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The monthly newsletter of South Downs Astronomical Society Issue: 404 - April 2008 Editor: John Simper April Meeting Dates: ۞ Friday April 4th. Main Meeting. Main speaker will be Prof. Ian Morison: “Proving Einstein Right.” In the Assembly Hall of the Chichester High School for Boys, Kingsham Road, starting at 7.45pm Ian is the Gresham Professor of Astronomy and a past president of the Society for Popular Astronomy. Based at Jodrell Bank Observatory, he teaches astronomy at the University of Manchester and in 1990 helped found the Macclesfield Astronomy Society. He lectures widely on astronomy, has co-authored two books for amateur astronomers, and writes regularly for Sky at Night and Astronomy Now magazines. He has also been honoured by having asteroid 15,727 named after him! ۞ Very few members now meet mid-month at the Planetarium, so the meetings have been discontinued, with immediate effect. ********************************* In the News: ۞ Nearly three months earlier than predicted, the 4th January appearance of a tiny sunspot (numbered 10981 by NOAA scientists) at solar latitude +27° with a magnetic polarity opposite that of the larger sunspot group near the equator, has marked the onset of solar activity Cycle 24. Forecast to start in March, the early arrival of the cycle has divided scientists on likely strength and timing of the cycle maximum. Some predict a strong cycle of about 140 spots, peaking in October 2011, whilst others suggest it will be weak with only about 90 spots, peaking in August 2012. Much more information is available at: http://www.noaanews.noaa.gov/stories2007/s2847.html A stunning detailed view of the new sunspot can be found at: http://antwrp.gsfc.nasa.gov/apod/astropix.html ۞ On February 4th, technicians at JPL beamed The Beatles' song Across the Universe into deep space in an unprecedented commemoration of several cosmic-themed anniversaries. Aimed at North Star Polaris, 431LY away, the event marked the 40th anniversary of the recording of the song as well as the 50th anniversary of the formation of NASA and The Beatles group. Two other milestones being celebrated are the 50th anniversary of Explorer 1, the launch of the first U.S. satellite, and the 45th birthday of the Deep Space Network, an international network of antennas that supports missions to explore the Universe. ۞ Following the outcry of anger and disbelief at what was seen to be a short-sighted decision to withdraw from the Gemini Observatory project, the STFC has reconsidered its decision and has now confirmed (end-February) its continued commitment to remain a partner in the project. Accordingly, the Agency has re-instated the UK observing time allocation for 2008. A little more information can be found at: http://www.scitech.ac.uk/PMC/PRel/STFC/ GemUpdate.aspx ۞ In early March, newspapers carried articles about Jodrell Bank, and the proposal to cut £2.7million a year from the budget of the UK's large radio telescope observatories, including Jodrell Bank's 250 ft steerable radio telescope, one of the e-Merlin consortium of radio telescopes. The STFC have now (5th March) classified e-Merlin as "Lower Priority" (the lowest of four categories) and propose to cut funding from April 2009, the planned date for the start of radio observations! Details of the STFC's prioritisation of UK astronomy research can be found at: http://www.scitech.ac.uk/STFCConsultation/sources/ STFC_PR2008.doc ۞ Welcome to two new members - Mr. B. Johnson (of Fareham) and Mr. J. F. Bigmore of Chichester - the Society now has 95 paid-up members (and is one of the largest in the South of England). ۞ If you, dear Reader, are one of the few who haven't paid this year's subscription, please note this (No.404) will be the final issue of the newsletter you will receive! How to Contact us: Editor - by email at: [email protected] Or by telephone: 01483 200286 Society - by email: www.southdownsas.org.uk BUILDING A HEAVENLY YARDSTICK Part 2: Our Star Neighbours & The Limits of Parallax Galileo embraced the 15th and 16th century ideas of Nicholas of Cusa and Leonard Digges, that stars were distributed through an infinite space, when he observed through his telescope many stars too faint to be seen by the naked eye. As for determining their distance, the quality of instrument available to Galileo was wholly inadequate to measure a stellar parallax. He could conclude, however, that the stars were extremely distant because, whatever magnification was used, they never resolved into anything other than pinpoints of light. In 1669 Robert Hooke used a purpose-built telescope to make the first determined attempt to measure