What Is an Astronomical Unit?

British spelling

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What is an astronomical unit?

We are dazzled by the sun's light and feel its incredible heat, but how far away is it?

One AU is the distance between the sun and the earth.

To put that distance into perspective, light leaving the sun, travelling almost 300,000 kilometres per second takes just over 8 minutes to reach the earth.

An astronomical unit is roughly 150 million kilometres or 93 million miles.

An aircraft can't fly in space, and it would burn up long before it reached the sun, but hypothetically, a Boeing 737 aircraft leaving the Earth would take over 18 years to reach the sun.

Now imagine 11,772 Earths lying side by side in a straight line; that distance would be equivalent to one AU.

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The sun is nothing special, it's just another star.

There could be roughly 200 billion trillion stars in the observable universe. Wow!

Looking up at the night sky, we see stars as pinpricks of light.

Our local star, the sun, differs as it’s very close to the Earth. We are dazzled by its bright light and feel its tremendous heat.

But what is a star?

A star is an enormous, luminous ball of gas made mostly of hydrogen and helium, all held together by its own gravity.

The reason it is bright and extremely hot is because of the nuclear reactions that are taking place in its core.

Photons of light generated in the core of a star can take up to 200,000 years to reach the surface. Then they can escape and travel through space at 1,080 million kilometres per hour.

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We all live on one, but why do planets exist?

So what does it take to be a planet in a star system?

It must be in orbit around a star; it needs to be big enough for gravity to pull it into a spherical shape; and the third requirement is that it can clear away large objects near its orbit.

Stars, including our sun, are created in massive interstellar molecular clouds of dust and gas. Over time, gravity pulls together large amounts of material that eventually form a star.

During that process, which takes millions of years, leftover material swirls around a new rotating star. It is inside this swirling disc of material that planets slowly form.

Today, we are familiar with our solar system, which has eight planets and countless other celestial objects in orbit around the sun, including our magical home, the Earth.

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A footprint left in the sand.

Considering the age of our planet, the human race has existed for a very short time.

Our local star, the Sun, is estimated to be just over 4.6 billion years old, and the magical world we call home is just slightly younger.

Billions of animal species have lived on the Earth, and most are now extinct, but there is still an abundance of complex life on the land, in the oceans, and in the lakes.

The Earth was less than a billion years old when primitive life arose.

Look at the image above; it has taken the evolution of life over 3.7 billion years before that human footprint could be left in the sand.

Evolution needs vast periods of time; our lives are not long enough to see the tiny changes taking place.

The end.

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