Human beings are able to keep time by observing the rotation of the Earth relative to the sun. But while Earth's journeys around its star are remarkable for life on our pale blue dot, compared to the vast journey that takes the sun - and our entire solar system - around the center of the Milky Way, that journey is very insignificant.
According to Keith Hawkins, an assistant professor of astronomy at the University of Texas at Austin, orbiting the Milky Way galaxy only once takes the sun between 220 million to 230 million Earth years.
In other words, if by this cosmic "clock" we were to calculate time, Earth would be around 16 years old (in cosmic years), the sun would have formed around 20 years ago, and the universe would be just over 60 years old.
The path of the solar system through the galaxy is similar to the orbit of Earth around the sun. But instead of orbiting a star, Hawkins said the sun orbits the supermassive black hole that sits at the center of the Milky Way. It wields a massive amount of gravity on objects near the center of the galaxy, but it is the gravity collectively exerted by the material itself in the Milky Way that holds the sun in its orbit.
The sun is traveling at enough speed, about 230 kilometers per second, around the equivalent of 500,000 miles per hour, as it continues to spin in a kind of loop around the middle of the galaxy, instead of being drawn into the black hole.
A galactic year reflects time on a grand scale relative to an Earth year - but it's not a clear calculation around the galaxy. What we Earthlings call a celestial year is unique to the position Earth holds in the spiral of the Milky Way.
"We would say that a galactic year is 220, 230 million years," Hawkins said. "Other stars in the galaxy, their galactic year is different."
Since the physics of planetary orbits are identical to the processes that form our solar system's orbit around the Milky Way, it is worth wondering if astronomers have worked out a galactic year's length. Hawkins says that in the early days of modern astronomy, it was really pretty simple science, which became apparent.
"Really it's about seeing stars move about the galaxy," he said. "You can see stars travel across the galaxy, and deduce from other stars' speed and direction."
