The earth as a planet - Carl Sagan's 1977 Christmas Lectures 1/6

he [Music] [Music] good afternoon I'm very pleased to be here at the Royal Institution to give this set of Christmas lectures on the planets this is 150 years after Michael Faraday gave his first Christmas lecture and there is a distinguished scientific Tradition at the institution which I'm very pleased to play a small part of my topic is the planets we of course live on one it's called the Earth and it is a tiny little planet made of rock and metal with a tiny thin atmosphere around it clouds water rocks mountains and life and one point

of interest is to what extent is this the case on other planets to what extent are things there different from here this machine is called an A and uh it is a kind of mechanical device to show how the planets move uh there were lots of them made a few centuries ago not so many these days the brass sphere in the middle is supposed to represent the Sun and if I turn this crank you can see the planets moving the innermost planets moving very fast the outermost planets moving extremely slowly the fastest moving one here

is Mercury the next one is Venus this one here is the Earth with the moon next to it then Mars Jupiter with four of its 12 or so moons shown and then over here Saturn with its rings and five of its 10 or so moons shown Beyond Saturn except it would be too big to fit on this aary are the planets Uranus Neptune and Pluto and uh perhaps even some others there's been a recent discovery of a tiny little planet out here uh between Saturn and Urus Perhaps it is a single small planet or perhaps

it is one of a great horde of little planets which might Circle the Sun out here there is a horde of small objects which circles the sun between uh Mars and Jupiter called The Asteroids we are just beginning to explore the planets we are only in the earliest stages of understanding them and I would like to begin by giving you a sort of uh Bird's eyee view of some of them the first picture here is intended for orientation uh we're on this third one and you can see what a tiny and insignificant Planet it is

and uh the solar system is mainly these large planets in the outer solar system we are on the debris the little tiny chunks of incidental matter in the inside of the solar system now people have long dreamt of traveling from one world to another there are various methods of doing it uh this is one suggested by a French artist named Grand V in the last century uh it's a lovely way to do it if only we could figure out how to build the bridge but uh we haven't figured that out and our method is quite

different it's to send spacecraft from the Earth to the other planets and the other objects in the solar system are really quite nice this is a photog PHH of the Moon you can see the dark smooth lowland regions the bright this is uh this is music for this occasion uh it is Japanese shakuhachi music and why we're using this music I will say in just a moment the bright rough areas of the Moon are Highlands and you can see that there are holes or craters which pit the moon and they are produced by the impacts

of large objects falling from space throughout geological time the solar system is very old 4 and a half billion years old and the scars of early catastrophes still exist on the lunar surface a close-up view of the lunar surface shows not just craters but also this remarkable Wiggly sinuous r whose origin is still being debated there are many craters here is a closeup of one and we will in a later lecture not only talk about craters we will make some in a generally non-destructive way and we will not not harm too many people as we

make a crater this is a very large crater the one you just saw we will make small ones later there are craters not just on the moon here is a mosaic of photographs of Mercury and uh this this detail was not known until very recently as a result of the Mariner 10 spacecraft to Mercury and you can see a great deal of detail but again including craters if we move outward from the earth we come to the planet Mars which we see here in a lovely Viking photograph and here the big craters that you see

are not made by impact by Collision these are volcanic craters Mars has among many most interesting features extraordinarily large volcanoes the largest of which is almost 30 kilm High 880,000 ft high that is a very large volcano Mars is a lovely beautiful interesting World in some ways very much like the Earth in some ways extraordinarily different and we will devote a few lectures later on to studying Mars Beyond Mars we leave the terrestrial or earthlike planets and come to the first of the great gas giants Jupiter everything we're looking at here is clouds and atmosphere

if there is a surface to Jupiter it is far below the region we can see the weather or meteorology on Jupiter is very different from the earth very interesting will teach us much as the studying of all the planets will do about our own planet Jupiter has a number of moons what you are seeing now are the very best Pioneer 11 photographs of one of the moons of Jupiter we can see almost nothing at all and this is the best picture we have at the present time in 1979 two spacecraft called voy will pass very

