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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.
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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.
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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.
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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
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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.