MIRA 40
Summer 1996



Lunar Observation


Drawing by Vaughan Cooper



1996 February 29
22.00 to 22.20 UT
Colong 39.9° — 40.0°
147mm f/10.7 Reflector x175
Ant. IV

Receding terminator over un-named ancient ring formation (in Oceanus Procellarum) within which lies a dome. Arrow pointing to Wichmann.






Unusual Observing Techniques
How to Observe a Pulsar on Your TV 
by Vaughan Cooper

Further to Mike Frost's unusual suggestion of using a cardboard box over your head to safely observe partial solar eclipses as well as using the device as a large pin hole camera to view your immediate terrain, the following may be of interest.
1/  Take one high tech box (your home television set) with an ordinary household antenna.
2/  Turn to a channel that has no programme and stare at the screen.  Every five minutes you'll get a lot of snow effect that covers about one third of your screen, this interference is coming from the pulsar in the Crab Nebula in which lies a neutron star about 6,000 light years away. Soon after pulsars were discovered, radio astronomers found the pulsar in the Crab Nebula was spinning at a rate of 33 revolutions a second and slowing down by 36 billionth of a second a day.  These facts established that pulsars are, as theory predicted, spinning neutron stars. The young age of the pulsar in the |Crab accounted for its great spin and also for the speed at which the rotation was changing and the only as far as is known that alternates its ordinary pulses with giant pulses that are about 1,000 times more powerful.  These giant pulses are among the brightest radio signals we have found in the universe which accounts for then being seen on your television set. Any other unusual observing ideas which members can offer would make a very interesting regular feature in the pages of this publication.


Does any member know if this will work?  I suspect it might if the TV antenna was pointed at the correct spot in the sky and the frequency was tuned in accurately.  Ed.






Some members of the society may remember me giving a talk on the Tides a few years ago.  I think I have learnt a little more about how they work since that lecture, so let me present a new, improved ...

Tides

By Mike Frost



My day job, putting computers into steel mills, often means I spend time at Port Talbot steel mill, on the South Wales coast. When I'm working there, I stay in the seaside resort of Porthcawl. Last summer, looking across the Bristol Channel, I observed the full Moon, high in the sky over Exmoor - at low tide!
What's going on here? Aren't the tides caused by the gravitational attraction of the Moon? Doesn't high tide follow the Moon round the Earth?
Well, yes, the tides in the oceans are mostly caused by the pull of the Moon (the Sun has an effect too) - but the situation is more complicated than many books care to admit! Let me explain...
I'll begin by considering the gravitational effect of the Earth on the Moon. Let me make the assumption, popular but unconfirmed, that the Moon is made of cheese. Cream cheese, to be precise, which is to say that I assume the Moon would hold together under it's own gravity, but would be unable to resist any deforming forces produced by the Earth's gravity. With the Moon, there is also the simplifying feature that the same face is always presented to the Earth.
Our starting point, as with almost all matters of motion in the Solar system, is Isaac Newton's law of gravity, which predicts the gravitational force between two masses M1 and M2 at a distance R apart:

          GM1 M2
F = - ——————

            R2

That is to say, the force varies inversly with the square of distance.  Now, when the Moon is at its mean distance of 384 390km from the Earth, the distance from the Earth to the near side of the Moon is 382 652km, and the distance to the far side is 386 128 km, so that the force on the near side of the Moon is

(384 390*384 390)/(382 652*382 652) = 1.009

stronger than at the centre, and the force on the far side is

(384 390*384 390)/(386 128*386 128) = 0.991

weaker than at the centre.  This is the essence of the tides - the variation in the strength of gravity across an object.

What effect does this differential force have?  First subtract out the mean gravitational force - this produces the Moon's orbit around the Earth.  What is left after the subtraction of the mean force is a stretching force - towards the Earth at the side nearest, and away from the Earth at the side furthest.  If the Moon were made of cream cheese, therefore, it would be stretched into a shape like a rugby ball, or ellipsoid, pointing towards the Earth. (See Fig 1)

