Galactic Year
The most common large-scale time scale is millions of years (megaannum or Ma). However, for long-term measurements, this still requires rather large numbers. Using as a measure the time it takes for the solar system to revolve once around the galactic core (GY - not to be confused with Gyr for gigayear), approximately 250 Ma, yields some easily memorizable numbers. In this scale, oceans appeared on Earth after 4 GY, life began at 5 GY, and multicellular organisms first appeared at 15 GY. Dinosaurs went extinct about 1/4 GY ago, and the true age of mammals began about 0.2 GY ago. The age of the Earth is estimated at about 20 GY.
Read more about this topic: List Of Unusual Units Of Measurement, Time
Famous quotes containing the words galactic and/or year:
“When the galactic sea was sucked
And all the dry seabed unlocked,
I sent my creature scouting on the globe,
That globe itself of hair and bone....”
—Dylan Thomas (1914–1953)
“No matter how corrupt and unjust a convict may be, he loves fairness more than anything else. If the people placed over him are unfair, from year to year he lapses into an embittered state characterized by an extreme lack of faith.”
—Anton Pavlovich Chekhov (1860–1904)