close to the moons of Jupiter and we will obtain 50,000 photographs of Jupiter Saturn and their 20 some odd moons and we will have turned our almost complete ignorance about these moons into great knowledge these Moons by the way are not like our own some of them are icy some of them are covered with sodium and other salts uh as if there was a great o which lost was lost into space these are very strange places and they will be very interesting to explore Beyond Jupiter and its moons is Saturn the great ringed planet the

nature of the Rings is something we'll say a word about later The Voyage of spacecraft will also pass by Saturn and its rings it will in fact then pass perhaps by Uranus if it is still working and will inexorably leave the solar system it will it is condemned to wander forever in the dark between the Stars its transmitters will eventually die and it will be silent but still the most distant Emissary of mankind we thought that in the remote chance that there are other beings on planets of other stars who wander between the stars and

capture derel spacecraft from backward civilizations like ours that it might be nice to send a message and so there are phonograph records on each of the two Voyage of spacecraft with instructions for use written not in English since it's unlikely they speak English and uh those records contain some spoken and written greetings great many photographs scientific information sounds of the earth uh some greetings from whales as well as people and uh an hour and a half of the music of the world among which was the Japanese shakuhachi piece that you just heard so we are

in a very uh preliminary way trying to say hello to the cosmos now on the outskirts of the solar system in fact halfway between here and the nearest star are the Comets they are the closest thing to Interstellar matter that we are likely to uh be able to examine in the near future they're an almost complete mystery and uh another major objective of space exploration now this picture of the solar system uh is relatively recent for most of human history people thought that the Earth was at the center not just of the solar system but

of the universe and that not only all the planets went around the earth once every day the Earth was not imagined to rotate but also the sun went around the earth and that was a kind of astronomical egotism in which we thought we were Splendid and uh all the other objects in the universe somehow secondary to the earth and its inhabitants well what we have found is just the opposite that we are a very humdrum and ordinary local in the universe and one way in which this was found was to consider the question of the

phases of the planets now we know that the moon goes through phases we see full moon Half Moon quarter moon Crescent Moon new moon but what about the planets now in the old Earth centered view thank you the planets in this case Venus were imagined there's a lovely old book which has moving Parts unlike books today which only accidentally have moving Parts this one is moving on purpose Venus was imagined to go around the earth but also to have a little internal motion of its own and that view with the Sun out Beyond Venus made

a certain prediction about the phases of Venus you would sometimes see partly illuminated Venus sometimes see a full illuminated Venus but no one had a telescope to see whether Venus went through phases and if so what kind now here is the Earth centered view of the universe with the Earth here and the planets and the Sun going around the earth this is the supposed orbit of the Sun and in the sun centered view of the universe the planets including the Earth go around the Sun and as this old picture shows go through phases well in

the sun centered view or hypothesis let's try and see what the phases ought to be like what I'm going to do is to have someone be Earth and someone be Venus and I will try to be the Sun and uh Ju Just because somebody big has to operate the light and um let's see what the phases of Venus would be like so could we have a an earth person thank you this is an excellent Earth person and could you please stand by let's say a few steps up by that camera and we will we will

be eventually viewing things on the television screen from the point of view of that camera and that Earth person and now could we have a Venus person Venus is a planet completely covered with clouds although it's a very nasty place on its surface and so we have this completely clean sphere representing Venus and could could you just stay there for just one moment and what I'm going to do is uh point this light representing the Sun at Venus now if this were a real Sun it would be shining in all directions of course but then

everybody in the audience would be Blinded By the Light from this and you couldn't see what was happening here so I'm going to Simply follow the motion of Venus around the Sun as seen from the earth now the Earth really ought to be moving around the Sun Also but that would be a little awkward and the Earth would have to step over people's knees and so on so we're having the Earth stationary but the point will still be made so now Venus could you please slowly Circle the sun uh you can do it a little

faster than that actually it takes Venus 225 days to go around but we will do it somewhat faster than that and now notice how we are going into half moon or half Venus Crescent Venus and now Venus is passing by the Sun and now the Crescent comes around again and Venus is now nicely walking backwards into a half Venus and now growing the phase grows and soon we will be back near full Venus thank you very much m Venus thank you very much Miss Earth now that kind of motion is exactly what Galileo found Venus

