also be the usual Beginners and Equipment Breakout sessions, and no doubt much more besides. Come early to get a parking space!

Until the next meeting, Dark Skies to All!

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