# Monty Python's 'Galaxy Song'

Eric Idle's 'Galaxy Song', which appears in Monty Python's last film, The Meaning of Life, is often touted as a musical and scientifically accurate tour of the universe. It's certainly musical, but just how accurate is it?

### Verse One

Just remember that you're standing on a planet that's evolving...

Without doubt, the best explanation for life on Earth1 is evolution, as originally proposed by Charles Darwin. Put very simply, the gradual process of genetic mutation leads to changes in organisms that better allow them to survive in their current environment and, perhaps more importantly, respond to changes in their environment. Those that change most successfully will survive and pass their genes on to their descendants. While evolution is rarely visible over a human lifetime, except in experiments involving micro-organisms, it is almost certainly going on all around us.

...and revolving at 900 miles an hour

To work out the speed of rotation of the Earth, first we need to know the circumference at the Equator. One of the first estimates of this was made by an Ancient Greek mathematician called Eratosthenes in around 200BC. As we don't know what units he was using, it is difficult to know how close he came to the real answer, but his reasoning may have gone something like this...

When the sun is directly overhead in Syene, it is at an angle of 7.2° in Alexandria, 492 miles due north. The ratio of the angle between the towns to that of the whole Earth (ie, 360°) must be the same as the ratio between the distance between the towns and the circumference of the Earth. Therefore, if he had used these units, Eratosthenes would have measured the circumference of the Earth to be 24,600 miles, which is close to the true value of roughly 24,900 miles (39350 km). Estimates of how close Eratosthenes actually was range from 0.5% to 17%. Not bad for a man living over 2000 years ago!

So, having the circumference, and knowing the time it takes for the Earth to revolve (24 hours), we can calculate the speed: 1037.5 mph. This is a little way off Eric's 900 mph, unless...

Maybe Eric wasn't thinking of the Equator. The speed of revolution depends on where you stand on the Earth's surface. The further north or south you go, the shorter the distance around the Earth, but it still has to revolve in 24 hours. So, for a revolution speed of 900 mph, you would have to be standing at a latitude of around 30° north or south2. Places at this latitude include:

• North - Cairo, Bermuda, Florida, Shanghai
• South - Perth, Santiago, Cape Town, Porto Alegre

Maybe Eric was on holiday when he wrote the song.

That's orbiting at 19 miles a second, so it's reckoned...

The speed at which the Earth orbits the sun is calculated from the time taken - 365.25 days3 - and the orbital distance: 940,000,000 km. This works out to 2,573,580 km/day.

• 2,573,580 km/day = 30 km/s4
• 30 km/s = 18.8 mps5

Spot on!

...a sun that is the source of all our power

Directly or indirectly, the sun provides us with all the energy that we use.

Plants, which we eat, get their energy from photosynthesis. The animals that we eat get their energy from plants. When plants and animals die, they may eventually become the fossil fuels that we burn to generate electricity.

Solar power obviously comes from the sun, but what about other energy forms? Wind power depends on the weather, in which the sun plays an important role. With nuclear power and wave power, the link may be a little more tenuous but, when it comes down to it, the composition of the Earth and the arrangement of the planets are due to the effects of the sun at the time the solar system was being formed. Another point to Eric.

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

The speed of the sun through our galaxy is 220 km/s. However, its speed relative to the stars around it (standard solar motion) is 19.5 km/s. This works out to be 43,600 mph, 1,050,000 miles per day or, in Eric-speak, a million miles a day.

Our galaxy was named the 'Milky Way' because, on a clear night, it can be seen across the sky as a milky-white cloud of indistinguishable stars. With the naked eye, it is estimated that we can see around 10,000 stars from Earth, which is located, as Eric notes, in an outer spiral arm of our swirly galaxy.

### Verse Two

Our galaxy itself contains a hundred-billion stars

It is very difficult to work out how many stars there are in the Milky Way. Estimates vary from a hundred-billion - as Eric quotes - up to a few trillions. However, if the galaxy contains a few trillion stars, then by definition it must contain a hundred-billion. So we can give Eric that one.

It's a hundred-thousand light-years side to side

The Milky Way has a radius of around 15 kiloparsecs. A parsec is 3.262 light years, which gives a radius of 48,900 light years. This is a diameter ('side to side') of 97,860 light years. As near to a hundred thousand, give or take some artistic licence, as makes no difference. Another one to Eric.

It bulges in the middle, 16-thousand light years thick...

The 'galactic bulge' has a radius of roughly 1.5 kiloparsecs. This gives a diameter of 3 kiloparsecs, equating to 9786 light years. Not quite 16,000, sadly.

