MIRA 55
Christmas 2000



Observation of the Un-Named Lunar Feature Near Schickard & Inghirami

By Ivor Clarke


LOCATION            42°S  66°W  near to the large crater Schickard
DATE                   Sunday 16, 2, 1992
TIME                   20.35 UT to 21.35 UT
MOON                 13 days old
LUNATION           855
COLONG             70.6°
SEL. LIBRATION  Long.-1.27  Lat.3.15
LOCATION           Bedworth,  Warwickshire
CONDITIONS       Excellent, seeing Ant.IV to V
INSTRUMENT      102mm Refractor, 7.4mm Plössl +1.8 Barlow=180x & 243x
 

Folk with long memories may recall a cover drawing I had on MIRA, 10 years ago (MIRA 27; Jan. 1990) of this strange ridge on the Moon.  At that time one of the members, Rob Moseley, suggested that this was an undrawn lunar object which was not recorded on the lunar maps.  As far as I can tell, it still isn't on any map but is an interesting feature.  This is still, I'm afraid, the best drawing that I have of this ancient crater and ridge feature.  It can be seen in The Hatfield Photographic Lunar Atlas in Plate 12b as a line leading away from the crater Schickard across a ruined crater a little below the Inghirami valley line.  The Lunar Orbiter IV spacecraft photographed this area on 1967 May 24th on frames HR172/2.  Also this feature was observed by Harold Hill, the well known lunar observer on 1991 November 19th at a similar sun and moon angle to this observation.
It was drawn in much the best seeing conditions that I have experienced with this feature.  There was very little shimmer, excellent transparency and with the Moon high in the sky, this observation was a joy to do.  The sun rising over this part of the Moon had not long picked out the ridge and was just starting to illuminate the rear walls of the crater.  During the time it took to do this drawing the shape of the rear walls and the faint background markings had altered considerably.
The colong of 70.6° is a mid-point for this observation and is, I feel, the best figure to give for a drawing which takes an hour or so to do.  Drawings are not the exact lighting conditions at any one moment, but a mixture of many moments spread around the area of the drawing.  In drawing an area as large as this, the eye and the pencil move from point to point checking and re-checking alignments and proportions and many different shapes of shadow and highlights combine to form an impression of the subject.  So to give an exact range of values is misleading because the reader is not in a position to know what parts of the drawing is the exact colong for (and the observer will not remember anyway) and with drawings taking longer than a few minutes the lighting will change.
The top of the ridge looked quite flat and level but the bottom was uneven, possibly caused by shadows from small hills across the crater floor which was in deep shadow.  Between the start of the ridge and the rear wall of the crater was a gap of some distance.  The unevenness on the floor was faintly visible as dark markings merging to black.  The hills on the top of the ridge were starting to catch the sun, but not showing too much of their correct shape, while those below where starting to show as craters filled with shadow.  In the centre there was just one well defined crater and a longer hill.
The part I had called a Birds Head previously did not look like a birds head this time, it was easy to see what it was, a large shallow crater in the wall of the main formation (center foreground).  This appeared to mark the end of this portion of the main crater wall, with no sign of it any further north.  Next to this Birds Head crater seemed to be two shallow craters joined together.  Behind them a small depression ran along the top of the crater rim.  The dark interior floor of the main crater behind had a striped appearance which ran back to the bright ridge.  The ridge was lighter in tone than the rear walls and I did not see any breaks in its length; even though it was nearly broken where it crossed the wall of the main crater in the south east.  This looked like a collapse in a small section of the cliff caused by a small crater on the top edge.  After leaving the crater the ridge took on a different appearance, looking twice as broad, running on towards Schickard through lighter coloured ground before ended in a group of four or five shallow craters.  This part of the drawing is a little compressed to get in the end of the ridge.
Whether the ridge is caused by material thrown out by the Mare Orientale impact or is the result of a fault or landslip it may not be possible to tell from Earth.  We may have to wait for a more detailed survey of this area from close orbit or even from a geologist on the lunar ground.



