Whenever you look up at the night sky, almost every object you are able to see belongs to our galaxy. There are between 100 and 400 billion stars in the Milky Way and at least another 100 billion planets. Although it is by far not the largest galaxy, that is still a staggering number. It spans around 150,000 to 200,000 light years in diameter and is believed to harbor a supermassive black hole in its center. About a quarter of the way from there orbits our own Solar System.
We travel at around 220 kilometers per second and it takes our small planet 225 to 250 million years to complete one revolution. To put that into perspective, the last time the Earth was relatively in the same position, it had just started the Jurassic Period. The continents were just beginning to split apart and the first dinosaurs were roaming their surface. It lasted more than 50 million years and, in that time, the first birds took flight and the earliest lizards and mammals appeared.
After it for another 80 million years spanned the Cretaceous Period. This was the time of the mighty T-Rex and the cunning Velociraptors and at this point the first flowering plants appeared. Its end is marked by the asteroid impact, which killed off the giant reptiles and it happened on the opposite side of the galaxy.
From that moment up until today stretches the Cenozoic Period, or the era of “new life”. This is the age of mammals, adapting and evolving into the world we all know today. About 4 million years ago the Australopithecus evolved in Africa, marking the birth of the first humans. Around this time the Sahara Desert started to form. And finally, about 320,000 years before today Homo Sapiens appeared. In that time, we have travelled less than 0.15 percent.
There are actually only three objects in the sky, observable with the naked eye, which are not from our galaxy, and all three of them are themselves galaxies. In the Northern Hemisphere this is the Andromeda, our closest neighbor, and in the south the Magellanic Clouds – two dwarf galaxies, orbiting the Milky Way.
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