...but out by us it's just 3-thousand light years wide

One estimate for the thickness of the galactic disk - the area that contains our solar system - is 700-800 parsecs. Again using a conversion factor of 1 parsec = 3.262 light years, 800 parsecs works out at 2610 light years. Rounded up to the nearest thousand, that makes 3000.

We're 30-thousand light years from galactic central point...

Well, we used to be 30-thousand light years from galactic central point. At least, we used to think we were. Astronomers have since refined their estimates of the size of the Milky Way and the current value is in the mid-20-thousands.

...we go round every two-hundred-million years

Assuming we are 25,000 light years from the centre of the galaxy, then the path taken by the sun as it orbits the centre is given by the formula for the circumference of a circle: 2πr = 157,000 light years. As one light year is equal to 9,500,000,000,000 km, and the sun moves at 220 km/s, this all works out to an orbital period of roughly 215-million years. Perfect.

And our galaxy is only one of millions of billions
In this amazing and expanding universe

No-one really knows how many galaxies there are. The Hubble telescope can see around 80 billion, and there are undoubtedly many more waiting to be discovered. 'Millions of billions' might be a slight exaggeration. Then, again, it might not...

### Verse Three

The universe itself keeps on expanding and expanding
In all of the directions it can whizz

The current scientific consensus regarding the origins and current state of the universe is that it all began in an enormous explosion called the big bang and is still expanding outwards. This has been worked out by examining light from distant stars, which turns out to have a 'redshift' (ie, its light is redder than would be expected from the components of the star). Based on the Doppler Effect - the property of waves that causes the change in pitch of a fire engine siren as it passes by - this redshift means that the universe must indeed be expanding.

Whether or not it keeps on expanding for ever depends on the amount of 'dark matter' in the universe. If there is enough of it, gravity will eventually act to stop the universe expanding and crunch back down to nothing. If there is not enough dark matter, it will keep expanding forever.

As fast as it can go, at the speed of light, you know...

Here, Eric is probably wrong, although the precise rate is unknown. The rate at which the universe is expanding (expressed in km per second per megaparsec) is defined by the 'Hubble constant'. Fans of The Hitchhiker's Guide to the Galaxy may be amused to know that a group working at Cambridge University in 1996 calculated that the Hubble constant is 42 km/sec/MPc. Sadly, this is unlikely to be the final answer - the value will become more accurately refined as astronomical methods and technology improve - currently, the Hubble constant is believed to be 71 km/sec/MPc. In addition, the expansion of the universe may in fact be speeding up.

...12-million miles-a-minute, and that's the fastest speed there is

The speed of light, one of the fundamental constants in the universe, is 300-million metres-per-second. This equates to 18-million km-per-minute or 11.2 million miles-per-minute. It's not quite 12-million miles-a-minute, but '...11-million miles-a-minute...' doesn't scan.

If Einstein's theories of relativity are correct - and so far no-one has managed to prove them wrong - then the speed of light is indeed the fastest speed there is. Some rather esoteric experiments have shown that certain waves can be made to move faster than the speed of light, but central to Einstein's work was the tenet that, for useful information, the speed of light is the ultimate speed limit. So far, this seems to be the case.

So remember, when you're feeling very small and insecure
How amazingly unlikely is your birth

During a single menstrual cycle, the chance of fertilization in a 'normal' couple6 is around 20-25%. Of those eggs that are fertilized, as few as one-third survive to be born.

Add to this the fact that the precise combination of genes to make you who you are have been inherited, generation by generation, from your ancestors over thousands of years, and the birth of any given individual seems unlikely in the extreme.

And pray that there's intelligent life somewhere up in space...

The dream of finding intelligent life, or life at all, on planets other than Earth is one that a lot of time and money is spent chasing. For example, the SETI project ('Search for Extra-Terrestrial Intelligence') involves computer users all over the world, who can download software that allows their computer to use its spare processing power to analyse astronomical data in the hope of detecting messages from friendly (or otherwise) aliens.

...'cos there's bugger-all down here on Earth

Quite.

1Unless you happen to belong to a fundamentalist group believing in Creationism, in which case your definition of 'best explanation' might be a little different.2The formula is Length of Parallel = Length of Equator * Cos(Latitude).3The '.25' explains why we need leap years.486,400 seconds in a day.51 km = 0.621 miles.6Although what constitutes 'normal' in this sense may be a sensitive issue. See 'The Psychiatrist' episode of Fawlty Towers for an example.