to do when he looked at it with the first telescope uh several hundred years ago and Galileo found that the waxing and waning the changing from Crescent to full back to Crescent of Venus did exactly what the sun centered hypothesis predicted and not at all what the Earth centered hypothesis predicted that's one of the lines of evidence that the Earth is not the center now if we actually uh look at uh a picture of Venus we see that uh it does go through phases this is a telescopic view of Venus and I just show you

this so you will believe me if I say Venus goes through phases actually uh the Earth also goes through phases and uh in this picture is a lovely picture of the Earth as seen from the Moon the Moon is in the foreground and the Crescent above the Moon is the Earth as seen from the Moon and it goes through phases if you figure it out for just the same reason that Venus or the moon does as seen from the earth now these are the this is the geometry of the solar system but we haven't said

anything about scale how big is it the solar system there are lots of ways of finding out the simplest of which is by radar you send a radar pulse to a planet it hits the planet comes back you measure how long it took you know how fast radio or light travels travels at the speed of light and uh right that was easy 186,000 m a second 300,000 kilm a second and so half of the transit time gives you the distance and in this way we can determine for example that uh the Earth is uh about

150 million kilometers 93 million miles from um the sun uh I hear I hear an echo which we're not supposed to hear quite yet um and what we'd like to do is to give a sense of the scale of the solar system by having someone in the audience uh say something like uh let's say Hello Universe seems an innocuous enough remark um and let us see how long it takes for that remark to get to various places starting with the moon um who would like to say hello to the universe uh that's good lots of

people would like to say hello to the universe so so would we uh that's why we sent the Voyager record could you please say Hello Universe Hello Universe Hello Universe now that was a one and a qu second time delay which is by accident just how long it takes for light or radio to travel from the earth to the moon now we have here a diagram which has just been uh started out um to indicate the rate of progress of a signal sent by radio from the earth into space so here's the orbit of the

earth and we will see how long it takes for that signal to get to the orbits of the other planets Venus Mercury the two interior planets Mars the next planet out here's the Sun and Jupiter and Saturn will be out here and U so in the next few minutes we will follow the progress of a radio signal and this is the fastest than anything can travel so what we are going to see is the fastest travel time that is possible now what I would like to do now is to while we're waiting for the light

to get to other planets to begin a discussion of a particular planet we would like to study in some depth a strange and exotic World let us imagine that we know nothing about this world we have no prior information about it and we approach it with absolutely open minds we have no prejudices about whether it's inhabited or not whether there are volcanoes or rivers or if there are inhabitants what they look like maybe they are all squamous blue ooids with 30 tentacles and Float one meter above the ground or perhaps that's what we look like

um perhaps we're from some other planet let us say and uh we will examine this distant and exotic World which by accident is the Earth and uh first we will look at it from telescopes from our world and it will be little we will not see much detail and then we will come closer and closer and look with ah we just heard another Hello Universe because our radio signal has just gotten to Venus and we will shortly be interrupted by another Hello Universe when it gets to Mars it's just keeping reminding us of the progress

of our radio message we will look mostly with photographs with first from our planet as I said then we will put cameras on imaginary spacecraft which will fly by the Earth then go into orbit about the Earth and then land on the earth and look around and see whatever is to be seen now in addition to looking we can use scientific instruments to determine other factors about a planet for example we could pass the sunlight reflected off the planet through a device called a spectroscope which divides the light up into individual colors or wavelengths and

if there are colors or wavelengths missing subtracted by the passage of sunlight through the planetary atmosphere we can by appropriate laboratory comparisons determine something of the chemical composition of the atmosphere of the earth and we would find that it was composed of nitrogen and a rare and poisonous gas called oxygen um some carbon dioxide some nitrogen small amounts of water vapor ozone some other gases all of that could be determined remotely without ever going there hello it has just reached Mars uh another thing we could do we know how far the Earth is from the

