From our ocean-covered rocky planet, humanity has long measured time by Earth's orbit around the Sun—one full revolution equaling one year. Yet Earth resides within the Solar System, which itself orbits the Milky Way's center, inspiring the concept of the galactic year.
While Earth's annual journey sustains life here, it pales against the Solar System's vast voyage around the galactic core—a trek that underscores the immense timescales of the cosmos.
Astronomy professor Keith Hawkins from the University of Texas explains that the Sun takes 220 to 230 million Earth years to orbit the Milky Way's center. On this galactic timescale, Earth is roughly 16 years old, the Sun formed about 20 years ago, and the universe is around 60 years old.
Just as Earth orbits the Sun, our star orbits the supermassive black hole at the galaxy's heart. Though this black hole wields immense gravity nearby, it's the collective mass of the Milky Way that maintains the Sun's path. "The Sun travels at about 230 kilometers per second—fast enough to trace a circular orbit around the center rather than spiraling inward," Hawkins notes.
A galactic year dwarfs an Earth year but isn't uniform across the Milky Way, which spans 100,000 light-years with Earth positioned 28,000 light-years out. "If the galaxy were a city, we're in the suburbs," Hawkins illustrates. Stars nearer the central black hole complete orbits faster, while those in our outer spiral arm take longer.
This mirrors planetary orbits: Mercury circles the Sun in 88 Earth days, Uranus in 84 years, and Pluto in 248 years. Astronomers calculate the galactic year's length through stellar motions, leveraging precise modern observations for reliable data, as Hawkins affirms.