From the Editor


One of the joys of astronomy for me is the fluidity of ideas about the universe we live in at any one time.  For instance, when I was a lad it was pretty well known that the canals of Mars were most likely old river beds which carried small amounts of spring melt water from the poles into the lower latitudes.  Even then it was most unlikely that the lines where artificial, built by a struggling civilisation to water crops!!!  Soon after, the Mariner 6 & 7 Mars probes swept past and shot a few low resolution pictures which showed a cratered planet with NO straight lines on it.  Another dream dead.  Mars became a dead and dried-up world, with only small amounts of water at the poles locked up with frozen carbon dioxide frost.  All the later Mars orbiter's showed the craters and canyons and sand dunes and volcanoes but no sign of water.  Only very old traces of river valleys and fans from the flanks of the huge volcanoes.  But now a turn round, water may still flow on Mars.  But only underground! The Mars Global Surveyor has seen evidence of small channels running off the sides of the canyons where water has spilt out from the sides.  Underground the pressure and temperature are high enough to enable water to stay a liquid.  If it brakes through to the surface and a quantity gushes out forming a channel in the dry sand, it will not go far before it boils away in the thin atmosphere.  But what a change round; from a small amount, to none, to maybe plenty, in 30 years.  Another big debate is starting to hot up.  Has the speed of light changed during the life of the universe?  Well if it has, it would explain a lot of the problems with the Big Bang theory and the weird way inflation is used to make the maths work.  All I have ever read about this idea of inflation in the first fraction of a second of the universe smacks of grasping at straws in an attempt to make things add up. But changing light speed makes it all work splendidly.  Is this correct?  Who is right?  Keep reading the news in the years to come!  We live in interesting times.

Ivor
Editor


Telescopic Thoughts

by Ivor Clarke

The seasons for looking at the night sky are now here.  Now is the time to get that new telescope you've been promising yourself, or get out the one you already have and dust it off.   Here are a few thoughts on the tricky subject of what telescope is best.


These joined arrows show how adding a x2 Barlow lens to each eyepiece doubles its power and gives a wide spacing for a range of 3 eyepieces of 26, 15 and 9.7mm. 

I wonder how many members will have a new telescope this year?  Maybe for a birthday or for Christmas?  The first astronomical telescope you own will forever be remembered, hopefully for the exciting discoveries and views you had with it.  The first views of ragged lunar craters and smooth mare, the first views of Saturn's rings and the cloudy Jupiter with its four dancing moons and dusky Mars.  The bright crescent of Venus and the smoky Ring Nebula.  Bright colourfull double stars like Abero and sparkling clusters, the Orion Nebula and M31.  All of these are visible through any scope, plus thousands of other delights.

Telescopes are personal things and observers use them for many reasons.  Both large and small scopes have their devotes who swear by them.  But remember: There Is No Such Thing As The Perfect Telescope.  Each person wants to look at different things, so therefore, no one telescope is perfect for all types of viewing.  Large aperture scopes are the only type which will pull into view faint galaxies and nebula.  But large scopes cost more and may need to housed in purpose built observatories if they are to be used successfully.  Also as time goes on and observers grow more experienced, a more advanced or different telescope may be needed.  You may start to specialize in a certain area of astronomy and need the scope to help you see and record the data you need.  Also tastes and interests change and a sometimes a new instrument will rekindle interest in a subject.

Just because you have a telescope now, don't think you have to have it forever.  Equipment used a few years ago by just a few of the most advanced amateurs is today being used by many as the price falls and the technology moves on.  Last year I sold my 4inch Vixen refractor which I'd had for a number of years and bought a little Meade ETX-90EC scope with the proceeds.  The Vixen was a fine telescope with excellent optics on a good tripod with an excellent mount.  It was quite easy to polar align the equatorial mount and I enjoyed many fine views of the Moon and planets through it.  The optics were excellent and crisp images were the order of the day.  But I felt like a change; I wasn't using it much and some times you just would like a change!  Changing your telescope is a bit like changing your car, you can out grow then, get fed up with them, just get to dislike it.  Some folk change their cars almost every week and others keep them for years, its your choice, you must decide.  You may need something bigger / smaller, more expensive / cheaper.  You may just feel like a change, why not?  I did.  How many folk buy a new car and run it into the ground and change it 15 to 20 years later?  Not many!  So don't feel that a scope is for life, it's for using!

I never used my Vixen much to look for faint misty blobs hiding in the overhead orange glow which is supposed to be the night sky.  I suppose they where there, lost in the sodium glow, but to find such diffuse objects takes a much less light polluted sky then mine here in the Midlands.  Really dark skies don't exist in the UK as far as I know.  If they do, I have not seen one.  But I do know a man who has!!  One of our members has taken some first class photographs of the Milky Way from central Wales, so maybe it is still just possible!

From my back garden, double stars showed up OK in the 4•Vixen, as well as open star clusters and globular clusters, often showing the colours of many of the stars.  Some of the brighter planetary nebula such as M57 are easy to see, so was the Orion nebula, while M31 the Andromeda galaxy looked just like a large bright fuzzy star.  It was difficult to imagine, in this case, that you are looking at a galaxy bigger and brighter than our own Milky Way galaxy!  Remember that I was looking through a 4 inch telescope rather than a 14 inch one, so you can see only the brightest parts of the inner core of the galaxy with the smaller telescope.  If you want to find and see faint fuzzes, size matters, as the car ads keep telling us.