a stellar annual parallax. Choosing Draconis because it passed high overhead (minimising atmospheric refraction) and for its brightness (hopefully meaning it was close), his measurements gave a hopelessly large result of several tens of seconds of arc (its true value is less than one thousandth of his result). The error derived from problems with the telescope (the lens tube suffered from thermal movement and distortion as it passed through the roof of his house), and his measurement procedure (he made fewer than 10 observations over 6 months,). He also chose to measure star right ascension and declination rather than, more conveniently and accurately, differences in angular position of the star relative to another visually close and fainter (background) star. Perhaps even more importantly, insofar as the effects were not peculiar to just his own observations, Hooke was unaware of the need to make corrections for the effects of image displacement (caused by stellar light aberration), and nutation (changes in the direction of the Earth's axis in relation to the celestial sphere.) Both effects were identified and explained by the 3rd Astronomer Royal, James Bradley, in 1728 and 1748 respectively. Ignoring the parallax method, James Gregory developed a photometric line of reasoning to estimate distance. In 1668 he compared the brightness of Sirius with Jupiter at opposition, and after making certain geometrical corrections, he calculated a distance of about 83,000 AU (1.3 light years ((LY)). Isaac Newton adopted a more philosophical approach, arguing (1686) that Sirius and Saturn had about the same apparent brightness (!) and thus, by adopting appropriate values for planet size, distance and albedo, he could calculate how far away our Sun would have to be to illuminate Saturn to the same apparent brightness as Sirius. His figure of 800,000AU to Sirius (12.6 LY, corresponding to a parallax of 0.26'') was of the correct order of magnitude, but arrived at using a wrong value for planet albedo and an erroneous primary assumption - Saturn has an apparent magnitude between +1.2 and -0.2 and Sirius has a value of -1.5. In 1698, also following a photometric line, Christiaan Huygens (one of the greats of observational astronomy) calculated the distance to Sirius by comparing its brightness to that of the Sun. By an ingenious experiment involving making a small pin-hole into a dark room, and magnification of the Sun's image, he concluded the star was nearly 28,000AU distant (equal to 0.43 LY, and about one twentieth the actual distance). In the early 1700's, John Flamsteed undertook a systematic survey of the heavens, cataloguing the position of more than 3000 stars to an accuracy of 10'' (compared with Tycho's accuracy of about 1'). When Edmond Halley took up the search for stellar parallax in 1718 he compared Flamsteed's angular positions with those recorded by the ancient Greeks, generally noting good agreement except for four bright stars (Sirius, Arcturus, Betelgeuse and Alderbaren) which were different by up to half a degree. Checking against Tycho's catalogue, compiled just 130 years earlier, confirmed that Sirius had indeed moved relative to other local stars. The difference in position in such a relatively short time (the star proper motion) vindicated the idea that stars had independent motion, being neither fixed in space nor moving in concert with each other, and further stimulated the search for stellar parallax. In the mid-1720's, James Bradley's parallax search again concentrated on Draconis. From measurements made with the 12 1/2-foot Zenith Sector, he noted the star transit was fractionally lower each night, reaching a minimum declination, and then progressively higher in the sky. Over 12 months the total movement was about 20''. If due to a parallax effect, the minimum and maximum declinations should have been in December and June, but were observed to be 3 months out of phase. Bradley correctly reasoned the difference was due to the finite speed of light and transverse velocity of the Earth relative to incoming starlight giving rise to a vectorial relative speed of light, an effect he termed aberration of light. After correcting for this effect, his failure to detect any parallax meant that the star was at least 200,000AU (3.2LY) distant with an annual parallax of less than 1'' (current estimates place the star at nearly 150LY, with a parallax of 0.02"). More than two centuries after the invention of the telescope, and despite numerous attempts and observations, astronomers were little closer to reaching an accurate estimate of the distance to even a single star. But what was clear was that any estimate based on