Humans are used to keeping time by measuring Earth’s movement relative to the sun. But while Earth’s trips around its star are noteworthy to life on our pale blue dot, that journey is pretty insignificant when compared with the epic voyage that carries the sun and our entire solar system around the center of the Milky Way.
Orbiting the Milky Way galaxy just once takes the sun approximately 220 million to 230 million Earth years. In other words, if we were to measure time by this galactic clock, Earth would be about 16 years old (in galactic, or cosmic years), the sun would have formed about 20 years ago and the universe would be just about 60 years old.
The solar system’s journey around the galaxy resembles Earth’s orbit around the sun. But rather than orbiting a star, the sun circles the supermassive black hole that lies at the center of the Milky Way. It exerts a tremendous amount of gravity on objects near the center of the galaxy, but it’s the gravity exerted collectively by the material in the Milky Way itself that keeps the sun in its orbit.
The sun is moving with enough speed about 230 km a second about the equivalent of 500,000 miles per hour that it continues to revolve around the center of the galaxy in sort of a circle instead of getting pulled toward the black hole.
Compared with an Earth year, a galactic year represents time on a grand scale but it’s not a consistent measurement across the galaxy. What we Earthlings call a galactic year is specific to Earth’s place in the Milky Way’s spiral. We would say that a galactic year is 220-230 million years. For other stars in the galaxy, their galactic year is different. The galaxy is about 100,000 light-years across and the Earth is about 28,000 light-years from its center.
But, how astronomers have figured out the span of a galactic year. It’s mostly about watching stars move around the galaxy. You can watch stars move around the galaxy and deduce from the speed and direction of other stars.