At our level, the Sun takes between 225 and 250 million Earth years to make a complete tour of our galaxy. If we were to measure time with this galactic clock, then the Earth would be about sixteen years old, the sun would have formed about twenty years ago and the universe… about sixty years ago.
Our system’s journey around the galaxy resembles Earth’s orbit around the sun. However, rather than circling around a star, the solar system circles around Sagittarius A *, the supermassive black hole at the center of the Milky Way. This cosmic ogre is as heavy as four million suns. Our star is moving at a speed of about 230 kilometers per second around it, completing one revolution of the galaxy in about 225 million years.
If you take into account our current position in the Milky Way, it means that the last time our planet was in this location, the dinosaurs were just about to begin to emerge.
If we then think of our galaxy as a clock and our current location at the 6 a.m. mark, then Stegosaurus, which appeared about 155 million years ago, would be at 9 a.m. Giganotosaurus, some of the largest carnivorous dinosaurs of all time, emerged at noon. The Cretaceous finally ended around 4 p.m. (66 million years ago), with dinosaurs like the T-Rex and the Triceratops.
It also stands to reason that what we Earthlings call a galactic year is specific to our place in the galaxy. “If you imagine the Milky Way as a city, our planet is positioned somewhere near the suburbs,” says Keith Hawkins, assistant professor of astronomy at the University of Texas (Austin). “For stars that orbit near the black hole, in the ‘city center’, a galactic year is relatively short”.
For example, the star S62, one of the closest to the supermassive black hole in our galaxy, circles it every 9.9 Earth years. A galactic year therefore lasts less than ten years. During its journey, the star steadily approaches the black hole within 2.4 billion kilometers. For comparison, this is closer than the average distance between Uranus and the Sun.
Similar rules control the variability in the length of a year between planets in our system. Mercury, for example, performs a complete orbit around the sun in about 88 Earth days. On the other hand, Pluto, the distant dwarf planet in our system, takes 248 Earth years to complete an orbital cycle.