We now know that Earth and all the other planets in our solar system orbit the sun, but this wasn’t always the case. For centuries, it was thought that the sun and everything we see in the night sky orbited Earth. This Earth-centered, or geocentric, view of the universe made sense based on the simplest of observations possible by humans. Watch the sun over the course of the day and it appears to move across the sky while we stand stationary. Similarly, the motion of the stars supports this notion. A contemporary of Archimedes in Greece, Aristarchus, may have been the first person to write down reasoning as to why Earth orbited the sun. Unfortunately, Aristarchus’ work has been lost to history, but Archimedes did mention it in one of his works.
About 17 centuries later, Copernicus disrupted the western world by putting forth his model of a sun-centered, or heliocentric, solar system. In a draft of his work he mentioned the work of Aristarchus, but that reference was not included in his final published work.
Since the acceptance of the Copernican model of the solar system, we have come much further than the understanding that Earth is not the center of the solar system. We know now that our solar system isn’t even near the center of our galaxy, the Milky Way, and certainly nowhere near anything that could be called the center of the universe.
When it comes to our galaxy, it appears we are located on one of several spiral arms, and we are about two-thirds of the distance from the center of the galaxy to its outer edge. All the stars we can see in the night sky are part of the Milky Way, and we know that our galaxy is one of at least 100 billion galaxies, and as many as 200 billion galaxies in the observable universe.
In today’s photo, you can see the band of the Milky Way extending across the sky above Bryce Canyon National Park in Utah. The band-like nature of our view of the Milky Way suggests we are looking toward the center of a galaxy whose stars are contained within a rather flattened, disk-like arrangement. This leads astronomers to hypothesize that the Milky Way is a spiral galaxy, but of course there is no way for us to know that with complete certainty without being able to get outside the galaxy and look back at it.