measurement of angles had to be accurate to at least one second of arc, roughly equivalent to the width of your thumbnail viewed at a distance of 2.5 miles! By the 1830's, astronomers had developed a set of three guidelines to identify promising parallax candidates. Wilhelm von Struve, using the largest refracting telescope then available, chose to study Vega, a star that was bright (apparent magnitude 0.03), exhibited large proper motion, and had a close optical companion (against which differential angular measurements could be made). His observations over 14 months in 1838/9 indicated Vega had a parallax of 0.125'' (corresponding to distance of 26LY and very close to the actual figure of 25.3LY). However, his subsequent measurements indicated the parallax to be more than double his original result! At about the same time, Wilhelm Bessel used a heliometer (a split image instrument with a relatively wide field of view and able to accurately measure very small angles) to study 61 Cygni. Although only of apparent magnitude 5.2, its very large proper motion (it has the nickname The Flying Star) and close visual proximity to two faint background stars made it a suitable candidate. At the end of 1838 Bessel announced he had measured a parallax of 0.31'' (10.6LY), close to the actual value of 0.29'' (11.4LY). Shortly after, Thomas Henderson, Director of the Cape of Good Hope Observatory, measured a parallax of 1.16'' for Centauri - corresponding to a distance of 2.8LY (compared with an actual parallax of 0.74'' and distance of 4.4LY). At last, by the mid-19th century, astronomers had proved the validity of the parallax method of distance measurement, albeit only to three nearby stars. As to how far the parallax method could reach, the availability of larger and better quality instruments, and refinement of observing techniques, led to greater ability to measure very small stellar angles with increasingly high reliability and accuracy. By the turn of the century, reasonably accurate parallax measurements for some 40 close stars had been made, and by 1912 a catalogue of stellar parallaxes listed 244 stars. Interestingly, 111 results were obtained with heliometers, 39 by photographic means, 8 by filar micrometers, 3 by spectroscopy and the remainder by meridian transits. The limits of ground-based measurements of stellar parallax are now claimed variously as being between about 100 and 300LY (the latter figure equivalent to a parallax of about 0.01''); reliable distance measurement by the parallax method may be limited to half these distances. But even so, the reach of parallax was still only about one fortieth of one millionth of 1% of the size of the Universe! To extend measurement further into the Universe, other observational methods and analytical techniques would be necessary, and these we will start to examine next month. ************************************* New/Recent Publications: ۞ In mid-January this year, NASA highlighted a new book Touch the Invisible Sky, by Noreen Grice, Simon Steel and Doris Daou, specially written for able sighted and blind readers. With 60 pages of NASA color images of nebulae, stars, galaxies and some of the telescopes that captured the images, Braille and large-print descriptions accompany each of the book's 28 photographs, and Braille markings have been added to the illustrations. It includes spectacular images taken by the Hubble and Spitzer Space Telescopes, the Chandra X-ray Observatory, and from ground-based telescopes. Each object is presented as it appears through visible-light telescopes and in different spectral regions, from radio to infrared, and ultraviolet to X-ray. ۞ The latest astronomy/cosmology book by the very readable John Gribben, The Universe: A Biography (published by Penguin in January) makes cosmology accessible to everyone. Exploring the latest frontiers of scientific discovery he tells us what we really know about the history of the Universe, how it began and how its structure developed; and what emerged to hold it all together. He describes where the elements came from, how stars and galaxies formed, and how life emerged. And finally, he looks to the future: is the history of the Universe going to end with a Big Crunch or a Big Rip? ۞ For those readers who enjoy a science fiction view of life and its place in the Universe, Ian M. Banks latest science fiction novel Matter (published by Orbit in February) is now in the shops. Nearly 600 pages long, the theme is the conflict between members of the Culture world, where nobody wants for anything, and the members of other lessprivileged societies. For those readers more interested in Banks' non- sci-fi writings, he publishes