MIRA 100
2017-1


As you may have noticed this is MIRA 100.  It must be some sort of milestone when the history of the C&WAS gets written up.  I have now been the Editor since February 1993 with the No. 29 issue, so 71 issues and nearly 25 years later I would like to thank everyone who has contributed, especially Mike Frost who has produced about 110 articles, stories and quizzes over the years.  Without their help hardly any of the 100 issues would have happened.
Thank you all.  Ivor Clarke, Ed.





Mira – The Wonderful Star
By Mike Frost

I’m sure most people who read this journal know that it is named after a star.  Mira is the popular name of omicron Ceti, in the southern constellation of the Whale, Cetus. 

If you know anything about the history of Mira, it will probably be something along the lines of “Mira was one of the first variable stars to be discovered, by David Fabricius in 1596.”  Like so many statements often repeated in popular astronomy books, this over-simplifies the story.  A 1996 paper by Dorrit Hoffleit, speaking to the American Association of Variable Star Observers in commemoration of the 400th anniversary of Fabricius’s observations, tells the fascinating story of the discovery of Mira’s variability.
David Fabricius (1564-1617) was a minister from Friesland, on the present-day German-Dutch border.  A capable astronomer, he was trying to map the course of the planet Mercury across the sky.  Remember the great intellectual revolution taking place in the sixteenth century, toppling the Earth from the centre of the solar system to replace it with the Sun.  The two planets Mercury and Venus clearly never moved far from the Sun in the sky, and there were two competing hypotheses.  Did these two planets orbit the Sun in a near-circular orbit, as the Earth was meant to do?  Or did they orbit epicyclically around a point between Earth and Sun?
The problem was that the orbit of Mercury could not easily be reconciled with either theory.  Although it never strayed far from the Sun in the sky, its motion was wildly unpredictable; on some apparitions it was much easier to see than others.  We now know that there are two complications to Mercury’s orbit – first, its eccentricity is much higher than any of the other seven planets; and second, the orbit is tilted to the ecliptic.
Throughout the 1590s, Fabricius mapped the course of Mercury.  But the star charts of the time were not good enough.  In addition to plotting the positions of Mercury, Fabricius also had to plot the star fields the planet was moving through.  Fabricius first noted down the position of Mira in August 1596; so, as you can see, he was primarily interested in it as a background star for his investigations of Mercury.
Actually, it’s more complicated than that.  Mercury was nowhere near Mira when Fabricius drew his star chart.  Some people have suggested that Fabricius had, on this occasion, mistaken Jupiter for Mercury, and was mapping stars in the wrong area of the sky.
Fabricius first noted the star on August 3rd 1596.  When he next tracked the position of “Mercury” he noted that he thought the comparison star had grown a little brighter.  Over subsequent weeks, it then became fainter again.  By October it was no longer visible to the naked-eye.
Disappearing stars were not completely unknown at the beginning of the seventeenth century.  In the year 1572, a new star, in Latin a “stella nova”, had burst into the sky.  Tycho Brahe had observed it and written about it, and to him we owe the name “nova” – actually now we would call it a supernova.  Another such nova would burst on the scene in 1604 (the last visible to the naked eye until a supernova in the Small Magellanic Cloud seen in 1987).  Both these stars blazed in the sky for several months, then faded gradually from view.  Fabricius surmised that he might have seen a fainter example of a nova; one which made it to second magnitude, and so not very notable, then faded away completely a few months afterwards.
That might have been the end of the story, but fifteen years later, in 1609, Fabricius was again observing in Cetus. To his surprise, he found that the star he had seen vanish fifteen years earlier had reappeared!  This was something completely new, and completely unexpected.
Surprisingly no-one took much notice of Fabricius’s rediscovery.  Perhaps this was because he only lasted until 1617, when he met one of astronomy’s more unusual demises.  From his pulpit, Fabricius accused one of his parishioners of stealing his geese.  The angry parishioner struck him on the head with a shovel and killed him!
Fabricius was doubly unlucky, as his untimely death came just after the invention of an instrument which would have helped his observations immeasurably – the telescope (Fabricius and his son did observe the Sun using a rudimentary scope, and so were among the first to see sunspots by projection).  In the case of Mira, magnification allowed the star to be observed down to much fainter magnitudes, eventually all the way through its cycle.  Additionally, the telescope triggered a resurgence of interest in the sky, so more people were observing.
So during the 1630s a number of people started to observe Mira.  First was Nicholas Holwarda, another native of Friesland.  He was able to discover that the star brightened periodically, reaching its brightest every eleven months.  Then came the great Polish astronomer Johannes Hevelius (Jan Hevel).  Hevelius built some of the finest telescopes of his generation, and he swept the skies repeatedly and assiduously.