Sun we know how much sunlight is absorbed by the Earth we could calculate how hot the Earth ought to be from the sunlight which reaches it we could test that idea by uh putting a heat sensitive device a thermocouple say at the focus of a large telescope and see how much infrared or heat radiation the Earth gives off to space and in that way we could begin to get some idea of the physics and chemistry of the Earth's environment well I'm going to now begin this exercise and I would ask you oh yes Mercury it's

just reached Mercury is it yes and I would ask you to pretend that you do not come from the earth you are from somewhere else you don't know about the Earth you wish to find out well here's our first view it's not very much hard to tell much about the strange planet Earth from this view we see a half Earth we already know it goes through phases we see white with some patches it is in fact the clouds of the earth that we see the Earth is on the average about half Cloud covered we obviously

have to have much better observations in order to determine more about the Earth occasionally we find the Earth in a strange local in this case just passing in front of the sun it makes for a very beautiful picture does not tell us much about the earth the earth is the completely dark thing uh in the foreground which is almost obscuring the Sun as we get closer to the Earth we can observe more detail here you can see the wispy lovely cloud patterns uh and you can see that on the average they do cover about half

the Earth we can see down at the bottom a uh the Antarctic ice cap we can see a continent underneath the clouds that is the clouds come and go but underneath them are features which don't seem to come and go the continents are the same every time they appear over the surface of the rotating Earth and there are also dark Regions near the continents which eventually we would discover were oceans and that would be very strange because nowhere else in the solar system at least today are there oceans and uh if we looked in the

extreme upper right hand portion of this picture we would see the Horn of Africa the Arabian Peninsula the Sinai Peninsula there would be no sign for example of tank battles in this region uh and the reason for that is that our resolution our ability to see find detail is only about 100 kilm in this picture we could not see anything smaller than 100 kilm across and therefore if we were to see a tank it would have to be 100 kilm long and that it's a very large tank which fortunately does not exist on the earth

so to examine more closely we must get better pictures and here we see a closeup of the same region this is the uh region of the Nile Valley and the ismaila canal you can see this beautiful uh Cloud which runs from lower left to upper right it is a jetstream Cloud you can see lots of wispy clouds at this point our meteorologists our experts in weather would start getting very interested our geologists our experts on the surface features of the planet would get very interested biologists would be dreadfully bored they would not see anything of

Interest no hint of the Earth being inhabited uh although they might argue that liquid water was good for Life there would be others who would dispute that here is a lovely picture of a cyclone which would make our meteorologists extremely happy they would study this picture for a long time here well there's a strange object in the foreground which I ask you to ignore that of course would indicate the presence of life um this is a Skylab spacecraft but again underneath it uh all sorts of little wispy clouds here is another incidental spacecraft in the

foreground this is gmany Again by the way look at the edge or limb of the earth you can see this little blue band following the uh Horizon and that is the atmosphere of the earth seen Edge on the U tall stick with a cylinder on top of it is the means by which uh astronauts communicate to the ground I sometimes like to think that it's an old tomato soup can connected to Houston Texas by a long string which you can see a little of the other end of the string is another can and uh they

talk to each other that way um the trouble is that the string gets wound around the earth many times and it becomes a little awkward uh that's not actually the way the communication is done it's done by radio that's the radio antenna it just looks like an old can in a string that's all a parenthesis I couldn't resist let's go back to our extraterrestrial guys here is a lovely volcano we are looking at the volcanic Calera the hole in the top of the volcano which you see this dark thing right there uh this is in

fact the island of Hawaii and is alleged to have a great deal of life on it but you can't see anything of it here all you can see is a volcano here's another volcano I think this is Etna and again much the same situation a beautiful mountain no sign of nearby life and here is another sign of geological activity uh on the left in color is the San Francisco Bay area and the uh great San Andreas fault coming down from the bay which uh is a sign of great GE iCal activity there are many earthquakes

there in fact eventually California will drift out into the Pacific Ocean because of a split or tear along that line now that will not happen soon uh in some people's minds regretfully um but will happen uh in the next few million years uh also in that picture on the right is another similar feature the Great East African Rift Valley which is beginning here at the Dead Sea and goes all the way down to South Africa it is 5,000 kilm or 3,000 Mi long and is a sign of great recent geological activity on the earth when