But any size of scope cannot show you things washed out by a bright sky glow from numerous street lights and security lamps.  Only from a dark sky will you get down to the lower magnitudes and see the fainter galaxies and nebula.

One of the main reasons for my purchase of the Vixen, was because I was getting more interested at that time, in observing the Moon than anything else, so size didn't matter.  But sharpness and clarity did.  Refractors score in these departments, and size for size a refractor will beat other types of telescopes, but at a price!

Other types of scopes can offer much larger apertures to grab all these photons coming in from a distant object.  Scopes like large Dodson's and Newtonians with a mirror up to a meter in diameter are available to serious amateurs.  All telescopes approaching this size need to be installed in a permanent specialist building.  This can cost a lot of time and money and only you know what you want to look at and can afford.

All types of telescopes have some bonuses and some disadvantages when compared with other types and the user must choose which are the best features for themselves.  This is not an easy choice by any means.  Don't forget about ease of use.  Big Dobsonian's and Newtonian's can have eyepieces high in the air when pointed upwards, while refractors can have you grovelling on the ground when pointed to objects overhead.  Big scopes are heavy and may need help to set up and some are not portable at all.

I used the Vixen for several years, but it was difficult to manoeuvre a long tube around on a German equatorial mount and finding other sky objets was sometimes tricky.  Especially if you were not sure of the correct location of the object.  One old trick is to use a pair of binoculars to search the general area of sky for the object, even if it's below the magnitude limit of the binoculars.  Finding the right patch of sky an object is in from your star charts is a great help.  Then you can use the scope to zoom onto the object, after knowing its position relative to the other stars in its neighbourhood.  Some of the local stars around the object will form patterns you will see in your star atlas, most times the object can then be found easily with the scope.

I did have an electric battery powered RA drive on the Vixen and it was excellent, very quite, very smooth for tracking the stars and planets; but it wouldn't go at the Lunar speed which is slower than star speed!  Very annoying!   The Moon orbits the Earth in 29½ days moving through constellations in a couple of days and its own diameter in about 100 minutes.  So if you where trying to draw a particular crater or feature on the Moon, it was necessary to keep slowing down the drive with the hand controller to let the Moon catchup' with the drive.  If you intend to do a lot of Lunar observing, it's a good idea to make sure the drive will run at the correct Lunar speed, I only found out later after I was using it that the drive only ran at the sidereal rate, plus its two faster slew speeds.

One of the hardest questions a beginner has to answer with a new scope is, what are you going to use it for?  If you know the answer before you buy, so much the better.  You have a much better chance of geting the right type of scope you need for the job in hand.  But for most first timers and for most beginners, it is better to have as much versatility as possible.  Beginners want to try to see everything.  And why not?  It does not matter if the telescope is new or second hand, (providing it performs well) and it does not matter what size it is so long as you are happy with it and it shows you the parts of the sky you want to see.

Buying a second hand telescope can be a worth while money saver.  Telescopes don't deprecate with the same same speed as cars, so a telescope 5 or more years old would cost about half the new value depending on the type and make.  Looking after your equipment will save you money in repairs and it will save you money when you do decide to sell by commanding a higher price.  Not many people will buy a telescope that looks like its had a hard rough life with dents in the tube and marks on the mirror or lens.

If you are buying secondhand always try to see and use the telescope before you buy.  If the seller doesn't want you to try it out, walk away. But if the seller is at the other end of the country is it a risk?  This is most unlikely as I found when I sold my Vixen from an ad. in Astronomy Now's classified section.  I had around 20 to 25 phone calls from all over the UK over a two month period.  I sold it within two days of the Astronomy Now magazine hitting the shops, but calls continued to came from folk as far away as Northern Ireland and northern England and I talked to some very interesting people who trusted my description of the state of the scope and made offers.  In the end it was sold on trust and parcelled up in the original packing and sent by mail.

Selling; the lessens are: (1), keep the original box and packing if possible, it is much easier to send all the components in the original makers box made for the job, than make one up yourself, (2), try to keep the scope clean and free of damage if possible, no scratches and dents, (3), don't modify it, (4), ask a sensibly price, (5),  tell customers exactly what they are getting and how much you want.

If you are buying secondhand do remember, (a), its not new, someone has used it,  (b), make sure you know what make and modal of telescope it is you are talking about,  (c), know how much it cost when new,  (d), how old it is and how much use has it had,  (e), if you can go to see it, do so, (f), make sure that the instruction manual is available.  Finally (g), make sure you get the magazine FIRST hot off the press and ring immediately you see the instrument you want or you will be to late for that bargain.  People are always buying and selling telescopes so don't buy the first one you find, buy the one you want.

