Perched on our small rocky planet covered with oceans, we learned to measure the passage of time based on the trajectory of the Earth around the Sun. A complete revolution corresponding for Man to one Earth year. However, the Earth is included in the Solar System, which itself orbits the center of the Milky Way. This fact quickly led to the notion of a galactic year.
Humans are used to measuring time via the motion of the Earth relative to the Sun. But while Earth's journey around its star is important to life on our little planet, it's pretty insignificant compared to the long journey that orbits the Sun — and our entire Solar System — around the center of the Milky Way. .
Orbiting the center of the Milky Way takes the Sun between 220 and 230 million Earth years, according to University of Texas astronomy professor Keith Hawkins. In other words, if we were to measure time by this galactic "clock", the Earth would be around 16 years old (in galactic or cosmic years), the Sun would have formed around 20 years ago, and the Universe would have about 60 years old.
The Solar System's journey around the galactic center resembles Earth's orbit around the Sun. But rather than orbiting a star, the Sun orbits the supermassive black hole that sits at the center of the Milky Way.
It exerts enormous gravity on objects near the center of the galaxy, but it's the gravity exerted collectively by the material of the Milky Way itself that keeps the Sun in its orbit. "The Sun is moving at a sufficient speed — about 230 kilometers per second — to keep it spinning around the center of the galaxy in a kind of circle, instead of being pulled towards the black hole says Hawkins.
Compared to an Earth year, a Galactic year represents time on a large scale — but it's not a universal measurement across the galaxy. What we Earthlings call a galactic year is specific to Earth's place in the spiral of the Milky Way. The galaxy is about 100,000 light-years across and Earth is about 28,000 light-years from its center.
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“If you imagine the galaxy as a city, Earth is somewhere near the suburbs ". For stars orbiting near the black hole — the center of the “city” — a galactic year is relatively short. In the "suburbs", where our Solar System is located, "galactic years are a bit longer .
Similar rules control the variability in the length of a year between planets. For example, Mercury, the innermost planet in our Solar System, completes a full orbit around the Sun in approximately 88 Earth days. Uranus, the seventh planet from the sun, orbits the Sun every 84 years, by Earth standards. And the distant dwarf planet Pluto takes 248 Earth years to complete an orbital cycle.
While the physics of planetary orbits is similar to the mechanisms that shape our solar system's orbit around the Milky Way, it's worth wondering how astronomers figured out the length of a galactic year. Hawkins says it's actually a pretty basic science based on the movement of stars, made simple with modern astronomy.