It is to Johannes Hevelius that we owe the name Mira, “wonderful”, and I think that tells us how highly Hevelius regarded his discovery.  When Hevelius was born, arguably only one other example of a variable star was known – Algol, “Al Ghoul”, the demon star, in Perseus, which dips in brightness every three days.  The name Al Ghoul suggests very strongly that Algol’s regular dips in brightness were known to the Arabic astronomers who led the scientific world around the turn of the last millennium.  It’s possible that other, earlier, cultures might have spotted that Mira comes and goes – the Chinese, who recorded many “guest stars” or novae, mentioned a number of stars in the vicinity of Cetus, but their descriptions are not clear enough for us to say that one of them was definitely Mira.
You might remember me telling you about the first Englishman to observe Mira, the Northamptonshire clergyman John Palmer in the 1640s (see MIRA 82).  Palmer had heard about Hevelius’s observations and made his own over a period of months.
In a letter dated 1st April 1st 1664, from John Wallis to Johannes Hevelius, Wallis wrote “As for the marvelous star in Cetus that appears and disappears from time to time (about which you wrote a commentary), which my countryman John Palmer has observed from the year 1639 onwards, and others upon his instigation, I have nothing to add to what I imparted to you in the letter I sent last year, except that (which is also remarkable) he advised me that for some years he inquisitively investigated that star, he could never see it in the western hemisphere even when it was visible in the eastern hemisphere.  For after he had observed it in the east and had noted its approach toward the meridian, when the star reached that point it unexpectedly disappeared and could not be seen beyond it.  And it is quite true that after he had sought for it vain over many years, at last he saw it even beyond the meridian towards the west sometime, but only rarely.”
As I pointed out in my earlier article, Palmer’s observation that it could only be seen the eastern hemisphere is, at first sight, bizarre.  I put forward a possible explanation, based on the fact that Mira’s period of variability is only 25 or so days short of a year.  If Palmer only observed Mira at the same time each night, then year-on-year Mira would be at its brightest at approximately the same time of year, and therefore in the same part of the sky; however, this would change slowly over the years.  If this is what happened, it does rather suggest that Palmer was not an all-night observer!
Over the following decades, more and more observers contributed their data.  It became clear that Mira and Algol were different beasts.  Algol’s period was an unvarying 68 hours; Mira’s seemed to vary slightly around 333 days. Algol’s magnitude dip was a constant 1.3 magnitudes (2.1 to 3.4); Mira’s maxima and minima could vary from cycle to cycle (at its brightest 2.0 to 4.9; at its faintest 8.6 to 10.1).  In the 1680s another star was found, Chi Cygni, which exhibited similar behaviour to Mira; and omicron Ceti is now known as the prototype of a whole class of Mira-type variables.  During the 1740s, the York astronomers Edward Pigott and John Goodricke found two examples of another category of variables, now called Cepheids after Goodricke’s star delta Cephei; Pigott’s discovery eta Aquilae was of the same type.  These two stars were naked-eye stars known since antiquity, but their variability had not been spotted previously because the range was small.
The astronomers of the seventeenth and eighteenth centuries struggled to explain what was causing these variabilities.  There were two competing theories. Perhaps the stars were genuinely changing in brightness; or perhaps the starts were unchanging, but something obstructed our view of them.  Algol seemed to be the best candidate for an eclipsing variable as its regular dips in brightness could easily be explained by another object moving in front of the star every sixty eight hours.  Indeed, Algol is indeed an eclipsing binary, two stars orbiting each other.
The eclipse model didn’t work so well for Cepheids; the period of variability was very predictable, but the brightness variation was asymmetric.  For Mira, things were even worse – the changes in maxima and minima over different cycles might be explained by, say, an eclipsing cloud rather than another star or planet, but the changes in periodicity were much more difficult to explain.  The explanation had to wait for the development of astrophysics and the understanding of the fusion processes that drive the luminosity of stars.
Mira, it turns out, is a star in the later stages of its life, a cool red giant on the asymptotic giant branch of the Hertzsprung-Russell diagram, about 300 light years away from us.  It’s in the thermally pulsing phase.  Every 10,000 years or so, Mira undergoes a thermal pulse lasting around 10 years.  After each pulse Mira increases in luminosity.  The pulses cause a dynamic instability of the star, which we see as the eleven month variations in luminosity.
How can Mira undergo such huge variations in brightness?  Actually, the total luminosity across all wavelengths varies by much less; in the infra-red, only by a factor of two.  In fact Mira outputs most of its energy in the infra-red, so a small reduction in temperature will cause a big drop in visible light.  It’s a bit like a hot plate, which glows brightly on full, but can still output heat on a lower setting even though it no longer radiates visible light.
There are some surprises, too.  Mira is actually a double star, although not an eclipsing binary.  Mira B is a high-temperature white dwarf, about 70 astronomical units from Mira A.  The two are physically connected and the Chandra X-ray observatory has tracked mass-loss from Mira A to Mira B.



