The Galactic Year (Cosmic Year)

The sun is one of hundreds of billion of stars in our galaxy, the Milky Way. The galaxy is composed of gaseous interstellar medium, neutral or ionized, sometimes concentrated into dense gas clouds made up of atoms molecules, and dust. All of the matter -- gas, dust, and stars -- rotate around a central axis perpendicular to the galactic plane. The centrifugal force caused by the rotation balances out the gravitational force, which draw all the matter toward the center.

The mass is located within the circle of the Sun's orbit through the galaxy is about 100 billion times the mass of the Sun. Because the Sun is about average in mass, astronomers have concluded that the galaxy contains about 100 billion stars within its disk.

All stars in the galaxy rotate around a galactic center but not with the same period. Stars at the center have a shorter period than those farther out. The Sun is located in the outer part of the galaxy. The speed of the solar system due to the galactic rotation is about 220 km/s. The disk of stars in the Milky Way is about 100,000 light years across and the sun is located about 30,000 light years from the galaxy's center. Based on a distance of 30,000 light years and a speed of 220 km/s, the Sun's orbit around the center of the Milky Way once every 225 million years. The period of time is called a cosmic year. The Sun has orbited the galaxy, more than 20 times during its 5 billion year lifetime. The motions of the period are studied by measuring the positions of lines in the galaxy spectra.

How old is the Earth? This question preoccupied first philosophers, then scientists, for many centuries. Today, we know that the age of the Earth is approximately 4.54 billion years, with an error range of about 50 million years (4.54 × 109years ± 1%). This number is based on evidence from radiometric dating of the oldest-known terrestrial rocks as well as lunar rock samplesNotes 1 and meteorites.

The Earth is actually a fast-moving spacecraft. It orbits around Sun with an average speed of about 30 km/s (67,000 mph or 108,000 kmh), but what’s more, our galaxy, the Milky Way also rotates. It makes one rotation every 250 million years or so. As a result, our planet is approximately 18 galactic years old. Only a “galactic” year before, there was a supercontinent called Pangaea or Pangea. The Age of the Dinosaurs (Mesozoic Era) was just started, and mammals had yet to evolve. And, dubbed as the “mother of all extinctions” or “the great dying”, the Permian–Triassic extinction event, the most profound mass extinction in Earth’s history, was in progress.

For comparison, the Milky Way galaxy is approximately 13.2 billion years old, while the universe itself has been dated to 13.8 billion years.