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Space is really big!

In the words of Douglas Adams (author of the excellent Hitch Hikerís Guide to the Galaxy series):

“Space, is big. Really big. You just won't believe how vastly, hugely, mindbogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space!”

The distances between even the bodies in our own solar system can be hard to comprehend.

As a thought experiment, Iíd like to a walk you through an exercise to highlight just how big space is.

The Earth as a Tennis Ball

Ever seen the movie Despicable Me? This movie features a shrink ray that Gru uses to shrink the moon. Letís repeat this exercise.

Letís start by first shrinking the Earth to the size of a Tennis Ball.

A tennis ball has a diameter of approximately 6.7 cm. The average diameter of the Earth is 12,750 km, so this equates to a reduction scale factor of around 190 million to one. Zap! Done!

If we wanted to reduce the moon to the same scale as our tennis-ball Earth, what size would it become? Applying the 190 million to 1 scaling to the moonís diameter of 3,474 km, we get a new diameter of just 1.8 cm – the size of a marble!

Just how far away from the tennis ball should we place the marble moon? On average, the moon is 384,403 km away from the Earth, which is just over 2 meters in our new scale (the height of a door).

So, if we shrank the Earth to the size of a tennis ball, the moon would be the size of a marble a doorís length away. Thatís a lot of space inbetween!

Interesting side note — when Apollo 13 traveled to the far side of the moon in 1970 carrying astronauts Lovell, Swigert and Haise they reached a distance of 400,171 km from the Earth (albeit unscheduled – 100 km futher than than the planned mission). Then, and to this day, they remain the people who have voyaged the furthest from our planet.


When the Shuttle blasted off into space, it typically reached an orbit of ‘just’ 200 miles altitude (classified as Low Earth Orbit). Using our scale factor of 190 million to 1, this equates to a distance of just 2 mm. This is less than the ífuzzí on a new tennis ball!

Geosynchronous orbit occurs, on our model, at a radius of just over 22 cm (A Geosynchronous orbit, popular with many communication satellites, is an orbit in which the time period of satellite rotation around the planet matches the rotation of the planet itself; A satellite placed in geosynchronous orbit around the equator will, thus, keep the same position in the sky at all times).

Here comes the Sun …

The sun is quite a bit bigger than the Earth. In fact, the diameter of the sun is a whopping 1.391 million km. Applying our scale factor this equates to a diameter of 7.3 m in our model.

(This is more than three times the distance from the Earth to the moon!)

The diagram to the right shows just how big the sun is on our scale. It would dwarf a person stood next to it. If you look closely, this person is holding a tennis ball to scale!

Does that make you feel small? That tennis ball is the Earth!

Donít stand so close to me

Whatís wrong with the picture above? The answer is the correct spacing.

If a person were to hold the tennis ball Earth to our scale, they would not be standing so close to the sun.

The Earthís average radius from the sun is 149 million km. Applying our scale factor, this equates to 783 m. Thatís like a person holding a tennis ball seven football fields away. Thatís a lot space between them.

Yes, space is a very, big place!

How about some of the other planets? Mercury with its planetary diameter of just 4,880 km (not much larger than our own moon) would be rendered as a large marble. Being 57.9 million km away from the Sun in real life would be position at a distance of 304 m in our simulation.

Mars and Venus, at 207 million km and 107 million km respectively, would appear at 1,085 m and 564 m away from our model sun.

After the relegation of Pluto, Neptune is now, officially, the planet in our solar system that is the furthest from the sun. With a diameter of 50,000 km and an orbital radius of 4.5 billion km, to render at our tennis ball scale, weíd need to place a cantaloupe melon sized planet a distance of 23.6 km away from the sun. Yes, my non-metric friends, thatís nearly 15 miles away!

No wonder Neptune was not discovered until 1846. You need a pretty powerful telescope to spot a cantaloupe from a distance of 15 miles. Yes, space really is big.

Star light, star bright, first star I see tonight

The nearest star to the Earth (after the sun of course), is Proxima Centuri, which is 4.2 light years away. (A light year is a measure of distance that is exactly as it sounds; the distance that light travels in a year; it's about 5.9 trillion miles).

Even using our tennis ball scale, and shrinking things down 190 million times, the distances to even the nearest stars are impossible to meaningfully comprehend. On our scale, 4.2 light years converts to 209 thousand km! (And thatís just our very nearest star).

Space really is a very big place. Can you imagine traveling from a tennis ball for 209 thousand km? (Thatís like getting on a plane from New York to Seattle 54 times!)

Speaking of big things

Our sun, whilst massive to us, is not particularly large. There is a class of stars called Red Hyper Giants. The largest Hyper Giant discovered, to date, is VY Canis Marjoris, which has been determined to have a diameter over 2,000 times that of our Sun. (It's located 4,900 light years away).

On our reproduction scale that is 14.6 km high, the equivalent of 33 Empire State Buildings stacked on-top of each other. A star that size dwarfs our (mostly harmless) lowly tennis ball planet. If this star were in our solar system, it would extend past Saturn!

Remember that's 33 Empire State Buildings when reduced in size 190 million times! Space really is very, very big.

Mindbogglingly big

On a clear dark night, with the unaided eye, it's possible to view approximately 2,500 stars. This, however, is just a tiny fraction of the stars in our galaxy, The Milky Way.

Astronomers have estimated that there between 200 – 400 billion stars in the Milky Way.

Of course, the Milky Way is just one of many, many galaxies.

How many?

It's estimated there are over a 100 billion galaxies in the visible universe!

A trip to the chemist's really does seem like peanuts …

Always carry a towel

“Protect me from knowing what I don't need to know. Protect me from even knowing that there are things to know that I don't know. Protect me from knowing that I decided not to know about the things that I decided not to know about. Amen.”

Douglas Adams 1952-2001


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