Perhaps the single most counter intuitive thing in this description is the bulge away from the Earth.  Why?  Isn't the Earth attracting the Moon towards it?  Yes it is, but not as strongly on the far side of the Moon.  If the Moon were cheese then the cheese on the far side of the Moon would be able to "relax" outwards under the influence of the Earth's gravitational field.  This can be viewed as a centrifugal effect; if you were to attach a string to the cheesy Moon and swing it around in a circle, the Moon would deform into an ellipsoid in much the same way.
I'll drop one of my original assumptions.  As the Apollo missions proved, the Moon is not made of cream cheese, but of rock, which does not take too kindly to being stretched.  The Moon is therefore not in practice stretched into the shape of a rugby ball.  The differential effect of Earth's gravity is strongly resisted, setting up all sorts of stresses within the Moon's surface, which occasionally lead to moonquakes, as the Moon readjusts itself internally.  Nevertheless there is a tidal effect present.  In addition to the centrifugal bulge of 4km caused by its own rotation, the Moon bulges, by 4km towards Earth. The size of the bulge suggests that it formed before the Moon solidified completely.
So, I have established the nature of the tidal force, by considering the effect of the Earth on, first a ball of cream cheese, and second, a ball of rock.  How about the effect of the Moon on the Earth?  I hope you can see that the same arguments apply.  The gravitational attraction of the Moon on the Earth is rather less strong than that of the Earth on the Moon, but it is nonetheless present, and moreover varies across the Earth.  The near side of the Earth is 378 012 km from the Moon, the centre 384 390 and the far side 390 768, so the near side is attracted 1.034 times more strongly than the centre and the far side 0.968 as strongly.  So, although the force is smaller, the differential is greater because the Earth is larger in size.
However, there is another feature to the Earth which means that tides are far more noticeable than on the Moon - the Earth is covered in fluids.  Not cream cheese, admittedly, but 70% of the planet's surface is covered in water, which is far less able to resist the differential gravitational attraction of the Moon.  The Moon would like to squeeze the Earth into a rugby ball shape.  The Earth itself resists strongly, but the oceans are only too willing to comply.  The result - an Earth which is spherical, covered with oceans which are drawn into an ellipsoidal shape.  These are the tides we are familiar with. (See Fig 2)
There is one further element to add to our understanding of the tides.  The Earth, unlike the Moon, rotates rapidly - once every twenty four hours.  So the tidal bulges are in a fixed position relative to the Moon, and the Earth rotates into these bulges - giving us twice daily tides.  I say "twice daily", but if you consult tidal tables you will find that high tides come twelve hours twenty five minutes apart.  Why the extra time?  Because the Moon is rotating around the Earth with a period of 28 days.  As the Earth spins round into the tidal bulges it has to spin for an extra twenty five minutes to catch up with the Moon's more sedate progress in the meanwhile.
So, does this explain everything? In many explanations of the tides, this is the finishing point.  The Moon raises two tides beneath it, on opposite sides of the Earth, which the Earth spins into giving high tides 12.4 hours or so apart.  There then follows a quick word or two about solar tides (which I'll come to in good time) and the matter is considered closed.
Unfortunately, however, this picture is almost completely wrong!  My observation from Porthcawl shows that there is a lag between the Moon being overhead and the arrival of high tide.  A glance at the tide tables published in the newspapers indicates that the situation is considerably more complicated still.  High tide times might be expected to differ only by an hour or so, reflecting the fact that all of Britain lies essentially in the same time zone.  Yet actual high tide times occur virtually round the clock.  Why is this?
There are two major flaws in my simple explanation for the tides.   First, the rotation of the Earth is far too fast to allow a tidal bulge to build up to anything like the size of an ellipsoid.  If the Earth, like the Moon, had one face permanently locked in position, the oceans could be drawn into a permanent bulge over a period of perhaps years.  On the rapidly spinning Earth, however, the tidal bulges are whisked away before they have time to establish fully.
Second, and perhaps more important, the Earth is not completely covered with oceans. There is land in the way, blocking the path of the tides.  How is the development of tides affected by the changing depth of the oceans, irregular coastlines, oceanic currents (such as the Gulf Stream) and atmospheric weather conditions such as the prevailing wind?  To understand the tides on a local scale we have to consider the local conditions, and, as you can imagine, calculations soon become hideously complicated.
Nevertheless, it is still possible make some general observations.  In place of the tidal bulge we have local tidal currents, which, unlike permanent ocean currents, ebb and flow with the 12.4 hour periodicity of the Moon, though not necessarily in phase with it.  Indeed, tidal currents can move in the opposite direction to the motion of the Moon.  Multiple tides are also possible, when tidal currents can arrive from different directions - for example, Southampton has four high tides a day, as tidal currents can flow from either side of the Isle of Wight.
In the middle of large oceans the range between high and low tides is quite small - tens of centimetres only.  Far from moving across oceans longitudinally, high tides tend to sweep round oceans like the hands of a clock.  This is driven by the Coriolis forces which produce similar circular movements - cyclones and anti-cyclones - in the atmosphere.  The centres of the circles are called amphidromic points and have no tidal variation at all.  Large oceans have several such points.
In enclosed seas and lakes, the response to the tidal pull is more like a sloshing motion, where the water body moves coherently in a standing wave or seiche.  This, as you might imagine, is a rather smaller amplitude response.  In the Mediterranean Ocean, for example, the range of tides is only centimetres.  All lakes (even puddles!) have tides in theory, but the amplitude of the tidal response may be vanishingly small.
In bays and oceanic inlets, the tidal amplitude is enhanced if the tidal force causes a resonance.  The largest tides, one might guess, are where the tidal currents are funnelled into an increasingly narrow estuary or bay.  Sure enough, Porthcawl, on the Bristol Channel, has large tides, up to llm usually, and further up the Severn estuary tides are even higher, culminating occasionally in the Severn bore, where the tide arrives "all at once" in a solitary wave heading up the tidal River Severn.  The world's largest tides occur in the Bay of Fundy, Canada. Here the tidal currents which have been flowing up the eastern seaboard of North America are caught in between the Nova Scotia peninsula and the New Brunswick "mainland", resulting in tides of up to 15 m.
I promised a while back that I would discuss the tidal effects of the Sun.  You might think that because the gravitational force of the Sun is overwhelmingly greater than that of the Moon, the tidal forces caused by the Sun might be much greater.  This, however, is not the case.  Tidal forces are caused by the variation in gravity across the Earth, and because the Sun is much further away than the Moon, the differential forces or tides are rather smaller. In fact, the size of the solar tide is approximately half that of the lunar tide.  On top of the twice daily tides, the amplitude of the tides varies over a lunar month, as the tidal pulls of Sun and Moon go in and out of phase.  You may know the extremes as spring and neap tides.  In spring tides, the tidal pulls due to Moon and Sun coincide, producing a higher than average tide.  In neap tides the pulls are ninety degrees, or six hours, out of phase, and tend to diminish the amplitude of the high tide.
Finally, both the orbit of the Moon round the Earth, and the Earth round the Sun, are elliptical, and the tidal forces are greatest at perigee and perihelion respectively.  The magnitude of the tides is great interest to those who live behind sea walls, and consequently the conditions for the "worst case" spring tide are often computed.  The Moon's closest approach to the Earth moves through the calendar, but Earth is closest to the Sun in early January.  If there's a spring tide in early January, when the Moon is at it's nearest point and storms are forecast, make sure you have plenty of sandbags to hand.

