00:00A long time ago, we believed that the entire universe revolved around our little blue planet.
00:08This is what we call the geocentric model.
00:11We thought that the sun, the stars and all these sparkling celestial bodies danced around us.
00:17This theory was very popular in Greece and Ancient Rome.
00:21Famous geniuses like Aristotle and Ptolemy really loved this idea.
00:26But why did people believe that?
00:29Well, first of all, our ego is huge.
00:31Then, some elements seemed to support this idea.
00:34For example, if you stand on Earth and you look at the sun,
00:38it will seem to you that the sun revolves around you once a day.
00:42The moon and the planets seem to do the same thing.
00:45The obvious conclusion is, yes, all this is probably revolving around the Earth.
00:53Then there is the fact that the Earth seems relatively stable when you stand on it.
00:58You don't really feel like it's moving.
01:00And because of this stability under our feet, people thought the Earth was still.
01:06But at the same time, some Greek and Roman philosophers discovered something very interesting.
01:11They associated this geocentric model with the idea that the Earth was a sphere floating in space,
01:17and not a disc or something flat.
01:20And they started to see it more clearly.
01:22After a while, the astronomer and mathematician Harry Stark of Samos had a revolutionary idea.
01:28He thought that the Earth might not be the center of everything that existed.
01:33And he suggested that everything revolved around the sun.
01:35It is funny to note that at the time, this idea was considered nonsensical.
01:44This is why, for a long time, most people have remained faithful to their geocentric view.
01:49It took us several centuries to finally accept the heliocentric model
01:54where all the planets of our solar system revolve around the sun.
01:58This idea was highlighted by characters like Copernicus, Galileo and Kepler.
02:04This change was not easy for everyone.
02:06It was difficult to give up our centrality.
02:10But little by little, we gave up.
02:13And by giving up this idea, we discovered a whole new world.
02:17This journey led us to an amazing realization.
02:20Not only are we not the center of the universe,
02:23but we are only a small particle within it.
02:26We are a microscopic point in the immensity of the galaxy,
02:29which is called the Milky Way.
02:32The discovery of the Milky Way begins in our gardens.
02:35If you have already contemplated the stars by a clear night,
02:38you may have noticed a pale and luminous band stretching very far,
02:42like a sparkling celestial ribbon filled with stars.
02:46This is our galaxy.
02:47We named it the Milky Way,
02:49because it looks like milk spilled on a road.
02:59It was perfectly described in the 90s by Voyager 1.
03:03NASA's space probe took a picture called a pale blue dot.
03:08This tiny little pixel, almost impossible to see, is our planet.
03:13The picture was taken at a staggering distance of 5.86 billion kilometers from the sun.
03:19At the scale of the universe, this distance is nothing.
03:22But for us, it's unimaginable.
03:25Thus, when scientists discovered the Milky Way,
03:28they quickly equipped themselves with telescopes.
03:31Their goal was to map the unexplored territories of the night sky.
03:35And, of course, to find our place in this gigantic world.
03:41To do this, we first had to dig into the secrets of the structure of our galaxy.
03:45Well, we clearly see that it is in the form of a kind of elongated band.
03:50This means that our galaxy is not a big round ball like planets and stars.
03:55In reality, it looks more like a huge pancake or a flattened disc.
04:00And we can see that we are neither above nor below.
04:03We are well at the level of the disc.
04:06The next step is to travel the Milky Way to map it.
04:10However, there is a small problem.
04:12To do this, we would have to go through thousands of light years through this pancake-shaped object.
04:18To put things in perspective, let's go back to Voyager 1,
04:22the machine that took the picture of the pale blue dot.
04:25This space probe has been traveling in space for almost half a century.
04:29It left the solar system several years ago.
04:32Do you know how many light years it has traveled?
04:350.002.
04:37And how many light years does the Milky Way measure?
04:40100,000.
04:41You got it.