we look at Mars later we will find that there is a perhaps similar feature there here is a crater in North Africa Perhaps it is of impact origin perhaps not um such craters are formed on the earth as on the moon but the Earth has very efficient erosion wind and water rub things out and uh for that reason we have at any given time very few craters even though big rocks fall from the sky on the earth about as often as they do on say Mercury or the moon here is uh in the Arabian Peninsula

a set of lovely sea sand dunes which it would be one sign of evidence that uh the Earth Earth's atmosphere is active and stirs up material and makes sand storms uh once we knew something of the atmosphere of the earth we could deduce that uh indirectly as well as this direct evidence notice all this time there's not a hint of life we're looking at the Earth reasonably uh closely we're looking often at kilometer resolution we're seeing things a little more than half a mile across and yet in all these spectors up to now there hasn't

been a hint of anything alive nor in this picture you can see at left a lovely snow has happened in this mountainous region to the right you can see the mountainous region without snow these are in fact the Rocky Mountains in the United States in the vicinity of Denver Colorado here is uh part of the St Lawrence Seaway in Canada and you can see great floating blocks of ice breaking up uh this is one of many pictures that would indicate the presence not just in oceans but uh Inland of liquid water here is a beautiful

picture of ice choked sinuous rivers in the Arctic and uh this picture as many others show the Earth to be extraordinarily beautiful and I think long after our scientists got totally bored with the Earth our artists would continue to be interested in it here now is a uh closeup picture of a particular River you may see two numbers a five and a six on the earth they have not actually been written on the earth uh that's just been put on for another reason we should ignore that if we saw a large five on the surface

of the Earth that would suggest that there was life on the planet of course um especially if we used Arabic numerals but what we see is uh the sinuous tributary wandering feature which is a characteristic feature of flowing liquids and we can also look at the mouth of uh this River the Colorado River mouth is up here at the center and uh follow yes here's here's the mouth of the river uh and we can follow the entire course of the river by the way in this picture there is right within the box here a uh

sort of cork screw on the ground uh some miles long and uh it looks very strange and perhaps artificial we might think that perhaps that was produced by intelligent beings turns out not in the least that's an old Stream Valley here now is a photograph of the east coast of the United States at kilometer resolution and uh if you look closely at um say New York City uh over here or at Washington DC over here you find no sign of Life intelligent or otherwise um at kilometer resolution uh you do find that the cities are

discolored uh the places where there are cities turn out from orbit to be dirty but it would be a very intelligent extraterrestrial who would deduce that wherever there's dirt there's intelligent life and apart from the fact that cities are generally dirty there's no indication of the existence of cities much less their inhabitants at this resolution scale now here is a picture which clearly shows the presence of life if only we were wise enough to recognize it because each of these peculiar Pacific Islands is a coral ATL is a product of Life uh that one say

or that one this one over here um they are produced by Colonial organisms Carl hordes of animals all working together to make a caral island but unless we know beforehand about life on Earth it would not be apparent that that's the case so here's a sign of life but we wouldn't recognize it now here is the Sultan sea in California and just below it a region called the Imperial Valley which um in this picture gives a sense of structure there's a kind of model pattern there if only we could see it better we might know

something about what it's like well we can look more closely and here uh if we look still more closely at this red pattern the ground we have a sense of structure of squares and rectangles just barely apparent if we look elsewhere at improved resolution we find that the Earth is covered with squares and rectangles when we look at the Earth at 100 met resolution there are squares everywhere for some reason the Earth is into squares now there is no way that this could be of geological or meteorological origin these squares could only be of biological

origin and so we would have to deduce that there's some strange form of intelligent life on the earth which is passionate about car the ground up into squares and rectangles for reasons which we canot in the least imagine but there's no doubt that that's what it likes to do and so now we must make a search for the square people here this is a place called Chicago different kind of squares and rectangles are apparent here what their function is not so obvious this is Denmark where they're more into rectangles than into squares it's a a

nice departure here is France where the uh the arrangement is not nearly so regular and also each pattern is separated from the next one by a dark line which is in many cases due to Hedges uh so here we have not just a passion for ukian geometry but also a passion for territoriality and here is uh still another pattern this place is into rectangles but is also into up uh in this place great structures have been built vertically as well as horizontally this is New York City and uh how these stag mites of intelligent origin