Any book on astronomy for beginners will have chapters on all the different types and sizes of telescope and what you can be expected to see through them.  Read them and understand the differences in the different makes and models so that you don't make an expensive mistake.  If you are getting a book to read about telescopes, make sure it's up-to-date.  The change in telescope design over recent years is astonishing, new scopes seem to appear in every monthly issue of Astronomy Now and similar magazines.

One issue that is not covered very well is eyepieces, or rather which to buy.  By that, I don't mean which make or price but what will give you a good range of magnification.  What you want is a wide range of powers from low to high depending on what type of scope you have and what you want to look at.

Eyepieces are rated by their focal length, the smaller the number the more they magnify, and the larger the number the greater the field of view.  So a 10mm eyepiece will magnify more than a 25mm one.  Most eyepieces are made 1¼ inch in diameter and will fit into almost all telescopes.

Lets look at a general range of magnifications for most purposes.  A small telescope like mine, both my old Vixen and the new Meade have a similar focal length, even though they are very different in design.  The 4 inch Vixen was of 1,000mm focal length and the smaller Meade ETX-90 has a 1,250mm focal length.  Telescopes such as 6 inch Newtonians have similar focal lengths while a 8 inch may have a focal length of 2 meters.  Bigger mirrors will have longer focal lengths of cause.  The Meade comes with a 26mm eyepiece as standard which gives a magnification of x48, (1250 ÷ 26 = 48).  Great for looking at the Moon, star clusters and nebula, hopeless for Jupiter and Saturn and double stars.

So if you are a planet hunter or need to see the Lunar Rills or split close double stars you need magnification, sometimes lots of it!  A range of good eyepieces can cost as much as the telescope, so some care is needed in purchasing the ones you really need rather than to try to cover all eventualities.

One way to cut the number and cut the cost is to buy a Barlow lens.  A Barlow lens is one accessary worth buying from the start.  It is a tube with a negative lens at the bottom and drops into the telescope eyepiece opening, before fitting the eyepiece in after it.  The pair stick out of the eyepiece holder some way but do increase the magnification.  These come in various powers and most now seem to be x2 or x3 power, doubling or trebling the magnification.  So a set of three eyepieces and a x2 Barlow will give, with a bit of care, a full range of six magnifications.  Thus saving buying two eyepieces.  While a x2 will double up the power of an eyepiece, but with some loss of brightness, a x3 will drop the illumination even more.  If all eyepieces are from the same manufacture, swapping the eyepiece in the Barlow should not shift the focus to much.

It pays to buy good quality camera lenses for serious photography and astronomical eyepieces are no exception.  Make sure all the glass surfaces are coated, inside and out.  Multi coatings are best.  If you have a good quality mirror in the scope, you must use quality eyepieces to complement it.  If you don't, why did you spend the money on a good mirror in the first place?  Most of the major companies now make excellent eyepieces, some costing hundreds of pounds for the extreme wide angle versions.  But don't worry.  I'm not saying you've got to buy them!

In the graph at the top is a suggestion for a set for a small scope.  If it comes with a 26mm eyepiece then the magnification for a 1,250mm focal length telescope will be x48 as we have seen.  Add a x2 Barlow and you get x96.  The Moon now looks only 2,500 miles away!!  A purchase of a 9.7mm eyepiece will give x129 and with the Barlow x253.  Now you are flying over the Moon at less than 1,000 miles!!!  So two eyepieces and a Barlow give you four separate magnifications, nicely spaced across the main observing range of this size of scope.  Buy a 15mm eyepiece and you get x83 and x166 with the Barlow, 6 different magnifications for the price of 4!!  Each magnification different and spaced so you have a range of magnifications depending on the subject and of cause, the weather.

If you intend to buy a set of eyepieces with your scope then you can work out a range for the particular focal length of your scope and try to have as even a range as possible.  But if you are more likely to buy eyepieces one at a time, more care is needed.  For while the magnification for a given eyepiece is different from the others in your collection, adding a Barlow can make the magnification the same as one you already have.  This wastes the original eyepiece by giving just one extra magnification instead of two.

Other eyepieces can fill in the gapes such as a 6.4mm giving x195 and x390.  I would not recommend using this magnification on smaller scopes, the image will be very dim.  This list of eyepieces are from the Meade 4000 series range and cover most of the average amateurs working range.  A few other manufactures I looked at make similar ranges from about 5mm to 30mm.  Then the price starts to rise rapidly for the exotic types with extreme wide angle views to take in large tracks of sky. Take care with the purchase and you can be the proud owner of a carefully selected range of eyepieces with a smooth step range from around x40 to over x250.  This will cover almost every requirement.