04:42But the absence of sophisticated technology did not stop us.
04:46In the 18th century, an audacious astronomer named William Herschel decided to explore our galaxy.
04:53With only a telescope, this Indiana Jones of astronomy began to map the stars in the sky.
05:00And he discovered Uranus, more than 2,000 nebulae,
05:04and created the first map of the Milky Way, which represented it as a disc.
05:08The map was not super-precise, but nevertheless very impressive.
05:15Unfortunately, he was not aware of what is called interstellar dust.
05:19It is like a space fog that can veil our view of the stars in the center of the Milky Way.
05:25This dust made the central region of the Milky Way appear less crowded than it really is.
05:30Let's move on to the 20th century.
05:32Henrietta Swan Leavitt, an American astronomer, was also very curious about the night sky.
05:38But she focused her attention on a particular type of star, the Cepheids.
05:43These stars had a unique peculiarity.
05:46They pulsated, that is to say that their burst varied according to a well-defined period.
05:51Leavitt's work at the observatory where she was employed was similar to that of a librarian.
05:57She cataloged these stars.
05:59And in doing so, she made an incredible discovery.
06:02She found a direct link between the brightness of these stars and the frequency of their pulsations.
06:07This discovery is now known as the Leavitt law.
06:11This means that by simply measuring the speed of the pulsations of these stars,
06:17astronomers can determine how far they are.
06:20These variable stars have then allowed to measure all kinds of distances.
06:30Before the 1920s, most scientists believed that our Milky Way was the only galaxy in the universe.
06:37But as telescopes improved, some astronomers began to realize that the Milky Way was not the only galaxy in the universe.
06:45We began to discover more and more galaxies.
06:48But if the Milky Way was only one galaxy among so many others, where were we exactly in all this?
06:54This is when a scientist named Arlo Chaplet came on stage.
06:58Armed with a powerful telescope, Chaplet turned his attention to globular clusters.
07:03These are groups of ancient stars that, densely gathered, form spherical clusters.
07:10By discovering them, he noticed something very interesting.
07:14The oldest stars around us were not scattered.
07:17They were grouped around the center of the Milky Way.
07:20And they pointed in the direction of certain constellations, such as Sagittarius and Scorpio.
07:26It turned out that our galaxy had started from the center.
07:30This was the heart of the Milky Way, and the oldest stars had settled there.
07:35This means that we are not even in the center of our own galaxy.
07:39It looks like our importance is decreasing more and more with each new discovery, doesn't it?
07:44Anyway, Chaplet discovered that we were located somewhere on the periphery of our galaxy.
07:50Chaplet's calculations were not perfectly accurate, but they were still quite precise.
07:56Fortunately, we now have very efficient and fascinating tools.
08:00Since then, we have been able to locate our location precisely.
08:04We are located near a partial arm of the Milky Way, called the Orion Arm.
08:08It is located about 26,000 light-years from the center of our galaxy.
08:12This is how we discovered where we are in the Milky Way.
08:20Do you think our story is over?
08:22No, absolutely not.
08:24Here is Gaia, the celestial cartographer of the Milky Way.
08:28Launched in 2013, Gaia embarked on a bold mission.
08:32She must map the Milky Way with an unprecedented level of detail.
08:37Not just a summary map, but a photograph of the most subtle nuances of our galaxy.
08:43Gaia assembles the positions and movements of about a billion stars.
08:47And that represents about 1% of all the stars of the Milky Way.
08:51But this tiny fraction is still a very important part of our galaxy.
08:56This tiny fraction is still a true masterpiece of cosmic cartography.
09:02What a great adventure!
09:04It led us from the belief that the Earth was the center of the universe
09:08to the realization that we are less than dust in the great order of things.
09:13But the most important thing is that this story shows how curious humanity is about the mysteries of the world.
09:19And there is always something new to explore.
09:23We never stop making new discoveries.
09:25So stay tuned and keep watching the night sky.
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