Demonstrate an understanding of how astronomers use 21 cm radio waves rather than visible light to determine the rotation of our galaxy.

It is more accurate to study the rotation of our galaxy by radio waves because we can only see part of the sky with visible light.

The 21cm (centimetre) range is known as the Hydrogen line which allows us to see past clouds of interstellar cosmic gas and dust. The centre of the galaxy has a lot of this.


The Sun is around 30,000 light years from the centre of the Milky Way. This is about two-thirds of the way from the centre, along one of the spiral arms.

Like the planets orbiting the Sun, the Sun in turn orbits the centre of the Milky Way along a spiral arm.

It takes 226 million years to make a galactic orbit.

The Sun is positioned on a spiral arm called the Orion Arm.


Animation New Window

Milky Way animation Milky Way

Image New Window

Links New Window

SEDS Milky Way Data

Starchild Milky Way


  • Why do we use 21cm radio wavelengths to measure the galaxy’s rotation?
  • How long does it take for the Sun to orbit our galaxy?
  • Assuming the Sun is 5 billion years old, how many orbits has it made in its lifetime?