The set of eyepieces I use now, I first bought when I had the Vixen many years ago; I kept them when I sold the telescope.  I couldn't see the point of selling them with the telescope and having to buy another set again, the new scope was so similar in focal length to the old, I would have been buying a very similar set which would have cost a great deal more now.  Eyepieces don't ware out by looking through them and if kept properly will last a lifetime (I hope).

Don't forget that the quality of the eyepiece will effect your enjoyment of the sky.  Good eyepieces give sharp pinpoint stars to the edge of the field, if the stars near the edge are starting to get long and thin or soft and squashy, your eyepiece may not be up to it and adding a Barlow will only make things worse.

Don't be temped to pile on too much magnification, it won't work.  On a small scope like the ETX-90 or the 4 icnh Vixen and 6 inch Newton, x300 is about the absolute maximum you can use, and then very rarely.  Don't forget that the more you increase the magnification the dimmer the image will get.  The old rule of using a maximum x50 for every inch of aperture is still valid today!  Not only do you need more aperture to get more magnification but you need very little atmosphere turbulence either.  Most nights you are lucky to get over x150 to x200 before the seeing degrades the image to much and it is better to use a lower power eyepiece.  Low power eyepieces can give great views of clusters and nebula but don't forget it may be better to use a pair of binoculars on extended objects and with binoculars you are using both eyes so increasing the amount of light going into your brain.
So what are you waiting for?  Apart from clear weather?  Get out there and observe the heavens, write up your observations and send them into MIRA so we can all share them.

And how am I getting on with the little Meade?  Well it is much easier to carry around and set up than the Vixen.  It is light and it all fits into a small airline size carry-on bag (apart from the tripod).  You can buy a special tripod top plate which fits onto your tripod and the base of the Meade screws onto.  You can use this telescope in equatorial mode, or use it without a tripod on a sturdy table top in altazimuth mode with the computer finding and tracking stars, all you do is tell it what you want to look at and it will go and find it!!  Magic.  If it's on a tripod and polar aligned it will track stars or the Moon with the hand controller and it's an equatorial scope.
Do make sure the table or telescope support is firm and solid, if it is not firm enough to stop and dampen down all the nudges and bumps it gets through normal handling, it will vibrate the scope and the view will be a mass of blurred light.

The Meade has superb optics with a clever hand controller which governs speed and slew rates and with the addition of the AutoStar computer, which is only a little larger than the hand controller, will find 14,000 objects in the sky!!!  Its down side is its lightweight plastic base and poor drive and lock mechanism.  If only a few more dollars had been spent on the motors and drive train . . .  I suppose it would rapidly add to the cost if the engineering in the base and arms was made stronger and smoother.  As it is, it works fine with control over both the Dec and RA on either the hand control or the AutoStar hand computer controller, but it feels as if you need to be careful with it.  Altogether this is an amazing piece of kit and it's no wonder it's the best selling scope in the world.

So whatever telescope you are using I hope it gives you much to wonder at and you get clear skies this winter.









ATTRACTION OF GRAVITY

 By Paritosh Maulik
Like minded people or unlike charges attract each other, but the gravity does not discriminate - it attracts everything.  It is one of the fundamental forces.  Although it is very week, it has strong influence on everything around us.  It is holding the universe together.  In this article we shall discuss some of the properties and effects of gravity.
 

Both Velocity and Gravity can Change the Wavelength
We all know that the change of the siren of an ambulance is due to Doppler shift.  The velocity of the sound wave in air is of the order of 340 m/sec and that of the ambulance is 10 - 15 m/sec (40 - 45 km/h).  These two velocities are reasonably close and we can hear the effect of change in frequency or wavelength with velocity.  The velocity of the flashing blue light (a million times faster), on the other hand, is too high compared to the velocity of the ambulance to have any effect, that is, no change in its colour.

By 1920's it became clear to astronomers that optical spectral lines are shifted to a longer wavelength for a majority of stellar objects, but for a minority of these, the wavelengths are shifted to shorter values.  The only obvious explanation was, these are moving so fast, that like the siren of the ambulance, the frequency of light from these objects is changing.  If the observed wavelength is 5050Å from the rest value of 5000Å then the redshift is z = 5/500.  Now if we multiply z by the velocity of light 3x105 kms, the velocity of the object works out to be 3x103 kms. This is the Doppler redshift, i.e. slowing down of light (change in frequency and not the velocity) due to velocity of the source.  Conversely the decrease in wavelength, the objects and the observer are approaching closer and it is called blue shift.

For a wave, the velocity, frequency and  energy are interrelated.  Light can be treated as either particles, a photon, or as a wave.  As light particles leave a massive object, they will have to overcome the high gravitational drag; these will looses energy or slow down.  So the effect of gravity on the light appears as a change in the frequency rather than the effect on the velocity of light, which is constant.  Light particles cannot slow down, so light looses energy; a light with lower energy is light with longer wavelength  i.e.  redshifted.