Alternatively, buy a house in Rugby!


Source material/further reading:

Asimov on Astronomy (Ch 1) - Isaac Asimov (Coronet 1977)

Cambridge Encyclopedia of Astronomy (Ch 10) - ed Simon Mitton (CUP 1979)

Encyclopedia Britannica, Macropedia Vol 23 (Fifteenth edition 1994)

MacGraw Hill Encyclopedia of Science and Technology, Vol 18 (1992)





Quantum or Particle Physics and Matter
by Pam Draper


I don't think I personally know of anything in my life I've come across so confusing as trying to understand what matter consists of.
It started off with the atom, then the electron, followed by neutrons and protons.  Then there's the positron...  It turns out that every particle has an associated Anti-Particle with the same mass but opposite charge except the photon, which can behave like a particle and sometimes a wave!
Then we go into the nucleus of the atom containing protons and neutrons.  These contain point-like objects called quarks (pronounced as quarts).  Depending on what type of atom it is, hydrogen etc., dictates how many protons and neutrons are inside.  Quarks come in 6 known flavours (not as in taste though), up, down, strange, charm, bottom and top. 'These also vary in their properties, ie., charge and mass etc.
Quarks have what's called 'Colour Charge', an electric effect only felt at distances smaller than the size of a nucleus.  This charge is passed on by gluons. Gluons build a lattice to trap quarks within the nucleus of an atom.
Next came the confusing world of the families of particles such as the following. . . . mesons, or hadrons, muons, fermions, baryons, bosons, leptons, wand Z particles, pions and kaons and one day maybe the graviton waiting to be discovered. These families have stories of their own to tell.
Sorting out this confusion is my present task, these particle interactions are within what's known as 'The Standard Theory'.  All these families of particles participate in the four fundamental forces in the universe...