Finally, there is a loss of matter into interstellar space.  Dramatic pictures by the Hubble Space Telescope show a trail of matter stretching thirteen light years away from Mira.  The Mira system is moving through the interstellar medium at 130 kilometres per second, and the stellar wind blowing away from its surface forms a shock- or bow-wave as it slams into slower moving material.
Haven’t we come a long way in 419 years?  The wonderful star, discovered by Fabricius and named by Hevelius, is now largely understood by modern astrophysics.  Does our knowledge make it any less wonderful?  I don’t think so.  In fact, the very idea that you can track the year-long pulsations of a dying giant star through binoculars from your back garden is rather wonderful in itself.  The next time you take a look at Cetus, set aside a moment to find Mira, wherever it may be in its cycle, and wonder at the awesome events taking place out there in the depths of space.


“The History of the Discovery of Mira Stars”, Dorrit Hoffleit’s 1996 after-banquet address to the American Association of Variable Star Observers (AAVSO), can be found at

http://www.aavso.org/sites/default/files/jaavso/v25n2/115.pdf

Dorrit Hoffleit was 94 years old when she gave the address, and she lived to see her 100





The First Interstellar Voyagers
By Ivor Clarke

Anyone who reads Science Fiction and watches TV or films will know that when it comes to traveling to the stars, interstellar space ships are BIG.  Sometimes very BIG.  From Star Trek to Star Wars to Independence Day to Close Encounters, if you want to travel to the stars, large ships are de rigueur for the journeys of weeks or months.  The Star Trek ‘Enterprise’ (NCC-1701-D) in the Next Generation series was a 2,000 ft long, 4,500,000 ton vessel with a crew complement of over 1,000.  Reaching speeds of Warp Factor 9 it can run all around the local star systems in a few days or weeks.  In Star Wars, the Imperial II class Star Destroyer ship is the BIG one in the original film’s opening scene shooting at the smaller one with the Princess and the two droids on it.  The Star Destroyer is another big ship with over 46,000 military personal on the 1,600 m long vessel.  And they can all fly faster than light!  If only!
Other authors have even thought of bigger vessels such as the SF writer Alastair Reynolds with his 4km long ships with over 1,000 decks, shaped like a very thin cone to overcome wind resistance from the thin interstellar gas when travelling close to light speed.  Two-thirds the way down the hull are the drive units suspended out in pods on short wings with the ship then tapering to a point at the rear.  His ships get close to light speed and (for a change in SF) don’t break the light barrier and relativity makes the onboard journey time seem shorter with most of the crew in suspended animation.  It sill takes years in real time to get from one star system to another with the ship accelerating for half the journey, turning round and de-accelerating for the other half.
Other writers have imagined hollowed out asteroids a few kilometres in diameter with the thick rock of the outer shell offering protection from cosmic rays and dust when travelling at a good percent of light speed.  All of these are but babies compared with the vessels thought up by the late author Iain Banks.  His ships were BIG. The biggest over 50 miles across, a sliver oblate sphere shape with millions of people living on board in any way they like to (within reason).  He called them the Culture which contained numerous other alien civilisations within vast areas of the galaxy.  All the ships decisions are made by an AI Mind able to track everyone aboard the vessel, and all can connect to it.  Iain Banks’s ships had wonderful names, Serious Callers only, Wisdom Like Silence, Yawning Angel, No Fixed Abode, Tactical Grace and so on.  The largest ones manufactured other ships for their own use and travelled around the galaxy to see all the interesting bits like wars and super nova as well as helping local worlds with any problems that cropped up.
One of the first serious proposals for interstellar travel was the British Interplanetary Society’s Project Daedalus in the early 1970’s.  This was a large unmanned probe to Barnard’s Star 5.9 light years away, which would take about 50 years travel time at 12% light speed.  This too was a big ship weighting in at a starting weight of 54,000 tons of which 50,000 was fuel for its deuterium/helium-3 fusion rocket engine!  It was proposed that it would be a two stage ship with the first stage operating for two years to reach 7% LS before the second stage fired for another 2 years to reach the 12% LS.  Carrying 500 tons of scientific equipment and several rocket powered probes which would be launched a few years before the main spacecraft arrived.  Also onboard are radio and radar equipment dishes along with a 5 metre telescope to search the target system years before Daedalus arrived and using the second stage engine bell as the radio communication dish back to transmit findings back to Earth.   
So it comes as a shock when someone comes up with an idea exactly the opposite of BIG.  The Breakthrough Starshot idea is of a tiny light sail 4m square carrying a tiny chip such as in a smart phone to be humanities first interstellar spacecraft!  The idea is to build a few thousand of these solar sails and launch them aboard a rocket into a high orbit, then use a battery of ground based lasers to propel them to around 15% — 20% light speed.  They could reach Proxima Centauri in around 25 years and Alpha not long after.  The size of electronic components have reduced dramatically in recent years, look at the reduction in 20 years in the size of mobile phones.  Don’t forget in the olden days of 1995 you could only talk to someone!  Now the whole phone weighs in at only a few grams with screens, camera, computer, battery and case all built in the millions.  So it should be possible within the next 20 years for a fleet to set off to explore the stars.


