Last week I passed an important milestone and, as one does these days, I sent a WhatsApp to the family group stating that I was starting my sixty-first orbit of the Sun. What I thought was a very simple statement, caused great puzzlement amongst its (non-scientist) readers and, after explaining it to them, they found it a thought-provoking and interesting, not to mention, amusing way of expressing the simple fact that it was my 60th birthday.
Of course, a year is no more and no less than the time that it takes for the Earth to orbit the Sun. So, someone who, like me, has reached 60 has made sixty orbits of the Sun aboard Spaceship Earth. It is not rocket science.
Then, I started to think about what that number implies.
Everyone learns at school that the circumference of a circle is calculated by multiplying:
Astronomy makes the calculation simple for the Earth because we define the radius of the Earth’s orbit as being exactly 1 Astronomical Unit – 1AU, for short.
So, each year, you, me and everyone else on Earth travels:
2×3.142×1 = 6.284 AU
That is a pretty good distance. If you go 6.284AU outwards from the Sun, you go well past the orbit of Jupiter, two-thirds of the way to Saturn. Not bad for a year of travel. Better than most frequent fliers.
So, in my 60 years of planetary commuting, I have travelled
60 x 6.284 = 377 AU
That is more than 9 times the distance of Pluto or, if you prefer, twelve and a half times the distance of Neptune. It is also 0.006 light years.
Okay. Most people find it hard to envisage an Astronomical Unit and even less a light year. So let’s convert it into units that we understand better… perhaps.
1 AU = 149.6 million kilometres
So, in sixty years, I have travelled a modest
56 405 200 000 kilometres
That is 56.4 (American) billion kilometres. It starts to be a seriously impressive number.
However, that is not the full story, because not only is the Earth travelling around the Sun, the Sun itself is travelling around the centre of the Milky Way Galaxy, making a circuit every 225 million years.
Now, that sounds rather slow but, because our Galaxy is rather large and our Sun is in an outlying part of one of the spiral arms – the Carina-Sagittarius Spiral Arm – it actually converts to a quite decent turn of speed.
How decent? Well… 220km/s.
Now, there are 31536000 seconds in a year. So, in my sixty years, the Sun has travelled:
60 x 31536000 x 220 = 416 275 200 000 km
So, we sum the distance that Spaceship Earth has travelled around the Sun and the distance that the Sun has travelled around the Galaxy and we get a total of:
56 405 200 000 + 416 275 200 000 = 472 680 400 000 kilometres
However, there are two more movements that we have to take into account if we want to see how far I have travelled in my life: the rotation of the Earth and the Sun’s up and down movement in the Galaxy.
The Earth is spinning. It is the reason why the Sun rises in the east, crosses the sky and sets in the west. We are not generally aware of it, but that spin is the reason why weather systems on Earth (hurricanes, cyclones, high and low pressure systems, …) spin too.
The Earth spins at 1670km/h at the Equator (slower, as you go north and south, which is why countries go to some lengths to find launch sites as close to the Equator as possible, to take advantage of this boost when launching rockets into space – it is also why launching a satellite into a “wrong way” orbit, that is, east to west, is very hard work).
Let’s assume, for simplicity, that I have lived at the Equator. There are 8760 hours in a year. So, in sixty years, I will have rotated:
1670 x 8760 x 60 = 877 752 000 km
That is actually a rather small number compared to the Earth’s orbital velocity about the Sun and the Sun’s orbital velocity about the Galaxy. Still, we will add it in.
Similarly, the Sun wobbles up and down in the Galaxy as it orbits the Galactic Centre. At present, we are no less than 50 light years “above” the plane of the Galaxy. This upwards movement amounts to about 7km/s, which adds another quantity to our yearly galactic commute.
7 x 31536000 x 60 = 13 245 120 000 kilometres
So, our grand total is now:
486 803 272 000 kilometres
Are we finished? Well, actually not…
The Milky Way Galaxy itself is moving within the Local Cluster of galaxies, which includes the Andromeda and Triangulum Spirals and the two Magellanic Clouds, plus a host of faint, dwarf galaxies.
And our Local Cluster is moving within the Virgo Cluster of Galaxies, which is itself falling into the Coma Supercluster of galaxies.
It all adds up to 600km/s of movement of the Milky Way Galaxy that we must include to sum up all the different trajectories that we follow every second of our lives. So, let’s ring up that number too:
600 x 31536000 x 60 = 1 135 296 000 000 kilometres
So, how far have I travelled in my “60 orbits of the Sun”?
The grand total now is:
1 622 099 272 000 kilometres
0.17 light years
So, even if I have only travelled about 4% of the distance to Alpha Centauri in my sixty years, I can still consider myself pretty well travelled. Astronomically well travelled, in fact.
Maybe you have children. Just think that, when your child is 10 years old, he or she has already travelled:
2 703 517 000 000 kilometres = 0.03 light years
Your children can consider themselves well-travelled, in a Galactic sense, too! And, the amazing thing is that we are none of us really aware of the fact that we move so far through space every year, every hour, every second of our lives.