How far will you fly around the galaxy?


The galactic year is the length of time it takes our sun - and the entire Solar System that goes with it - to orbit the centre of the galaxy.

Space is, as Douglas Adams once said, really mind-bogglingly big. That's why the galactic year is correspondingly mind-bogglingly long: somewhere between 225 and 250 million of our puny Earth years. When you have something as big as our galaxy, it takes a long time to get round it. Even just once.

One galactic year, then, is a long time even on the geological scale. Loads of stuff can happen in 250 million years. Entire species can evolve, reach their peak, then decline. Mountain ranges can be built and eroded down again. Rivers can cut valleys, silt up, shift position, disappear and be replaced by deserts.

Just one galactic year ago, Earth was undergoing the Permian-Triassic extinction event. Scientists don't know for sure what caused it, but it was a catastrophic event for the life on the planet at the time. Most of everything that was thriving at the end of the Permian got wiped out. Good news for us, because we had the rest of that galactic year to evolve from whatever reptilian proto-mammals we were at the time, and become what we are now.

Earth itself has only existed for a little under 18 galactic years (this is assuming that Wikipedia's numbers are correct - a big assumption I know, but we have to make some assumptions somewhere for this). For the first three or four galactic years (about a billion Earth years), it was a lifeless ball of rock. Then water began to gather into oceans, and the fun stuff started.

The galaxy itself has only been around, says Wiki, for about 61 galactic years. That's the entirety of elapsed known time. Before that, there was the Big Bang, or whatever weird spacetime mashup existed before it.

Thinking about all this stuff makes me wonder something else: how far will a human being travel during their minuscule micro-fraction of a galactic year of life? If we assume our Solar System's orbit around the centre of the galaxy is a circle, how much of that circle will one human lifetime experience? A tiny, tiny fraction of course. But how tiny?

Before we go any further, you should know: maths is not my strong point. I'm a words man. Words make me happy. Numbers make me shiver a bit. So everything that follows and involves mathematics might be very, very wrong. (Corrections will be welcomed.)

On its page about the Milky Way, Wikipedia says:

The orbital speed of the Solar System about the center of the Milky Way is approximately 220 km/s or 0.073% of the speed of light. At this speed, it takes around 1,400 years for the Solar System to travel a distance of 1 light-year, or 8 days to travel 1 AU (astronomical unit).

One AU is the same as the distance from the Earth to the Sun. And our Solar System travels that far around the galactic centre in eight days.

In a year, then, the Solar System moves 45.6 AU.

And over the average human lifetime of 70 years, that comes to 3200 AU - about half way between here and the Oort Cloud. That makes my head spin a bit. Space travel is so staggeringly difficult when we attempt it deliberately. But here we are, travelling so far and so fast, and even at this velocity it still takes us geological-scale time periods to complete one circuit around our average-sized galaxy.

That is how really mind-bogglingly big space is.

My conclusion? Life is short, yes. But it goes a long, long way.

(15th November 2014. Thanks to Paul, Matt and Phil for correcting my mathematical errors.)

Note: If I'm still wrong, and you know or can calculate any different, please get in touch and tell me. I'll cheerfully update this page.