The facts below give a sense of how large one billion (taken as 109) is in the context of passage of time.
About a billion seconds ago, the parents of middle school children were themselves in elementary school. (One billion seconds is roughly 31.7 years.)
About a billion minutes ago, the Roman Empire was flourishing. (One billion minutes is roughly 1,900 years.)
About a billion hours ago, modern men and their ancestors were living in the Stone Age (more precisely, the Middle Paleolithic). (One billion hours is roughly 114,000 years.)
About a billion days ago, Australopithecus, an ape-like creature related to an ancestor of modern humans, roamed the African savannas. (One billion days is roughly 2.7 million years.)
About a billion months ago, dinosaurs walked the earth during the late Cretaceous. (One billion months is roughly 82 million years.)
About a billion years ago, the first multicellular organisms appeared on Earth. (The universe is now thought to be about 13.7 billion years old.)
In terms of distance:
A billion centimeters is about the distance from Chicago, Illinois, USA to Tokyo, Japan.
A billion inches is 15,783 miles, more than halfway around the world and sufficient to reach any point on the globe from any other point.
A billion meters is almost three times the distance from the Earth to the Moon.
One should not increase, beyond what is necessary, the number of entities required to explain anything
