Galaxy Song

by Eric Idle

From the movie "Monty Python's Meaning of Life"

(Annotated by S. C. Smith)

Whenever life gets you down Mrs. Brown, and things seem hard or tough, and people are stupid, obnoxious or daft and you feel that you've had quite enough...

Just remember that you're standing on a planet that's evolving
And revolving at 900 miles an hour

Speed of revolution should actually be measured in terms of angular velocity, since the actual speed varies for different points on the Earth. The angular velocity of the Earth's revolution is 1 rotation per day. The radius of the Earth, $R\approx 6378 \mbox{km}\approx 3827 \mbox{mi}$. The circumference of the Earth is thus $2\pi R\approx 40054\mbox{km}\approx 24032\mbox{mi}$. A point on the equator therefore travels a distance of about 24,000 mi in 24 hr (1 day) is thus traveling at a speed of  1000 mi/hr. Speeds are lower at other locations, and a point on the poles has a speed of 0 mi/hr.
That's orbiting at 19 miles a second,
So it's reckoned
The orbit of the Earth has a radius of $\sim 1.5\times 10^{11}\mbox{m}\approx 9.3\times 10^7\mbox{mi}$, which is defined to be 1 astronomical unit (AU). The Earth thus travels a distance of $2\pi R \approx 5.84\times 10^8\mbox{mi}$ in $3.16\times 10^7\mbox{sec}$ (1 year) at an average speed of  18.5 mi/sec.
A sun that is the source of all our power

The sun and you and me and all the stars that we can see
Are moving at a million miles a day
In an outer spiral arm, at 40,000 miles an hour
Of the galaxy we call the Milky Way

In fact, the speed is closer to 3 times that number, if you count only the rotational motion of the galaxy, and approximately 20 times that number if you count the motion of our galaxy with repect to the cosmic background radiation of the universe.

Our galaxy itself contains a hundred billion stars
It's 100,000 light years side to side

A light year is the distance light travels in one year. $1 \mbox{ ly}\approx 9.46\times 10^{15}\mbox{m}\approx 5.9\times 10^{12}\mbox{mi}$.
It bulges in the middle, 16,000 light years thick
But out by us it's just 3,000 light years wide

We're 30,000 light years from galactic central point

While this number is somewhat uncertain, ranging from 23000 ly to 32000 ly, the currently accepted value is approximately 28000 ly.
We go round every two hundred million years
Current estimates put the length of the galactic year at 225 million years.
And our galaxy is only one of millions of billions
In this amazing and expanding universe

The universe itself keeps on expanding and expanding

The universe is known to be expanding because all distant galaxies are seen to be receding from us, with a velocity proportional to the distance from us.
In all of the directions it can whiz
As fast as it can go, the speed of light you know
The observable universe expands at the speed of light. The actual physical universe may expand at some other velocity.
12 million miles a minute and that's the fastest speed there is
The speed of light is $\sim2.99\times 10^8\mbox{m/s}\approx186,000 \mbox{mi/s}\approx 11.2 \mbox{million mi/min}$. According to known physical laws, it is impossible for anything to travel faster than the speed of light (relative to local space).

So remember when you're feeling very small and insecure
How amazingly unlikely is your birth
And pray that there's intelligent life somewhere up in space
Because there's bugger all down here on Earth

Scott Smith