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Key points

  • The distances between objects in space are enormous. They are often so large that standard units of measurement, like metres or kilometres, are replaced by alternatives that can cover the huge distances in space.
  • Scientists and often use units of to talk about the distances to distant objects, such as and .
  • A light year is the distance travelled by light in one year and is approximately equal to 9.46 trillion kilometres or 9,460,000,000,000 km.
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Size and distances in space

The solar system

Sometimes metres and kilometres are used to measure objects and distances in space, especially if they are small or within our solar system. A light year would be an inappropriate unit of measurement in these circumstances.

The sizes of , , and can be measured in metres and kilometres.

It's even possible to measure the distance from a planet to its moon, or from a to the in metres or kilometres.

For example:

  • Earth has a diameter of approximately 12,700km.

  • The distance from Earth to the Moon is approximately 384,400km.

  • The distance from Earth to the Sun is approximately 150 million km (150,000,000 km).

Earth has a diameter of approximately 12,700km. The distance from Earth to the Moon is approximately 384,400km. The distance from Earth to the Sun is approximately 150 million km (150,000,000 km).

Beyond the solar system

The distance to the next nearest is millions of times larger than the distance between planets in the .

The distance between one galaxy and another is millions of times larger than the distance between the stars in a galaxy.

For example:

  • The distance from the Sun to , our next nearest star, is approximately 40 quadrillion kilometres (40,208,000,000,000 km).
  • The distance from our galaxy the , to Andromeda, the next nearest spiral galaxy, is approximately 24 quintillion kilometres (24,000,000,000,000,000,000 km).

This means that the numbers used to describe and measure distances in space can become too big to understand, compare and write down.

To get around this problem, astronomers and scientists often use alternative units of measurement, such as light years, to measure astronomical distances.

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The light year

A light year is a unit used to measure distance, not time. One light year is the distance travelled by light in one year.

Light travels very quickly at around 300,000,000 metres per second (m/s).

There are approximately 31,540,000 seconds in the average year.

We can combine this data with the formula connecting speed, distance and time to work out how far one light year is in metres:

\( Distance = speed \times time \)

\( Distance = 300,000,000 m/s \times 31,540,000 s \)

\( Distance = 9.460 \times 10{^1}{^5} m \) or \( 9,460,000,000,000 km \)

Therefore one light year is equal to approximately 9.46 trillion kilometres (9.46 quadrillion metres).

This unit is used for astronomical measurements, such as the distance to a star, the diameter of a galaxy and the distance between galaxies.

For example:

  • The diameter of the Milky Way, is approximately 106,000 light years.
  • The distance from the Sun to Proxima Centauri, our next nearest star, is approximately 4.25 light years.
  • Barnard's Star is a red dwarf star and is the fourth nearest star to the Sun at approximately 6 light years distance.
  • The distance from the Milky Way to Andromeda, the next nearest spiral galaxy, is approximately 2.5 million light years.
Distances in light years
Figure caption,
Distances in light years to other stars

Which is the closest galaxy to the Milky Way?

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Other units of measurement

There are other units of distance that are useful to scientists and astronomers when referring to space.

These include:

  • : The distance light travels in one minute. Light from the Sun takes approximately 8 minutes and 20 seconds to reach Earth, so the distance between Earth and the Sun is 8.3 light minutes.

  • : The distance from the Earth to the Sun (approximately 150 million km). Mars is around 1.5 AU from the Sun, and Jupiter is approximately 5.2 AU from the Sun.

  • : One parsec is approximately 3.26 light years. Scientists and astronomers sometimes use parsecs to express the distance to stars or galaxies, instead of light years.

The Andromeda galaxy
Image caption,
The Andromeda Galaxy is 220,000 light years across and is over 2.5 million light years from Earth
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Test your knowledge

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Play the Atomic Labs game! game

Try out practical experiments in this KS3 science game.

Play the Atomic Labs game!
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