Earth time
A day, an earth day, is defined as the time it takes the earth to make one full rotation on its axis. This is a fundamental value, in that it is not calculated. Anyone can figure out how long a day is by waiting for the sun to rise each day.
We have divided each day up into 24 hours, which is arbitrary division. We could just as easily have divided days up into 10 hours or 20 hours or 25 hours, but the Babylonians were found of base 12.
A year is the time it takes the earth to revolve around the sun, which is three hundred sixty-five and one-quarter days. Like a day, this is a fundamental value and we don’t have to do math to figure it out.
Galactic time
Scientists say the earth is 4.6 billion Earth years old. Galactic years (GY) are convenient to think in terms of. Without GYs, scientists would need to think of “billions of years,” but then there are only 4.6 of those, so the gradient wouldn’t be big enough. If scientists think in terms of millions of years, that’s too many to deal with. So, galactic years are the most convenient.
A GY is the amount of time it takes the earth to revolve around the center of our galaxy, the Milky Way. (Scientists believe a black hole lies in the center of the galaxy and powers it.) A galactic year could be considered a fundamental quantity, because if you lived long enough, you could measure it. As it is, because we have such short life spans, we can only estimate its length by calculating it. Estimates for a galactic year vary between 200 million earth years and 250 million earth years, but the most commonly used number is 225 million earth years.
Age of things
1. Dinosaurs. When we talk in terms of earth years, we refer to “years ago” or ya. The dinosaurs were killed off 65 million years ago. When we talk in terms of galactic years, however, we talk about how long it was since earth’s formation. So, the dinosaurs went extinct at 19.6 GY, and the current time is 20 galactic year.
2. The earth. The age of the earth in earth years is 4.6 billion years. The age of the earth in galactic years is 20 GY.
3. Life. Life began at 5 GY, or 3.6 billion years ago.