ever came to be would not be apparent to us much less why they would want to build up when there's so much unused ground to build sideways on there would also be vast engineering constructs which would be apparent to us this is the great arbo radio telescope in Puerto Rico about which we will say more in a subsequent lecture and now here is a candidate dominant organism on the planet uh each of these is a creature which uh for uh sake of definiteness I will call an automobile and here is a great horde of these

intelligent organisms all clustering together perhaps for warmth or perhaps perhaps it's an outdoor political Rally or maybe a religious revival meeting and uh uh one can in fact see that much of North America and Europe is constructed for the sake of these little organisms great straight ways which are designed for them and special buildings to which they go at night uh in ones and twos and uh occasionally one can see small automobiles which are obviously the Young and uh at this point we would assuredly think we had found a dominant organism but if we looked

closely we would see that there were small parasites which would come in and out of the Automobiles and uh we would think that we were now into the microbiology of the organism disease organisms and we had made great progress little imagining that the the real dominant organism had yet to be found uh we would be quite astonished by a picture such as this in which there are great organisms uh many uh tens of meters long every one of these little white things here is in fact a great whale and this is a picture taken in

uh the ocean and then we would see places like this this is a marvelous Sand Dune field and in these great sandunes we would see strange creatures like these and what could these be well we uh would have the following kind of argument we would uh say look in this place there are stilted lumps there's lumps about that size connected to the ground by four stilts and many of them stretching as far as we can see now how do you make stilted lumps well there are no weather ways to make it there don't seem to

be any geology ways to make it probably it's a biology way to make it not only that things get rubbed down on the Earth there are uh sandstorms which rub things down running water rub things down and we could calculate that in the period of only a few thousand years the stilts would all be rubbed away and the lumps should be on the ground so these stilted lumps have been made in the last few thousand years there's some Machinery on the Earth from making stilted lumps at least every few thousand years maybe much faster and

I think we would have to deduce that that was due to biology and perhaps these are the dominant organisms then we would look closer here is a set of interesting creatures which would then be gone about 2 minutes later this is in fact a flock of flamingos and uh if we now look at another picture here we have for the first time the organisms that we truly know to be the dominant organism human beings this is a great outdoor swimming pool each dot is a dominant organism and uh nearby there are blankets and on each

blanket there are one or two dominant organisms but to see what the dominant organisms are really like we would have to go still closer and if we really did that we would finally come to grips with the true nature of the dominant organism on the planet now I'd like to do a just for a moment a stepping back and uh let's uh examine still another picture we have up to now looked only at the Earth in reflected sunlight here's the Earth in emitted light up at the top we see this is North America up at

the top we see a very bright light which is in fact the Aurora Borealis but down here all these other lights are due to cities at night here for example is the New York to Washington Corridor and and so on so the Earth due to its intelligent life shines at night here is a remarkable picture made of the Earth in radio waves it shows the Earth iding space in radio radiation and look the map is exactly like in the geography books and the people who wrote the geography books were never in space so they did

a very good job and here this is part of a plaque on a longlived spacecraft and uh here we have the configuration of continents some 200 million years ago when they were all almost together and touching due to what is called uh plate tectonics or continental drift the uh continents move apart here is their present configuration and some 8 million years from now here's what the continents will look like uh Britain seems to be still here Italy seems to be missing uh seem to be a lot of new places in the Indian Ocean and as

