Distance to the planets is measured by bouncing radar signals off them
and timing how long the signals take to get there and back.


  • nearby stars distance is calculated by measuring the slight shift in angle of each star in comparison to stars far away, as the Earth orbits the Sun; this is called “Parallax Shift“.
  • parallax shift, it can only be used to measure nearby stars, so astronomers work out the distance to faraway stars and galaxies by comparing how bright they look with how bright they actually are.
  • middle distance stars  –  astronomers compare colour with brightness using the Hertzsprung-Russell (H-R) diagram. (photo)
  • beyond 30,000 light-years, stars are too faint for main sequence fitting to work.
  • distances to nearby galaxies can be estimated using “standard candles” – stars that astronomers know the brightness of, such as supergiants and supernovae.
  • expected brightness of a galaxy that is too far away to pick out its stars may be measured using the Tully-Fisher technique – based on how fast galaxies spin.
  • counting planetary nebulae (the rings of gas left behind by supernovae explosions [example] ) is another way of measuring how bright a distant galaxy should be.
  • third method of calculating the brightness of a distant galaxy is to gauge how mottled it looks. (diagram)

Back to The Universe page.

Comments are closed.