The Galactic Year: One Orbit of the Milky Way

What the galactic year is

Just as the Earth circles the Sun once a year, the Sun circles the heart of the Milky Way once a galactic year. Our galaxy is a flat, spinning disk of a few hundred billion stars, more than a hundred thousand light-years across, with a bright bulge at its center hiding a supermassive black hole. The Sun lives out toward the suburbs, about 26,000 light-years from the middle, on the inner edge of a spiral feature called the Orion Arm.

Out there the Sun sweeps along at roughly 230 kilometers a second, fast enough to cross the United States in under twenty seconds. Yet the circle it has to complete is so vast that a single lap takes somewhere around 230 million years. That whole journey is one galactic year, the longest cycle on this site, longer than the entire age of the dinosaurs.

The last galactic year on Earth

Because the lap is so long, the whole sweep of life as we tend to picture it fits inside the most recent one. Set the slider to the start, one galactic year ago, and the Earth is in the Late Triassic: the first dinosaurs and the first true mammals are appearing, and every continent is fused into the supercontinent Pangaea. As the Sun rounds the galaxy, Pangaea breaks apart, the great dinosaurs rise and then vanish in the asteroid impact 66 million years ago, mammals spread, primates emerge, and only in the final fraction of the lap do humans appear. The Sun has now completed one full trip around the galaxy since those first dinosaurs walked, having seen all of that go by just once. Because the orbit is not a tidy closing circle, it comes back to roughly the same distance from the center rather than to the very same point in the galaxy.

About twenty laps in a lifetime

The Sun and its planets are about 4.6 billion years old. Divide that by a 230-million-year lap and you get roughly 20 galactic years: the Sun has circled the Milky Way about twenty times since it first lit up. A comfortable human life of 80 years, by comparison, is around a third of a millionth of a single lap. You can see this longest of cycles set beside all the others, from the 24-hour day on up, on the Astronomical Cycles page, and find today's position in each on the live cosmic clock.

How well do we know it?

The galactic year is not a precise number. Estimates run from about 225 to 250 million years, because the exact distance to the galactic center is hard to measure and the Sun's path is not a tidy closed circle. As it orbits, the Sun also bobs gently up and down through the disk, rising above the midplane and sinking back roughly every 60 to 70 million years, like a horse on a very slow carousel. So the figure of 230 million years is a good round average for a real, if slightly wandering, journey, not a clock that ticks to the second.