Quasars are massive  objects, about 109 solar mass, light leaving such a heavy object can suffer loss of energy or gravitational redshift.  Thus, we can see that the change in the behaviour of  light can either be due to velocity or gravity.  We shall come back to it later.

Gravity and Acceleration are two sides of the same Coin
Let us imagine we are travelling in a big comfortable car, well insulated from the outside reality.  Suddenly we feel a drag, pulled back into the seat.  We think the car has suddenly accelerated, but what if we perhaps just happened to be passing by a heavy object and it is dragging us backwards.  We can say, the pull of gravity and acceleration have same effect.  This is the nub of general theory of relativity.


When we throw a stone up in the air, we accelerate it, up it goes and then it slows down, gravity pulling it back and falls to the ground. The trajectory it takes is a parabola ABC, Fig 1.  Since gravity is omnipresent, the stone is under its influence throughout its journey.  We can say that if the gravity was not there the stone would have taken a straight line AD.  In real life such a situation can not exist; the reality is, the path  taken by the stone is the shortest distance between the thrower and point of fall, and this path is a straight line drawn on a curved surface. Such a line is called geodesic.  In other words, because of gravity, the spacetime gets warped.  Other factors remaining the same, the higher the gravity, the higher is the curvature.

When we consider the travel of the stone as above, we have to consider not only the space in x, y, z axes, but time as another axis as well.  If the space gets bent/warped by gravity, then gravity should warp the time axis as well.  We can treat light both as;  i),  a particle, (for example, the photoelectric effect: when particles of light hit surfaces, the surface can give off electrons), and ii),  as a wave (for example, interference of light).

If light passes close to a heavy object, from the Newtonian gravitation theory, we can imagine light particles being attracted by the object.  Before Einstein, someone else worked out a method to calculate, the attraction of light by an object according to Newtonian model.  Einstein recalculated the sum, albeit with the help of his colleagues invoking non-linear (non-Euclidean) geometry to take into account the bending of space by the gravity of the object.  The effect of bending of space alone were similar, both according to Einstein and Newton.

However, when the effect of warping due to time was introduced in the calculation of bending of light by gravity, the bending according to Einstein was almost twice compared to the calculated according to Newton.  Within a few years of this calculation, Eddington confirmed the bending of light (like a lens bends light and hence gravitational lensing) during a total solar eclipse.  Now we know that the experimental scatter was higher than the effect they were attempting to measure, for example the bending of the star light due to refraction as it grazes past the Sun was not considered.  However the science was young, and the general theory of relativity was vindicated.

Einstein's work gave the theoretical justification of Newton's formula of gravitational force, proportional the product of masses but inversely proportional to the distance squared.  It modified the Newtonian theory.


How Long does it take to be Hit by Gravity?
We have already discussed the bending of spacetime.  Now let us see how long it takes to feel the effect of gravity.  We all know that, if the Sun was to disappear now, after about 8 minutes, we shall notice the darkness.  According to the Newtonian model, once the Sun ceases to exist, we shall be free from the gravitational pull of the Sun.  Instead of going round the Sun we shall fly off in a straight line, however we shall continue to see the Sun for 8 minutes.  In the real world the Sun cannot disappear in an instant, because of the conservation of mass and energy.  So if the Sun is to have a close encounter, we shall feel something strange happening for 8 minutes before observing any change in the appearance of the Sun.

Another important point to remember in gravity is, it is present everywhere, in a real universe; it is full of matter and radiation, these can exert gravitational attraction.

The Strange Behaviour of Gravity
Gravity as a force is different from other forces.  Imagine two balls connected by a spring.  When the spring is stretched, the energy of the system goes up, but if we help to reduce the tension in the spring by bringing the balls closer, the energy of the system goes down.  In other words, if we help the force in the direction in which it is acting (the spring trying to pull the balls closer), the energy of the system reduces. But now imagine, these two balls are not connected by spring.  The force acting on these is their mutual attraction of gravity; as we bring these two balls together, the distance between them closes and the force of gravitational attraction acting on the balls increases i.e. the energy of the system increases.  Gravity is the only one force, in which if we help in the direction of the force, the energy increases.

The repulsive force between two positive charged protons in an atom are very strong, but these are held in position by electrons maintaining an electrical equilibrium.  Two protons, however, as particles, are mutually attracted to each other due to gravity.  Gravity is a weak force compared to the electric force holding the proton - electron together.  The net electric force in an atom = 1036 x gravitational attraction between two protons.  This has interesting consequences.  Most of the objects we see around us are electrically neutral.  So, as the number of protons and electrons goes up, the mass increases and the electrical charge increases with it; the net force will stay neutral. But the gravitational force increases with mass and eventually, if the mass is too large, the gravitational force takes over.  Just imagine, to make the apple touch the ground, we have to summon the combined attractive forces of all the atoms of the Earth.