A Solar Sail concept launched into Earth orbit

The whole idea was first put forward by Philip Lubin and announced in New York in April 2016 by Yuri Milner, head of the Breakthrough Foundation along with some wealthy backers.  So far the project has US$100 million to start research and the first craft could launch around 2036.  After placing the the tiny spacecraft into a high orbit many (hundreds or thousands) ground based lasers would focus a tight light beam on them to accelerate them to the target speed in around 10 minutes achieving an average acceleration of 100 km/s2, until they reach a speed of 60,000km per second.  The craft  are called StarChip, and then they speed on their way with no further influence from the ground.  It would be impossible for the StarChips to manoeuvre anyway as they contain no means of propulsion other than the 4m square solar sail.  The laser system to provide the necessary energy would need to have a combined output of around 100 GW in the current proposal, far more than its possible at present.
Each StarChip would have to carry a camera or other scientific measuring equipment, a plutonium power source, a computer and a laser communication system which uses the solar sail to signal back to Earth.  All this in just a few grams.  All components must be engineered to withstand the acceleration, the cold of interstellar space and cosmic rays and space dust.   And last 25 years plus.  Easy job.
The StarChips would be aimed to pass within at least 1au (150,000,000 km) of the target planet and photograph the surface and any moons and other objects found.  Out of the thousand launched it would be hoped that several hundred would pass through the system at various distances.  All of them relaying back data to Earth.
The date of 2036 is surprising for the first attempt: why so long?  A trial system should be possible within 10 years even if the speed achieved is much less with lower power lasers.  Once the technology exists it can be used for a lot of other research, such as exploring the outer solar system.  While to reach the stars a very high speed is required in the low percent light speed bracket, to reach the outer solar system bodies would require far less speed.



Positions of many of the recently discovered objects in the outer solar system and the large collection near Jupiter