I mentioned uh over here you can see California having drifted out to sea uh that's merely to mention that the configuration of the continents of the earth is to only for now at another time it'll be quite different now to show some familiar geography you may recognize these small Islands uh picture taken from space with h quite uh a configuration quite like that in the geography books and uh let us do now another zoom in in this case let's try to uh zoom in on some places that most of you know uh here you can

see England in the rough vicinity of London uh if we look more closely we can see the Tam's Estuary I may have this upside down uh and the TS in the vicinity of London and then if we look still more closely we might see some more rectangles London rectangles in this case some strange buildings one particular doed building in central London and if we then zoom in to this strange building we uh can perhaps pick out a random dominant organism in London there are some dominant organisms now so we have now done uh twice a

kind of exploration of the earth and seen among other things how varied and interesting it is but also how difficult it is to look for life it is not easy to find life you have to look very closely on a planet to find life that's there now maybe I can have a globe of the earth thank you all of the things that we've been uh talking about uh are on in in the last few minutes are on the surface of uh of the earth and they are uh conveniently put together in Globes like this one

and uh on this globe there are National boundaries marked but on photographs of the Earth from space there are no National boundaries to be seen uh which is perhaps a useful lesson for politicians the Earth rotates from west to east uh so that the Stars can seem to rise in the east and set in the west otherwise it' be difficult to explain them and uh as it turns in this this way some parts of the earth plunge into daylight and others plunge into night and you can imagine for example a wave of toothbrushing which circles

the earth once every day uh at the Terminator the place where light and dark parts of the earth meet there are actually two such places but I'm talking about the morning one well we have so far been talking mostly about the solar system by now our Hello Universe signal has passed far beyond the orbit of Mars far beyond the sun is lovely yellow as you see um but if that original hello was given say half an hour ago the hello is just now something like reaching Jupiter how long before it reaches the nearest star that

will take another 4 and a half years the stars are astonishingly further away than the planets so all we've been talking about is a cluster of Little Worlds around the Sun and then some immense distance to the nearest star which may indeed have planets about it how is this discovered how is it known how far away the stars are we cannot bounce radar off the Stars has to be done by some other method and let me just give you briefly an idea of how one of the early very clever Minds in astronomy figured out the

distance to the Stars this is a man named christanis hin who lived in Holland thank you both in the uh 17th century and he argued as follows he said it looks to me as if the stars are Suns the sun is near that's why it looks so bright if the sun were some immense distance away it would be just a little pinpoint of light and seem very dim how far would I have to move the Sun away in order to make it as deep as some bright star at night let's say Sirus so hyans went

out at night and looked at Sirius for a while look look look remember how bright Sirius is then next day still remembering how bright cus was yesterday he went out and looked at the sun he had a brass plate painted black here so it doesn't shine badly to the television cameras his was shiny and he held the plate up to the Sun and looked through each hole until he could find a hole which was so small that the amount of light from the sun which entered was just as bright as he remembered serious to be

last night and then he happened to know the inverse Square law which I hope many of you know and was able then to calculate how far away you'd have to move the Sun and he came out with the answer of about half a light year um uh that is it would take light about half a year to go from here to sirius had Sirius in fact been intrinsically as bright as the sun he would have gotten exactly the right answer in fact Sirius is brighter so he uh got it a little bit wrong but he

came very close with an a factor of 10 um and uh it's an astonishing achievement that uh that Hans did and it suddenly made a universe vast and awesome Beyond ordinary human understanding many people like Pascal said that they were frightened by the great spaces between the stars because space is amazingly empty now I would like to close with a little discussion about the nearby Stars when we look out at night we see the constellations many of you have probably been forced to memorize uh certain patterns of uh as if they were somehow uh firmly

embedded in nature as opposed to arbitrary psychological projective tests forced on innocent children by astronomers um so for example many of you know what I call the Big Dipper and you know as I think the Great Bear or the plow look at that in America we think it's a thing that you pick up liquids with and in Britain you think it's either a bear or an agricultural implement now those are three quite different things that must be merely what's in people's minds and nothing to do with what's out there now what is a constellation constellation

is a configuration of dim but nearby Stars bright and more distant Stars which by accident happen to be in the same part of the sky when you look up now I've been so annoyed at this uh business of forcing everybody to learn constellation that I've made uh as strong effort as I've been able not to learn the constellations so I know only a few and those only by accident I didn't mean to learn them one of them I learned for this sort of lecture uh and uh what I've always wanted to do is to find

some new constellation and give some absolutely silly designation to it and make everybody have to learn that but that's hard to do because all the constellations have been named the ones in the southern hemisphere more recently so you'll find the constellation of the microscope the constellation of the telescope and I guess if they had been discovered still more recently you would have the constellation of the refrigerator and things of that sort well there are no new constellations to be discovered around here what we have to do is go to some other place where the constellations

