The first thing to do is to get your head around is the scale; space is big. The largest distances on Earth pale into insignificance. The equator is twenty two thousand miles around, but geostationary satellites are further away. The moon is ten times further away than that — a quarter of a million miles. Distances between the planets are measured in tens and hundreds of million miles. The nearest star is twenty trillion miles away (20,000,000,000,000). Our galaxy is five hundred quadrillion miles across (500,000,000,000,000,000). The nearest galaxy to ours is two thousand times further than that. The furthest object found so far, is ten thousand times further away than even this vast distance. That's 70,000,000,000,000,000,000,000 miles.

Space

Fairly obviously miles are a hopelessly small unit to use. Astronomers use two other units when they are talking about distances.

Astronomical Unit

An AU is the distance from the Earth to the Sun: 92,955,807 miles

Light-year

A light year is the distance light travels in one year: 5,878,630,000,000 miles

I'll say this carefully, because it has been confused by many people, some of them science-fiction movie producers. The light-year is a measure of distance, not time.

Let's explore the light-year in more detail.

• One light-nanosecond is three hundred millimetres, or about one foot.
• One light-microsecond is three hundred metres.
• One light-millisecond is three hundred kilometres, or 186 miles.
• One light second is 186,000 miles, or not quite as far as the moon.
• One light-minute is eleven million miles.
• The sun is about four and a half light-minutes away.
• Pluto is about five and a half light-hours away. If there was a space-ship orbiting Pluto it's radio transmissions would take five and a half hours to reach us.
• The nearest star is four and a bit light-years away.
• The galaxy is one hundred thousand light-years across.
• The nearest other galaxy is two and a half million light-years away.
• The furthest object is thirteen billion light-years away

Those numbers are easier to handle, but remember, if you've got the sort of car that just goes and goes and lasts ages and never breaks down, then in its entire life it might just make it as far as the moon. One point three light-seconds.

And if you take the entire automotive production of the planet for one year (70,526,531 vehicles in 2008), and drive each vehicle to death one after the other, then you would only get two thirds of the way to the nearest star when the last one wheezed to a halt. At an average speed of 60mph it would take you thirty three million years. To put that in perspective, thirty three million years ago is half-way back to the dinosaurs; there was no such thing as a cat or a pig, let alone a human being, and grass was new.

Time

Time is something else with big numbers; a million years here, a billion there. Fortunately that is where it stops; the Universe was created only some fourteen billion years ago. But stop from time to time to consider what those numbers mean on the human scale.

• All recorded history is only five thousand years old.
• Ten thousand years ago there was no agriculture
• For two million years we were using only stone tools.
• The dinosaurs died out sixty-five million years ago
• Life evolved about three to four billion years ago.
• The sun will turn into a red giant and swallow the Earth in six billion years time.

Six billion years. You could fit recorded history into that over a million times. We could evolve from apes and create modern civilization one thousand times. We could even blow the planet up into little pieces and have it coalesce into a new planet, evolve new life, develop a new civilization, and the new guys would still have over a billion years to find some way to escape.

Space and Time

The fastest speed you can go in an Einsteinian universe is the speed of light. That's very fast — two thirds of a billion miles per hour — but it will still take years to get between the stars. Remember the closest one is over four light years away. Over four years at the speed of light.

We measure acceleration in g. One g pushes you back in your seat with the same force as Earth pulls you down. That's around about 0-60mph in 2.733 seconds.

• At 1g it would take nearly one and a half years to get close to the speed of light.
• The space shuttle accelerates at 3g
• A Saturn V rocket (as used by the Moon landings) accelerated at 4g
• At 10g untrained people pass out
• At 15g even the best trained astronaut will be unconscious

If we assume that either our spaceship is un-manned, or we have magical "inertial compensators", then at 100g it will still take five days to get to 99% of the speed of light, but only a fiftieth of a second to get to 60mph.

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Solar Systems, Galaxies and Universes