If the concept works we could monitor all the outer bodies in our system by sending a few probes every few months to Uranus, Neptune, Pluto, Haumea, Sedna, Makemake and Eris plus any others we can find.  This would enable us to map the entire surface of these bodies.  We have at present only a shadowy view of Pluto's southern hemisphere and to will be a long time before that changes.  Likewise we have only visited each outer planet Uranus and Neptune once in the 1980’s with now obsolete technology.
The faster the StarChips fly, the less time is available to record the subjects aimed at.  If an exoplanet has a moon at about the same distance as ours, at 20% light speed the StarChips will cover that distance in about 12 to 15 seconds.  While it is possible to take several hundred shots in that time you would only see the parts illuminated by the star at that moment.  Then it’s past and gone with no way to get back or even slow down.  It will carry on until it hits something, perhaps in the far future.  Follow up craft hours later would be needed to cover greater areas if the planet was spinning or months later if its tidally locked to the star.  For planets and moons in our system we have seen quick flybys by probes after taking years to arrive, the closest approach to Pluto only lasted a few hours after 9 years of getting there!  Then there was the 5 1/2 hours wait to receive the signals back on Earth.
One of the major items to be built to make this work is the laser system to propel them.  A large laser assembly on Earth has to fire its beam through the atmosphere even if it’s perched high on a mountain somewhere.  And the Earth turns so there would be only about a 6 hour period each day when the system would be available for use.  So why have it on Earth?  By 2036 we should (hopefully) have luna bases so a laser facility on the Moon would be the obvious choice.  With no atmosphere to distort the beam, the power requirements would be less.  A base near the Moon’s equator would give almost full coverage of the heavens and because the Moon rotates so slowly it would be able to illuminate the solar sails for weeks at a time, always pushing them faster on their way.  If the sails have any way of moving their position it should be possible to steer them slightly to correct their heading.  The StarChips would be launched from Earth into a lunar orbit or into one of the Moon’s or Earth’s Lagrange L2 point were they could wait until needed or the position of the Moon and target was correct before launching. 
While the StarChips are heading out of the solar system into the Kuiper Belt and then into the Oort Cloud they need to be able to signal their findings back.  As they get further away the signal strength will decrease and be difficult to read.  One way could be for them to be launched a few weeks or months apart forming into a string.  All of them able to signal back to the next in line and so on.  By having them cascading the signal down a line of them may solve the problem of communication and would require far less power.  There would be no point to the exercise if we can’t get results back, so keeping in touch with the little craft is vital.  To get a signal back from the nearest stars will require a leap into the unknown in communication, but you can imagine the media interest in getting the first close-up view of an exoplanet.  Look what happened with Rosetta and Philae at comet 67P and the views the views from New Horizon at Pluto. 
We know that there seems to be a lot of undiscovered bodies in the outer reaches of the solar system, as we are finding more and more.  But we don’t have any idea at present of what’s out there, or what any of these objects look like, this may be the best way forward to explore the outer reaches of the solar system.  To see what these distant bodies are, we need to get close and the more we find the more difficult it is to send a conventual spacecraft to visit them as they are so far apart it is not possible to plan journeys to take in more than the primary objective.  How many large bodies does the Kuiper Belt contain and how large is it and does the Oort cloud really go out to 200,000 AU (about 3 light years) and does it contain millions of potential comets and does it have large bodies forming a sphere around the sun?  
All of these questions and many more may be answered by tiny space craft speeding along with the help of a light beam.  And we get a quick look at our nearest stars and their planets before the century is out.  The stars are starting to be much more than points of light in the night sky.






The Einstein Tower, Potsdam
By Mike Frost




On a recent visit to Berlin, I had chance to spend the day in Potsdam.  This lovely city, capital of the state of Brandenburg, is fifteen miles to the south-west of Berlin city centre, and your Berlin transport pass will let you travel there on a fast commuter train.
Potsdam is famous for the conference that took place shortly after the end of World War 2, when the Americans, Russians and British negotiated on how Germany was going to be administered after the war.  These days the city is a popular tourist destination, because of its beautiful buildings and parks, especially the delightful SansSouci park, a huge network of gardens and palaces, which is Germany’s largest world heritage site.
When I arrived in Potsdam, however, I set off in the opposite direction.  This took some doing as the nice people from the station tourist desk, who didn’t speak good English, were determined to direct me to the tourist hotspots.  Of course, I wanted to go to the Observatory. 
Potsdam is a university town, and one of its most important establishments is the Astrophysical Observatory Potsdam, sited in the Albert Einstein Science Park on the Telegraphenberg, to the south of the city.  There is a bus route there, but it only runs during the week, and I visited on a Saturday.  This caused further bewilderment amongst the nice people from the tourist desk – I think I was their most difficult customer that day.  Eventually I set off on foot – the Institute is only a mile from the station, albeit mostly uphill.