are different and no human being has named them yet how far away do we have to go we clearly have to go some distance comparable to the distances between the Stars themselves we must go many light years a light year the distance light travels in a single year is about 6 trillion miles it's a big distance now we can't yet do that Voyager is in fact on its way to the stars but it will take some 50,000 years to go the distance to the nearest star and that's a little sluggish for the purposes of this

lecture um so what we can do however is to ask a comp computer which knows the three-dimensional positions of all the nearby stars to draw us a picture of what the sky would look like from this place or that so let's start out looking at such a picture first from the standpoint of uh of the earth so that we can see some recognizable objects so here we have the constellations as seen from the Sun these are in the Northern hemosphere and uh this constellation right here is the Dipper bear plow uh maybe you can see

the handle of the Dipper in the bowl here I guess this is the body of the bear and the tail and I can't see how that's a plow maybe yeah maybe that's the plow and that's the rains and there isn't any horse maybe anyway everybody knows that constellation even I know that constellation and over here is the North Star by the way all these nice dashed lines and numbers are of course not on the sky if they were on the sky that would make all of this much easier but I would like to call your

attention to uh this configuration which is an up down up down as you can see and it's called copia and as everybody can plainly see that's a lady sitting in a chair upside down in the sky now let us ask the computer to look at the stars in the same region of the sky but from the vantage point of the nearest star a place called Alpha centu well when we do that we see thank you here is the Big Dipper plow bear exactly the same here is the North Star in exactly the same position why

are they the same why aren't they different why haven't we changed our perspective enough to have the constellations change as well and the answer is that we have not gone far enough there's in fact only a single change in this picture and that is in copia here's Cass appear and it is as before an up down up down the only difference is there's another up there's a new star which wasn't there before what star is that it's the Sun that is our star as seen from alpha centu of course the sun is not in our

night sky because if it were it wouldn't be night that's the difference between day and night so we have to go a bigger distance so let us now look at the constellations from in the vicinity of the Earth again but not in the uh Northern Celestial hemisphere but in the broad range of middle Celestial latitudes and I want to ask you to memorize all of these uh stars but in here are all the zodiacal constellations what what people ask you about when they ask you what sign are you uh I like to say slow school

zone ahead um but what they mean as astrological signs uh the sign of the water carrier or uh the scales or the lion the Virgin all of those are supposed to be in here I can't see any of them but um what we can ask the computer to do is to move a distance let's say to the nearest star like the sun that's a place called taai and uh ask it to draw the constellations in this part of the sky and when we do that we find that there are uh enough apparent motions of the

stars that we get new constellations and so I thought it would be fun then to finally put my exotic new constellation in the sky of taai and uh I thought I'd make up a constellation of the unicorn but since there are there's already a constellation of the unicorn in our Sky I figured I'd make a special six-legged unicorn uh so you'd know it was a fake unicorn not a real unicorn like the ones we have here and uh I thought that the artist would do the six legs two two and two like insects on the

Earth but as you can see what the artist did was a quite creditable three and three now the constellations are quite different here and so I am finally able to make my constellation unicorn the sun is in fact a star in this sky and by a strange accident it is in fact that star right right there now I imagine that there would be some problem for the scientist who lived on a planet of taai who proposed that there was life on a planet that circled the star which joined the unicorn's tail to its rump it

sounds unlikely you cannot tell just by looking at the stars which have planets which planets are inhabited or anything of that sort and as our last slide reminds us there are an immense number of stars there are visible to the naked eye only some thousands of stars in this picture there are uh perhaps 10,000 stars but in the Milky Way galaxy of which we are a part there are some 250,000 million stars were we to move only 30 light years away we would be unable to even see the sun with the naked eye and that

is 1,000th of the distance from here to the center of the Galaxy we live in a galaxy vast and awesome Beyond ordinary imagining we live on a planet which seems typical a star which seems typical in fact in a Galaxy which seems typical and the great question of questions is whether what we have here is indeed typical or is it in some way unique we will explore that in later lectures you