Question: I have a question about galaxies, how do scientist know about the galactic year and how long it takes? Well people think that are universe is only 14 billion years old has our solar system gone through a galactic year? — Lily
Answer: Well, we can measure the rotation speed of the Milky Way Galaxy, which we find to be about 270 kilometers per second (or about 168 miles per second). It takes about 200 million years for the Milky Way to complete one rotation. I believe that this is what you are referring to as the “galactic year” (I have also seen it referred to as the “Cosmic Year”). Since the universe is about 14 billion years old, our solar system has gone around the center of the Milky Way galaxy a few times. Note, though, that the solar system is quite a bit younger than the universe: its age is about 4.6 billion years. So, the solar system has gone around the center of the Milky Way galaxy about 4.6 billion / 200 million = 23 times.