Gates to the Potsdam Observatory

Perhaps because of the sterling efforts of the tourist desk, the Observatory receives few non-academic visitors.  Nonetheless, the gatekeeper was happy to let me in. Following his directions, I crossed the institute, past maintenance buildings, offices, and the 25m high dome which housed the Great Refractor of Potsdam, a 80cm (32-inch) refractor.  Eventually, and after a couple of wrong turns, I arrived at my destination – the Einstein Tower.
This peculiar and very distinctive building looks like an observatory as envisaged by Salvador Dali.  Einstein, when he toured it, described it as “organic”, perhaps because many of the walls of the building are curved.  It’s a four-storey tower, surmounted by a dome, all sat on a multi-level base.  The whole building is built in bright white concrete.
The observatory was the idea of Erwin Finlay-Freundlich, friend and colleague of Albert Einstein early in his career.  In 1911 Einstein published a provisional version of his General Theory of Relativity, which sought to extend his earlier ideas of special relativity to include gravitational effects.  One of the predictions of this early version of GR, which remained in the final theory, is that photons emitted in a deep gravitational well would appear to lose energy, as observed from the Earth.  In particular, spectroscopic emission lines in the solar spectrum, produced by an atomic transition at a very precise frequency, would appear at a slightly different frequency when observed from the Earth.  Finlay-Freundlich proposed building a solar observatory to measure this effect.
His vision was realised by Erich Mendelsohn, who built the Einstein Tower as a solar observatory between 1919 and 1921, becoming operational in 1924.  A coelostat in the dome reflects the solar image down the tower to the base, where a spectroscope splits the light into its constituent colours for analysis.  All the moving parts are at the top of the tower, and the height of the tower means that the effects of atmospheric turbulence at the top are negligible at the base.  The original plan to use concrete for the building was hampered by a lack of building materials and the building was eventually built in brick and stucco, leading to a redesign of some of the scientific layout, though without a loss of sensitivity.
Unfortunately, it turned out that the Einstein Tower wasn’t able to carry out its set task of verifying General Relativity.  The problem was the turbulent outer layers of the Sun, which added unpredictable redshifts due to velocity on top of the predicted redshift due to gravity.  It wasn’t until the 1950s that scientists were able to disentangle the two effects and measure the gravitational redshift of solar radiation. Nonetheless, the observatory was used successfully as a solar observatory for many years.  In particular, Walter Grotrian carried out work of international importance on the solar corona during the 1930s.  He determined that a particularly impressive spectral line in the corona, which had been suggested was due to a new element, “nebulium”, was actually due to heavily ionised iron.
Of course, the history of the Einstein Tower and the observatory cannot be seen apart from other historical events of the 1930s.  As Hitler rose to power, Einstein, from a Jewish family, fell out of favour, and the bust of him which stood outside the tower was taken down and hidden in the observatory.  During the second world war, the observatory and the Einstein Tower suffered heavy damage during allied bombing.  After the war the observatory was restored, but it wasn’t until 1999 that the Einstein Tower returned to full functionality.  Indeed, the current incarnation in concrete is closer to Mendelsohn’s design than the original 1920’s building.
And what of Erwin Finlay-Freundlich, who originally proposed the idea for the observatory? Well, his is an interesting story.  He also proposed observing stellar fields around the Sun during a solar eclipse, to look for bending of starlight by the Sun’s gravitational field.  Finlay-Freundlich even traveled to the Crimea to attempt to view the eclipse of 1915 – but, as you’ll note, this was not a good year for a German to visit Russia, and Freundlich’s party were arrested.  You can read a fictionalization of this story in Stuart Clark’s novelization of the birth of modern cosmology, “The Day Without Yesterday”.
The bending of starlight was, of course, famously measured by Arthur Eddington at the eclipse of 1919 (and at other eclipses subsequently).  But, curiously, Finlay-Freundlich never accepted the results of Eddington’s observations, and came to doubt General Relativity.  His later career took some very unexpected turns, which I hope to tell you about – perhaps sooner than you expect…
The tower is still in use, though not the day I was there, for measurements of the Sun’s magnetic fields.  I stayed long enough to eat my sandwiches, then walked back down the hill and into Potsdam centre. 
If you are ever in Potsdam, the Einstein tower is worth a visit. Don’t let the tourist desk talk you out of it!





Two Scottish Observatories
By Mike Frost

In September 2016, I travelled up to Dundee to attend the autumn “away-weekend” of the British Astronomical Association.  We had an excellent programme of speakers.  On the Friday evening Nick James spoke about observing eclipses, and Dennis Buczinski gave a talk called “More than just a hobby” in which he entertainingly compared astronomy with more conventional hobbies such as train-spotting and stamp-collecting.  On the Saturday we had four professional astronomers: Professor Clare Parnell of St Andrews, who climbed all Scotland’s Munro's whilst studying for her doctorate, gave a quite mathematical talk on solar magnetic fields, which fed nicely into Doctor Alexander Macinnon’s talk on space weather.  Prof Andrew Cameron from St Andrews spoke on the discovery and characterisation of extrasolar planets, and Professor Colin Macinnes from Glasgow gave a fascinating survey of the prospects for solar sail spacecraft. 
The emphasis on our Sun, and the solar wind streaming away from it, continued with a series of talks by BAA members.  There were talks by our two Scottish-based section directors; Lyn Smith on the work of the solar section, and our new aurora section director, Sandra Brantigham, who announced that the name of the section had changed, to embrace “aurorae and noctilucent clouds”.  Sandra showed excellent pictures of Scottish aurorae, whilst the previous section director Ken Kennedy concentrated on noctilucent clouds.  Finally, another former aurora section director, Dave Gavine, gave a very entertaining talk on the history of aurorae.
So, a very enjoyable conference.  On the Sunday morning, we piled into a coach and headed up to visit Dundee’s Mills observatory, in Balgay Park, to the west of the city centre.  Unfortunately, the coach driver balked at the prospect of entering through the park gates, and so we had to walk up the hill to the observatory (there is ample car parking at the observatory, which normally hosts coaches without problem).  Balgay Hill is the second highest hill in the city; the highest hill is the Dundee Law, which houses Dundee’s prominent war memorial.