The mutual attraction between the atoms of sand grains are negligible compared to the attraction of the entire Earth.  The mass of asteroids and small moons like Phobos and Deimos of Mars are small; the mutual attraction between the gains are too small to hold a rigid shape.  But as the mass increases, the gravitational attraction between the constituents increases and the objects get rounder.  Even our Earth and Moon are not rigid enough to combat the attractive pull of the constituent grains and become round in shape.  Once we get to masses above Jupiter the gravitational attraction may not be stoppable, the consequence being the object becoming smaller and smaller.  We better stop here, we are veering toward black holes.  In short, if the mass is small, gravity has small effect, but once the mass goes up, gravity is the king.


Looked at differently, if say in another universe the net electric force in an atom is say = 1030 x gravitational attraction between two protons, this would mean that the gravity in this universe is somewhat stronger.  We do not need that much mass to keep things together and the objects in this universe such as planets and galaxies would be smaller.  In this world we would be crushed by the high gravity, if we do not change our shape!

Time to Collapse
 If the role of gravity is to attract each object, what about one part of an object attracting another part?  It does happen and in this way the object can get smaller and smaller.  Theoretically we define an object as dust, which can be compressed to a smaller size and when compressed these dusts do not offer any resistance.  Let us imagine such a dust, similar in size to our Sun; if it is to shrink on itself, it would take less than 30 minute to reduce its size to zero.  At the beginning, it will reduce its size slowly, but soon it is down hill all the way.  This is a catastrophic collapse.  But in reality, once object gets smaller, one portion begin to collide with each other, and this creates heat, heat causes expansion and prevents further collapse.  This happens in stars.

Hiding the Information
There are two aspects to consider, bending of spacetime and the escape velocity. Spacetime is bent around an object by its gravity.  Every object can bend spacetime around itself such that it gets isolated from its the surroundings.  The area over which this effect occurs depends on the mass of the object.  For example, if this is to happen with the Sun, it has to be squeezed into a size of about 3 km radius and for the Earth it is a large pea about 8 mm across.  This is called the Schwartzschild radius.  R = 2 x G x M ÷ c2, where M = mass of the object; G = gravitational constant and c = speed of light.

Consider the case of a rocket is fired from the Earth, the energy on the rocket due to gravitation, E = G x M x m ÷ r, where G is a constant, M is the mass of the Earth, m is the mass of the rocket and r is the distance between the rocket and the centre of the Earth (we can assume this to be  approximately equal to the radius of the Earth).  If the velocity of the rocket is v, its energy is E = ½ x m x v2.

The rocket can only escape, if its energy is at least the same or greater than the gravitational energy.  By solving these two equations we can work out the minimum velocity needed by the rocket to take off and escape.  Thus, v works out to be v = √(2G x M ÷ r).  This equation tells us that higher is the gravity, higher the escape velocity has to be for the rocket to break free from the gravity.  For the Earth, the escape  velocity, works out to be just over 11 km/sec.  If we find an object, with a high mass (and hence G), such that a rocket needs an escape velocity equal or greater than the velocity of light, 300,000 km/sec, means we cannot get any information, via the fastest means available to us, back.  It would be a dark patch, a black hole.

Consider two astronomical objects A and B; A is stationary with respect to B and B is moving towards an object with high gravity.  A and B are in communication with each other and after a certain point, close to the heavy object, the gravitational effect on B becomes so much that in order to overcome the  gravity, and reach A, the message sent by B has to be at a speed greater than the velocity of light.  A would not receive any signal from B.  The boundary B has crossed is called the event horizon.  This term has been borrowed from the Earth's horizon.  If a ship moves far away from the light house it can not see the lighthouse because of the curvature of the Earth.


Now let us imagine B has not crossed the event horizon yet; still it can communicate with A.  B is however under a strong gravitational influence; it communicates with A by light.  B has a light source of wave length λ.  If A can detect the light and confirms that wave length is λ, all is well.  As we have seen with the case of throwing a stone, that space is curved due to gravity, then light although travelling in a straight line (albeit drawn on a curved space) will take longer to reach A; hence the time dilation or slowing down of time due to gravity.  Now as the light of wavelength λ, leaves the high gravity field, it looses its energy.  The light leaving B will be of lower energy i.e. with a lower frequency as detected by A.  The wavelength, frequency and velocity are related as frequency x wavelength = velocity, or ν x λ = c.