under the name Ian Banks (without the middle initial!) ۞ Although published some years ago, members may be interested to be reminded of Bernard Lovell's absorbing history of the planning and building of the Jodrell Bank telescope. Voice of the Universe: Building the Jodrell Bank Telescope (published by Praeger in 1997) chronicles the engineering and financial problems of the project, and how the determination and energy that Lovell brought to it has made it a world-class facility for nearly 50 years. April 2008 Sky Diary: (Chart for Chichester, mid-month, 9.00 pm.) Mercury will at last make an appearance at monthend, close to the horizon in Taurus. Venus is poorly placed for observation. Mars, in Gemini, has decreased in size to 6 arcseconds, fading to mag. 1.2 at month-end. Jupiter rises at about 03.00 at mid-month, brightening slowly through the month to mag. -2.3. Saturn, mag. 0.5, is high in the sky as it passes through Leo close to Regulus. Uranus and Neptune are not favourably placed for observation, being too close to the Sun. The peak of the Alpha Virginids meteor shower is expected on the 12th. And, on the night of the 22nd, the Lyrids will arrive; peaking at a rate of about 15 per hour, the shower consists of particles from Comet Thatcher. ******************************** Last Month's (March) Meeting: ۞ The subject of Spike Leggett's short talk was "Star Magnitude, What is it? And Why?" About two millennia ago, Hipparchus and Ptolemy estimated apparent brightness of 1080 stars, using a simple classification ranging between 1 (for the brightest stars) and 6 (for the dimmest stars, just visible to the naked eye.) In 1856, Norman Pogson noted that the difference in apparent brightness between stars of magnitude 1 and 6 was a factor of about 100, with a factor of about 2.5 between brightness of stars with an apparent magnitude difference of 1. Noting that this relationship was exponential, he developed a mathematical relationship between magnitude and luminosity of two stars: m2 − m1 = − 2.5log10(L2 / L1), where m is apparent magnitude and L is luminosity, for stars 1 and 2. As a final step, astronomers defined the absolute magnitude of a star as its brightness if it were at a distance of 10 parsecs, where a parsec is defined as the distance of the star from the Sun which would result in a parallax of 1 second of arc, equal to about 3.26 LY. only 3 wavelengths, current satellites can scan up to 12 wavelengths, able to identify water vapour, Land/Sea and cloud temperatures, wind speed, etc. The geographical coverage of various Meteosats was explained, from continuous observation of a particular area by geostationary satellites (at a height of about 36,000 km) down to Polar orbiting satellites (at heights of about 250 km). Quoted examples of the various features observable by weather satellites ranged from the development of major weather systems (such as tropical cyclones in the western Pacific and hurricanes in the Equatorial Atlantic/Caribbean region) through to detailed, semi-local, features such as Sahara Desert dust plumes into the Atlantic and local weather fronts embedded in larger weather systems. Moving on to an explanation of the causes (convection, etc) of the many and varied cloud types we can see (Looking Up), Storm explained how they could be interpreted to forecast short-term and local weather conditions - from the simple development of Cumulus clouds, occasionally leading to thunderstorms, through to more subtle effects, such as the increasing presence of high-level Cirrus clouds, composed of small ice crystals, pointing to the arrival of a cold front. ۞ Titled "Looking Up and Looking Down", Storm Dunlop's talk took us to the world of Meteorology. In the first part of his lecture (Looking Down), Storm reviewed the development of satellite imaging of Earth weather systems, and the way in which they have revolutionised weather forecasting. From early Meteosats, scanning at ********************************* Planetarium Shows in April: Fri. 4th 7.30 pm. All New Hubble's Greatest Hits Sun. 6th 3.30 pm. The Northern Lights Tues. 8th 3.30 pm. Prepare for Blast-Off! Thurs.10th 7.30 pm. The Stars this Month Sun. 13th 3.30 pm. All Aboard - A Tour of the Planets! Tues. 15th 3.30 pm. The Stars this Month Thurs.17th 3.30 pm. Those Magnificent Moons Fri. 25th 7.30 pm. Largest & Most Unusual Telescopes Sat. 26th 10.00-4.30 pm. One-day Seminar (see the Planetarium website for details) Sun 27th 3.30 pm. The Stars this Month Do remember! SDAS Members can watch planetarium shows at the special, discounted, ticket price. Always interesting and very good value at only £5. Booking by Telephone: 01243 774 400 or 07818 297 292.