Perception in Astronomy

  Talk given by Alan Drummond at the meeting on Wednesday 21st September 2005

  Alan Drummond introduced his talk by giving a warning that what we perceive visually can be misleading for various reasons.

    He 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 information.

      We saw graphs of Subjective Brightness in Brils against Actual Brightness measured in milli-lamberts on a logarithmic scale explaining the scale used for star magnitudes.

      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.

        Our eyes were then tricked into seeing things that could be described differently such as the well-known black and white picture of "two faces" looking at each other.  After a moment we all saw the same slide as a chalice.  Another slide only showed the shadow of the word "shadow", but there were actually no visible letters.

      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 telescope.


        The next meeting of the Society is on Wednesday 19th October 2005 when the speaker will be Peter Parish who introduces us to "Planets and Small Telescopes"

      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 now on.


      Wednesday 16th November 2005.  The speaker will be Gilbert Satterthwaite FRAS talking about Sir George Airy's Contribution to Positional Astronomy"

      Wednesday 14th December 2005. The Second Wednesday of the month!! A Society member, Dr. Bob Seaney is giving a short talk called "Shedding Light on Light", an interesting talk on light itself!  This is followed by the AGM and of course hot mince pies!


       Members of the Committee are reminded that there is  committee meeting at the Abergavenny Arms, Frant on Monday 10th October 2005 commencing at 8.00 pm.  As always any member is welcome to come along.




        MISSED IT?  For those members who failed to see the note on page 2 of the previous Newsletter...  our meetings will in future be held in Wadhurst Methodist Church Upper Room.  Where is it? - Situated on the other side of the road, almost opposite Uplands College, where we used to meet.  It has wall to wall carpeting, and can accommodate up to 80 people.  There are also catering facilities on site.  We have some interesting speakers who have not spoken to us before, already booked for the next six months.

      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 mince pies!


      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.


      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 ianking2112@hotmail.com, or let Ian Reeves or myself know.


  Where No Spacecraft Has Gone Before

by Dr. Tony Phillips and provided by NASA Space Place

        In 1977, Voyager 1 left our planet.  Its mission: to visit Jupiter and Saturn and to study their moons.  The flybys were an enormous success. Voyager 1 discovered active volcanoes on Io, found evidence for submerged oceans on Europa, and photographed dark rings around Jupiter itself.  Later, the spacecraft buzzed Saturn's moon Titan - alerting astronomers that it was a very strange place indeed! - and flew behind Saturn's rings, seeing what was hidden from Earth.

      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 space." 

      What's out there?  Stay tuned...

      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 .



Chairman          Tim Bance                   01732 832745


Secretary         Ian Reeves                        01892 784255

Treasurer         Mike Wyles                   01892 542863


  Publicity         Michael Harte                       01892 783292

& Web site                    michael@greenman.demon.co.uk

  Editor            Geoff Rathbone                01959 524727


Any material for inclusion in the November Newsletter should be with the Editor by October 27th  2005