Last year the Mills Observatory celebrated its 80th anniversary.  But the person it was named after, John Mills, lived in the nineteenth century.  He was a leading industrialist in the city, a manufacturer of linen and rope, and a keen amateur astronomer.  Dundee was not a well-educated city at that time, and Mills decided that the people of the city deserved a public observatory to educate and inspire them.  When he died in 1889, he left his estate, four thousand eight hundred pounds, one shilling and sixpence, for the establishment of an observatory.  For a number of reasons, including the Great War and the Great Depression, the observatory wasn’t actually built until 1935.
The original telescope was an 18-inch Newtonian Reflector built by Grubb Parsons.  They also built the dome, from papier-mâché, which is surprisingly strong and resilient; there are other papier-mâché domes around, including the Godlee Observatory in Manchester.  After World War 2, there was a change of telescope.  The professor of astronomy at St Andrews wanted to test a Schmidt camera, and somehow persuaded the trustees of the Mills observatory that a telescope that no-one could look through was ideally suited to a public observatory.  Who was this canny astronomer?  None other than Erwin Finlay Freundlich, Einstein’s friend, who had fled to the UK from Europe at the start of the war.
Eventually Freundlich decided that the darker skies of St Andrews were better suited for his Schmidt camera, and proposed that it be replaced in the Mills observatory by an 1871 Cook Refractor which had been used for training purposes by astronomy students at St Andrews.  This was a much better idea!  The Cook refractor, which of course pre-dates the observatory, is still in place, although showing its age a little.  Before St Andrews, the telescope had belonged to Walter Goodacre, a founder member of the BAA and leading lunar observer.


The roof of the observatory is a large flat expanse, ideal for setting up portable telescopes.  Dundee AS members had set up an impressive variety of solar scopes, so we had some great views of the Sun in hydrogen alpha, which is the best way of observing solar prominences, and in filtered visible light, which shows off sunspots.  There is also a sundial, where it’s the observer who casts the shadow; marks on the ground tell you where to stand at any given time of year, and there’s a plaque on the wall explaining the corrections to observed time due to the equation of time.  On the morning of March 20th 2015, the roof was full to overflowing with people who had come to observe the eclipse from the observatory.
Downstairs there is a planetarium, a shop, and a small display of astronomical objects.  Amongst these are the original mirror from the Grubb Parsons scope, a mariner’s astrolabe and small transit telescope, and a fragment of the Strathmore Meteorite, which fell in several pieces to the north of Dundee on December 3rd 1917.
Walk to the east of the observatory and you come to a more recent attraction.  In 2004 Dr Bill Samson built a solar system walk, consisting of a series of standing stones, whose distances apart correspond to the distances of the planets from the Sun.  As you might know, the inner solar system is relatively compact, so the stones for the Sun, Mercury, Venus, Earth and Mars are relatively close, in an open area on the eastern summit of Balgay Hill.  Jupiter is a bit further away, Saturn further still, and then the path makes its way through the trees, past Uranus and Neptune, back to the Mills observatory, where the telescope pier itself stands in for Pluto.
The tour of the observatory concluded Sunday lunchtime.  In the afternoon, I made my way across the Tay estuary and through the Kingdom of Fife to Stirling, where I stayed Sunday evening.  I had a particular reason for doing this – I am hoping to hold a BAA Historical Section meeting in Stirling in 2018.  I went to visit the prospective venue, Stirling’s Smith Museum and Gallery.  The museum offers an eclectic selection of exhibits – there’s an exhibition about the grave of Robert the Bruce; the world’s oldest football, dating from around 1560, found in the rafters of Stirling Castle (where it was presumably kicked and lost); paintings by German WW2 prisoners held in a camp close to Stirling, and many other things.



