WADHURST ASTRONOMICAL SOCIETY
OCTOBER NEWSLETTER 2005
INDEX: MEETINGS, OTHER NEWS, CONTACTS
began by explaining how the human eye works with the sharpest image along the
optic axis. The rest of the image on the visual cortex at the back of the
eye is not sharp but the brain uses this to indicate movement.
In the case of the Blind Spot at the point on the retina where the visual
nerves pass through the back of the eye the brain fills in the missing
We saw graphs of Subjective Brightness in Brils against Actual Brightness
measured in milli-lamberts on a logarithmic scale explaining the scale used for
Alan described, with the aid of some detailed slides, the construction
of the retina. Nerves at the front and blood supply at the back.
We looked at a cross-section of the eye's retina that revealed the rod
and cone receptors. The rods respond to light as intensity only, whereas the cones
respond to colour. There are four
different cone receptors; Blue, Blue/Green, Green/Yellow and Red.
These are accommodated in most of the retina.
The rods on the other hand are concentrated around the centre of the
visual axis and are very sensitive to light.
This explains why we see in dim light conditions but only in black and
white, although it was also pointed out that we can sometimes be aware of faint
stars off axis but when we look directly at them, we can't see them.
Another graph showed the colour response against sensitivity revealing
that rods peak towards the blue end of the spectrum and the cones have a wider
response curve, peaking in the Yellow-Green area.
Alan mentioned that astronomers interested in variable stars very often
have problems since they would be using mainly the rods and since variable stars
tend to be towards the red end of the spectrum they are more difficult to
observe in detail.
Members were interested to hear that tiredness and illness can affect
night vision but surprisingly alcohol doesn't!
Another problem for astronomers is the brain's response to fast changes
in brightness where we see false brightness changes as the brain tries to
compensate. Also the eye becomes
fatigued and less responsive when looking at an object for some time, but
readily detects movement even in peripheral vision.
Alan then dislayed a number of slides to show how the brain can give
confusing information such as a grey square within a black background compared
with the same grey square with a white background.
The eye quite obviously suggests that the square in the black background
is the brighter. Also the brain
groups objects automatically even if there is no reason to.
Wrinkle ridges on the moon could be either ridges or canyons.
The rays from the crater Tycho could be seen well over the other side of
the face of the moon - or were they?
We were shown a picture of the surface of Mercury.
I could only see domes even though I knew they weren't.
Alan inverted the slide and without any doubt we were now looking at
craters. He said there were very
many examples like this but the brain needed more information to resolve what
they eye was looking at.
Another slide showed the Apollo 14 LEM on the surface of the moon with
tracks leading to it. It was very easy to estimate its distance.
Obscuring the vehicle and its tracks made it impossible to say how far
away the contours adjacent to the LEM were.
Alan talked about some early misconceptions in astronomy such as the
canals thought to exist on Mars by Percival Lowell and the strange
"ears" of Saturn. In
early telescopes the M51 Whirlpool Galaxy was thought to be a galaxy and a star
by John Herschel but has subsequently been identified as a galaxy being torn
apart by what is now thought to be a very dense companion.
It was a thought provoking talk about illusions of vision we are often
aware of but it was put together in such a way that it went a long way to
explain some of the anomalies we come across when observing through the
The meeting commences at 7.20 pm but this will be held in the Upper
Room of the Methodist Church opposite Uplands College as will all meetings from
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NOTE FROM THE SECRETARY
Following the strong recommendation from a large number of members
attending meetings held earlier this year, the Committee signalled its unanimous
support for a change to adopt the calendar year for our financial and business
purposes. This rearrangement will
be voted upon at the next AGM in December.
Subscriptions for the year 2005/2006 will fall due in the usual way on
1st November in line with the terms of our existing Constitution.
Provided that a majority of paid up members support the change, the
2005/2006 subscriptions will stretch from the usual 12 months to 14 i.e. until
the end of December 2006. In the
words of a recent television ad. this is the new Treasurer quoting you happy.
It will of course be necessary for the Constitution to be amended to
accommodate the approved changes. Copies
of the draft for the fresh Constitution will appear in the November Newsletter
in order to give members ample time to consider it ahead of the AGM with hot
NIGHT SKY IN OCTOBER
On the 3rd October there is an annular eclipse of the Sun visible Spain
and Africa. In this country we see
a 57 % partial eclipse at maximum. It
begins here at 0850 BST with a maximum at 1002 and ends at 1120.
Projection is probably the best way of observing the event, but don't
forget to look at the dappled effect on the ground through the leaves on the
trees. (Cloud permitting)
Mars is now very favourable in the evening sky to the east, about ten degrees to the right of the Pleiades and its apparent movement is no longer retrograde.
VISIT TO HERTFORD UNIVERSITY
Ian King has very kindly offered to arrange a guided tour of Hertford
University's astronomical facilities at the Bayfordbury complex, just over a
mile south east Hertford town on the B158, provided enough members are
interested in going. At present a small number of members have said they would go
but we do need more. The visit
would take place on Friday 21st October 2005, meeting at the site at
around 7.30 pm. Members
would need to make their own travel arrangements or share cars to the centre.
Our guide would be Nik Szymanek so it can't help but be a very
interesting and informative evening.
Ian would like to know the names of new interested members as soon as
possible. His telephone number is 01892
836288 and his email address is email@example.com, or let Ian
Reeves or myself know.
Dr. Tony Phillips and provided by NASA Space Place
Beyond Saturn, Neptune and Uranus beckoned, but Voyager 1's planet-tour
ended there. Saturn's gravity seized Voyager 1 and slingshot it into deep
space. Voyager 1 was heading for the stars - just as NASA had planned.
Now, in 2005, the spacecraft is nine billion miles (96 astronomical
units) from the Sun, and it has entered a strange region of space no ship has
ever visited before.
"We call this region 'the heliosheath.' It's where the solar wind
piles up against the interstellar medium at the outer edge of our solar
system," says Ed Stone, project scientist for the Voyager mission at the
Jet Propulsion Laboratory.
Out in the Milky Way, where Voyager 1 is trying to go, the "empty
space" between stars is not really empty.
It's filled with clouds of gas and dust.
The wind from the Sun blows a gigantic bubble in this cloudy
"interstellar medium." All
nine planets from Mercury to Pluto fit comfortably inside. The heliosheath is,
essentially, the bubble's skin.
"The heliosheath is different from any other place we've been,"
says Stone. Near the Sun, the solar
wind moves at a million miles per hour.
At the heliosheath, the solar wind slows eventually to a dead stop.
The slowing wind becomes denser, more turbulent, and its magnetic field
- a remnant of the sun's own magnetism - grows stronger.
So far from Earth, this turbulent magnetic gas is curiously important to
human life. "The heliosheath
is a shield against galactic cosmic rays," explains Stone.
Subatomic particles blasted in our direction by distant supernovas and
black holes are deflected by the heliosheath, protecting the inner solar system
from much deadly radiation.
Voyager 1 is exploring this shield for the first time.
"We'll remain inside the heliosheath for 8 to 10 years,"
predicts Stone, "then we'll break through, finally reaching interstellar
What's out there? Stay
For more about the twin Voyager spacecraft, visit voyager.jpl.nasa.gov.
You can learn about Voyager 1 and 2 and their grand tour of the outer
planets at spaceplace.nasa.gov/en/kids/vgr_fact3.shtml
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Any material for inclusion in the November Newsletter should be with the Editor by October 27th 2005
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