In a galactic year, also known as a cosmic year, the Sun completes one orbit around the Milky Way.
Humans often measure time by observing Earth’s relative motion around the Sun. However, these rotations of Earth around its star are only significant for life on this small blue planet. Compared to the vast journey of the Sun and the entire solar system around the center of the Milky Way, the planet’s rotation is inconsequential.
In a galactic year, also known as a cosmic year, the Sun completes one orbit around the Milky Way.
According to Keith Hawkins, a professor specializing in astronomy at the University of Texas, USA, the Sun takes about 220 to 230 million years to complete an orbit around our galaxy. In other words, if we measure time using this “galactic clock,” Earth is only 16 years old (in galactic years), the Sun is 20, and the universe is 60.
The solar system’s journey around the galaxy is similar to Earth’s orbit around the Sun. However, instead of orbiting a star, the Sun revolves around a supermassive black hole located at the center of the Milky Way. While it exerts immense gravitational pull on objects near the galaxy’s center, the overall gravitational attraction of the galaxy’s matter keeps the Sun on its orbit.
Professor Hawkins explains that the Sun moves fast enough, at about 230 km per second, to continue orbiting the galaxy’s center in a circular path without being drawn into the black hole.
Our Position in the Galaxy
Compared to a year on Earth, a galactic year is much longer and represents a vast amount of time. However, this is not a universally applicable way to measure all motions within the galaxy. What humans call a galactic year is suitable only when measured from Earth’s position within the Milky Way’s spiral arm. We say that a galactic year lasts from 220 to 230 million years. But for stars in different parts of the galaxy, their galactic years can be entirely different.
Our galaxy spans about 100,000 light-years, and Earth is approximately 28,000 light-years away from the galaxy’s center. If you imagine the galaxy as a city, Earth is situated somewhere in the suburban area. For stars with orbits close to the black hole, which means closer to the “city center,” a galactic year is relatively short. Conversely, for those farther away in the “suburbs” where our solar system resides, a galactic year is slightly longer.
Similarly, the length of a year among planets varies significantly. For example, Mercury, the closest planet to the Sun in our solar system, takes 88 Earth days to complete one orbit around the Sun. Uranus, the seventh planet from the Sun, takes 84 Earth years to complete its orbit around the Sun. And the dwarf planet Pluto requires 248 Earth years to complete one orbit.
Although, from a physical standpoint, the orbits of planets are somewhat similar to the mechanism that forms the solar system’s orbit around the Milky Way, what’s intriguing is how astronomers determine the length of a galactic year. According to Professor Hawkins, there’s nothing overly complicated, and the fundamental science has been clear since the early days of modern astronomy. “The main issue is simply observing the stars’ movements around the galaxy. You can track the stars’ motions around the galaxy and deduce from the speed and direction of those stars,” he says.