 Hello everyone and welcome to our universe. Today we'll be answering the question, where is the Earth? So this might seem like a bit of a weird question, where is planet Earth? Well it's pretty obvious, planet Earth is right here and we're all standing on it so we already know the location. But the thing is, where is the planet Earth relative to the Milky Way galaxy? Now that is a good question. A long time ago, in this galaxy, astronomers believed that the Earth was the centre of our universe. This is basically due to our viewpoint. From everything around us it looks like everything is orbiting around the planet Earth, so it's quite easy to assume that the Earth is at the centre of the universe. It's only been known for just a couple of centuries that observations and calculations have shown that the Earth and all the bodies in the solar system orbit around the Sun. And this is pretty much the same of how the Sun orbits around the Milky Way galaxy. This means that we've only known for about a century that we are part of a larger picture. We are part of a galaxy. But now astronomers want to know, where exactly does the Earth lie relative to the Milky Way galaxy? So for starters, let's talk about the Milky Way size. And the Milky Way size is big. It's really, really big. In fact, if you measure it in light years, it can be 120,000 light years in diameter, and it can be about 1,000 light years thick. The Milky Way's current size and shape is the result of eating up other galaxies. And it has been doing so to this present day. In fact, the Milky Way galaxy has a dwarf galaxy, a companion galaxy, called Canis Major. This is the closest galaxy to the Milky Way, and its stars are currently being added to the Milky Way's disk. Our galaxy has actually consumed another dwarf galaxy in the past. This is known as the Sagittarius dwarf galaxy. But the Milky Way galaxy is still an average size galaxy, compared to the other galaxies in the local universe. Andromeda, for example, is the closest major galaxy to our own. But it is twice as large as the Milky Way, and it has an estimation of nearly 800 billion stars. And some calculations say that it could hold a trillion. So the structure of the Milky Way is a very interesting thing. If you could travel to the outside of the galaxy and look back, you would see that the Milky Way galaxy is a barred spiral galaxy. The spiral arms are formed from density waves that orbit around the Milky Way. These are basically made of stars and clouds of gas that are clustered together. As the density waves move through an area, they can press the gas and dust, and this leads to a period of active star formation within the region. The existence of the arms have been determined from observing parts of the Milky Way, as well as studying other galaxies in our universe. If we do look at our galaxy from a whole point of view, you'll actually see that they're not actually real. We've never been able to photograph the whole of our galaxy from the outside. These are either pictures of artist renditions, or they are pictures of other spiral galaxies that resemble our Milky Way. It is very difficult for scientists to actually gauge what the Milky Way really looks like, mainly because we're within it. It's basically like living on the surface of a frisbee. It has only been through decades of observation, reconstruction, and comparisons to other galaxies that we're able to make a clear picture of what the Milky Way really looks like from the outside. From the current research of simulations and telescopes, we can now estimate the amount of stars that are in the galaxy, the amount of spiral arms, the structural rigidity, and the structure of the dark matter itself in the Milky Way galaxy. The Milky Way's arms are made up of dust and gas. This matter makes up 10 to 15% of all the luminous matter. This is basically the matter that we can see, and the remainder of this would be the stars. Our galaxy is roughly 100,000 light years across, but we can only see about 6,000 light years into the disk in the visible spectrum. Light pollution can be quite an annoying thing for astronomers, but it's a dusty ring of the Milky Way that is constantly prominent and blocking our view of the galactic centre. But with infrared astronomy, we can see the heat of the stars and cut out the gas and the dust. This allows us to see the Milky Way galaxy in other wavelengths of light, and therefore understand its structure a lot better. The Milky Way is surrounded by a vast halo of dark matter, and this accounts for about 90% of its mass. The problem is with dark matter is that we know it's there, but we don't know what it's made of. Its mass can be inferred by the observations to how fast the galaxy rotates. Most importantly, it is believed that its mass is helping keeping the galaxy together. This means that the galaxy without dark matter would tear itself to shreds. The solar system is located about 25,000 light years away from the galactic centre, but also it's about 25,000 light years from the edge of the galaxy. This means that the solar system is roughly halfway between the centre and the edge of the galaxy. Astronomers are definitely in agreement that the Milky Way has two major spiral arms. These are known as the Perseus arm and the Scutum Centurus arm. There are also several smaller arms and spurs, but astronomers can definitely agree that our Milky Way has two major arms. The solar system is located in the region between these two arms, and this arm is known as the Orion Cygnus arm. This arm is about 3,500 light years across and about 10,000 light years in length, where it breaks off from the Sagittarius arm. In fact, if you look at the Milky Way in the night sky, it is rather equal from both hemispheres, and this probably means that the solar system lies relatively close to the galactic plane. The Milky Way has a relatively low surface brightness. This is due to the gases and dust that fill the galactic disk. This prevents us from seeing a bright galactic centre. The sun takes over 200 million years to make one galactic year. The last time that the solar system was in this position in the Milky Way, there were still dinosaurs on the planet Earth. So even though our solar system is located 25,000 light years from the galactic centre, it is still constantly moving in a galactic orbit, constantly revisiting the same places every 250 million years. As our understanding of the universe has expanded, we have come to learn many new things. Not only is the universe much larger than we ever imagined, but we are also understanding a lot more about the structure of the galaxy, what it's really made of, the location of the Earth and the Sun, and hopefully the future of our galaxy, the solar system, and maybe planet Earth. So I hope that's given you a great insight into where is the planet Earth? If you'd like to know any more about the Milky Way galaxy or our location within the galaxy, I'll put some links in the description below. And as always, if you've enjoyed the video, click the like button, and if you want to support the channel, click subscribe. Thank you for watching.