1/ The Strong Nuclear Force
2/ The Weak Nuclear Force
3/ Electromagnetism
4/ Gravitation
 

1,  The Strong Nuclear Force
This binds protons and neutrons together to form atomic nuclei and binds the elementary particles, quarks, together.  Quarks are not found in isolation in nature.  Gluons carry the strong force within the nuclei.

2,  The Weak Nuclear Force
Mediates the process of radio-active decay.  The time involved in particle decay can be about a thousand-millionth of a second.  This is very long on the scale of particle interactions, (the strong force acts a million million times faster).  As far as it is known the force operates within a particle and does not extend beyond its boundary.  The weak nuclear force includes the neutrino or little neutral one.  It has zero mass and zero charge and responds to gravitational force but not electromagnetic.  They are found in Cosmic Rays and see the Earth as transparent.

3,  Electromagnetism
This is the attraction of the particles with opposite electrical or magnetic charge for one another, it includes light, photons, the energy of which is measured in waves, the electromagnetic spectrum.  Long wavelengths called radio-waves and short wave-lengths of x-rays and gamma rays.  It is responsible for the structure of matter.  It is attractive between oppositely charged particles, pulling them together and repulsive between similarly charged particles pushing them apart.

4,  Gravitation
The attraction of all particles of matter for one another.  It holds each star and planet together, keeps planets in their orbits and retains stars within galaxies.  The acceleration due to gravity at any place is called 'g'.  Einstein's theory predicted that light as well as matter is affected by gravity.  Other predictions of the theory, such as the existence in space of Black Holes, in which the gravitational field is so strong that light cannot escape.
Well I've learned a lot in the past year about ionisation, plasma, various types of gas clouds and elements present in the universe, and I have just skimmed the surface here of a very exciting and confusing subject.  But I discovered that it is not as difficult as you would believe.  With perseverance and determination, the pieces gradually start to fit together and hopefully remembered more easily.  It makes the dynamics of a magnificent and highly complex universe more appreciable.






Get Hooked on the Coat Hanger
with Colin Hughes

My first visit to the society's meetings was in October 1995 and within minutes I was discussing my particular interests with a society member.  I happened to mention that amongst other things I was fascinated by the 'Coat Hanger' (nickname for Brocchi's Cluster or Collinder 399).  The other chap had not heard of this to my surprise, for being a newcomer, I expected items such as this to be 'run of the mill objects' to other members.
This short piece is therefore written for anyone who has not seen the 'Coat Hanger' as it makes a fascinating binocular sight and is well worth a long look.
The 'Coat Hanger' consists of a near perfect straight line of six stars with four more forming a curving hook shape down from the centre of the straight line.  The botton of the hook is marked by the 5th magnitude multiple star 4 Val. Hence an upside down coat hanger shape.  The ten stars are said to be between mags 5 and 7, however I have been unable to locate them with the naked eye.  Apparently the stars are not related and only form this unique extraordinary shape by chance.
It's location is approximately dec+20°N, RA 19h 20m in the constellation Vulpecula the Fox, which no doubt most of you will know is near to the 3rd mag. double star Albereo (Beta Cygni) in Cygnus the Swan.  I actually locate it not by Albereo, but by first finding the bright 0.77 mag. star Altair in Aquilla the Eagle.  From here I slowly sweep my binoculars up to the constellation Sagitta the Arrow.  Then I locate the 2 stars at the right end of Sagitta, (the top one being Alpha Sagitta), and from here the 'Coat Hanger' is roughly at the 2 o'clock position between the stars Alpha Sagitta and 1 Vulpecula.


At this time of year (Spring & Summer) it is only visible from around 11pm onwards, but will get earlier as the year progresses.  Many of you who are more familiar with the night sky than myself probably know all about this asterism, but if you've not seen it, then I hope I've contributed something worthwhile for your observations.