Since the velocity of light is constant, if the frequency drops, the wavelength will go up, that is the spectrum would shift towards the lower wavelength end of the spectrum.  This is redshift, and since it was caused by the gravitational effect, it is gravitational redshift.

The reverse can also happen. If B receives a signal, in our case a beam of light, from A, B will see it as a shorter wavelength, as if the beam is being attracted towards the high gravity field; this is blue shift.  Jets from certain objects appear to travel at a speed greater than light.  It is because, the jet is travelling towards us and this makes it appear faster than light.  But it could also be due to Doppler Shift, that is, the observed effect is due to velocity rather than gravity.

Gravitational Lensing and Einstein's Ring
Now that we have seen that bending of light is not just a figment of imagination, but is real and can have some unusual effects on observations.  A galaxy lying in the path of a distant source and the Earth can cause the light to bend so we can see multiple images of the source.  These images lies in a ring, called Einstein's ring and depending on relative positions, the observer can observes a multiple of images of the source and a ring or part of a ring. Such images have been seen, both as optical and radio images.


Parting of the Astronomical Objects
Sometimes the images may suggest that the objects are moving at a rate faster than light.  This is not permissible.  One possible explanation is as follows; these sources are moving apart, but the gravitational lensing may appear to make these two sources look to be farther apart and if the conditions are correct, these object may appear to move at a rate faster than light, this is called superluminal motion.

Gravitational Wave
In a binary star system, as two stars go round each other, the spacetime around these objects gets disturbed, this causes a loss in the  energy of the star system, the stars come close to each other and the angular speed increases and the orbital period around each other decreases.  This has been verified with pulsars.  The relativistic calculation predict correct results than the simple gravitational theory.

The effect of gravitational radiation, propagating as wave, has not yet been detected in the laboratory.  The reason is the magnitude is very small and the experimental sensitivity required is very high.  However it has not been abandoned; laser interferometer several kilometre long is being tried.  If a gravitational wave strike the detector, it will cause a change in the path length of the beam and therefore the interference pattern.  The interest has been renewed with the advancement of technology.  It is now being proposed that this interference set up can be mounted on spacecraft separated by a much wider distance.  One of the reasons the astronomers want to detect gravity waves is currently we have only the electromagnetic waves to study the processes going in astronomical bodies.  These electromagnetic waves range from radio to X-ray and gamma- rays.  But the problem with the electromagnetic waves is these scatter, which blurs the information, but gravity waves, if exists, should not scatter and therefore may provide a new tool to study astronomical processes. (See MIRA 40, Pam Draper, Summer 1996, MIRA 43, Winter, Mike Frost, Autumn 1997, for some thoughts on gravity.)

Does the Gravitational Constant Change with Time?
Some cosmological models predict change of G, the gravitational constant to change with time. If we assume the Earth to be at rest then there are two forces of equal and opposite magnitude acting on the Earth:
1) The gravitational attraction of the Sun, and this force is balanced by
2)  the attraction of rest of the universe.
This second force depends on the density of the rest of the matter of the universe and the distance of the universe.  Density of the rest of the matter depends on the expansion of the universe.  According to the Newtonian and the relativistic models the G is constant, but if the universe expands we can expect the G to decrease with time.  If we carry out the sums, it appears that in fact the G can be expected to change a few parts in 109 in say about 100 years.

Attempts have been made to examine if this has caused any change in the Earth - Moon rotation.  The argument being, if the G is low, the influence of the Earth on the Moon will decrease, the Moon will move away and its angular velocity would decrease.  One of the problems in carrying out experimental verification is that the gravitational pull causes change in the shape of the Earth and the Moon; this in tern causes a change in attraction.  The change in the shape of the Earth due to gravitational attraction causes the tides (Mike Frost, MIRA 45, Summer 1998)


The gravity slows down the ever expanding universe.  So the universe of today is smaller than what it would have been if the gravity was not there.  Then tomorrow, the universe will expand that little less.  This effect is cumulative, thus the gravity is keeping the universe younger.

The decrease in the gravitational constant has  interesting consequences in the Sun - Earth relationship.  It has been shown theoretically that the temperature of a star depends on the gravitational constant, the higher the value of G, the higher is the temperature.  So if the G was high the Sun would be hotter and if the Earth was also too close to the Sun — too hot to begin life.  As the G reduced, the temperature of the Sun dropped.  The Earth under a lower gravitational effect was allowed to expand, triggered off the continental drift and as they say, the rest is history.
 

Further reading

The Lighter Side of Gravity,   Narlikar, J V.   Cambridge University Press, 1996
Just Six Numbers,   Rees, M.    Weidenfeld and Nicolson,  1999




  

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