The galactic year, also known as a cosmic year, is the duration of time required for the Solar System

Solar System

The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 to orbit

Orbit

In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

 once around the center

Galactic Center

The Galactic Center is the rotational center of the Milky Way galaxy. It is located at a distance of 8.33±0.35 kpc from the Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius where the Milky Way appears brightest...

 of the Milky Way Galaxy. Estimates of the length of one orbit range from 225 to 250 million "terrestrial" years.

The galactic year provides a conveniently usable unit for depicting cosmic and geological time periods together. By contrast, a "billion-year" scale does not allow for useful discrimination between geologic events, and a "million-year" scale requires some rather large numbers.

Timeline of universe's and earth's history in galactic years

In this list, 1 galactic year (GY) = 225 million years

• about −40 GY: Big Bang
  Big Bang
  The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...
  
  
• about −39 GY: Birth of the Milky Way
  Milky Way
  The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
  
  
• 0 GY: Birth of the Sun
  Sun
  The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
  
  
• 4 GY: Oceans appear on Earth
• 5 GY: Life begins
• 6 GY: Prokaryotes appear
• 7 GY: Bacteria
  Bacteria
  Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
  
   appear
• 10 GY: Stable continents appear
• 13 GY: Eukaryotes appear
• 16 GY: Multicellular organisms appear
• 17.8 GY: Cambrian explosion
  Cambrian explosion
  The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...
  
  
• 19 GY: Great Dying
• 19.6 GY: K–T extinction event
• 19.999 GY: Appearance of modern humans
  Human
  Humans are the only living species in the Homo genus...
  
  
• 20 GY: Present day