The Highland Hotel, Stirling, showing the observatory perched on top and views of the 12½” f/9 Newtonian Reflector


I stayed in the Highland Hotel, on the road up to Stirling Castle.  The choice of hotel was deliberate, as the Highland used to be Stirling High School in the Victorian era, and on the top of the school buildings there is an observatory.  The observatory was founded in 1889 as a gift to the school by Henry Campbell-Bannerman, liberal M.P. for Stirling and eventually Prime Minister, but soon fell into disuse (there’s no record of use after 1906).  It was mothballed for decades, before being renovated by members of Stirling Astronomical Society in 1973.  When the school was converted into a hotel, the council stipulated that the observatory had to stay in use, for the benefit of hotel guests and the people of Stirling.
At 8:00 PM that evening, I joined Alan Cayless of Stirling AS in the hotel lobby, another Stirling astronomer, Bob, joined us shortly after.  We made our way on to the hotel roof and opened the door to the observatory.  For a Victorian observatory, the interior feels surprisingly modern. The telescope is a Newtonian Reflector, of focal ratio f/9, and a primary mirror of diameter 12½ inches.  It was designed, built and installed by William Peck, a talented young telescope maker from Edinburgh, and presented to Stirling High School by Laurance Pullar.
The weather was not promising, with cloudy skies.  We opened the dome, but as the Sun set, the only target available for the scope was the Wallace Monument, one mile to the north.  However, the Moon peeked out briefly from behind the clouds, and we got to see a nice view of the lunar terminator cutting across the Sinus Iridium, Clavius and Tycho.
I was impressed by both observatories.  Both the Dundee and Stirling Astronomical Societies do a great job of public outreach, showing off the skies to the citizens of their towns.  I look forward to returning to Stirling in eighteen months time.






Ocean Beach Camera Obscura
By Mike Frost





Top,  the Ocean Beach Camera Obscura showing its camera like shape.
Centre,  Ocean Beach Camera Obscura from the terrace.
Bottom,  the Santa Monica Camera Obscura.

For our summer holidays this year, my girlfriend and I spent an enjoyable couple of weeks touring California.  As I wasn’t travelling on my own, I didn’t concentrate on astronomical activities.  But I did manage to visit a couple of places of interest to me.
Here’s one of them.  The north end of San Francisco’s Ocean Beach is overlooked by the Cliff House, which hosts a restaurant and bistro.  On the lower terrace of the Cliff House is one of San Francisco’s oldest tourist attractions – the “Giant Camera” camera obscura. It dates from 1946, when the land behind Ocean Beach was the site of the Playland, a fairground remembered fondly by older inhabitants of the city.  The camera obscura was built by Floyd Jennings, and George K. Whitney, the owner of the Cliff House, suggested building it to look like a giant camera.  As the word, and indeed the very idea of, a “camera” comes from camera obscura, Latin for “dark chamber”, this was an inspired idea.
The building measures 25ft x 25ft.  You enter past a kiosk, which sits where the lens would be on a real camera.  In the Cliff House camera obscura, the light enters through a turret on the top.  A 10 inch diameter mirror diverts light downwards through two 8 inch diameter plano-convex condensing lenses, focussing the image of the outside world onto a parabolic screen 12 ft below; this gives a magnification of 7 times.  It’s difficult to explain to those who haven’t visited a camera obscura how captivating that image is.
The mirror can be rotated, so the camera has a 360 degree field of view, but the best vista is out to sea, in the direction of the Seal Rocks, where one can occasionally see seals.  I’m also told that the camera is a great place to watch out for the Green Flash at sunset.  Unfortunately the day we visited, one of San Francisco’s notorious sea fogs was in progress, so although we could see the rocks, and along the beach, light levels were low and not showing off the camera to its best.
Nevertheless I was delighted to visit this wonderful attraction, which I have known about for more than twenty years.  These days it is a National Monument, on the National Register of Historic Places.  It was a pleasure too to talk to the camera’s curator, Robert Taccheto, who has run the attraction since 1995.  You can see a video of him demonstrating the camera at
http://bcove.me/t0h9809e
California has a number of other camera obscuras, though none is of quite such historical importance.  A recent addition to the roster is in the Exploratorium, San Francisco, which, I’m told, has a great view over San Francisco Harbour, Alcatraz and the Golden Gate, fog permitting.   I did manage to take a quick snap of the camera obscura located on the seafront (Ocean Ave) at Santa Monica, as we swept past on a bus tour of Los Angeles. Finally, no visit to LA’s Griffith Observatory would be complete without a view from the observatory camera, built by George Keene of Tehachapi CA, see
www.cameraobscuras.com/
If you ever find yourself in the Golden State, take the time to visit one or two of them!