Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Early astronomical observations

The observations of early astronomers enabled them to determine the following: 

  • There is an unchanging pattern of stars, revolving daily round an axis through the Pole Star. 
  • Sun, Moon and planets share that daily motion, except they drift slowly backward through the star pattern. 
  • The paths of the Sun, Moon and planets fall in a narrow band of the star pattern called the Zodiac. 
  • ‘Freezing out’ the daily rotation, we find the Sun travels round the ecliptic, the central line of the Zodiac, in a year. 
  • The Moon travels round an orbit in the Zodiac, tilted at some 5° to the ecliptic, in a month. 
  • Mars, Jupiter, and Saturn travel round orbits in the Zodiac, making reverse loops, one for each Earth year, as they do so.
  • Mercury and Venus are only visible shortly after sunset and before sunrise – they remain close to the Sun. 
  • Jupiter completes an orbit of the Sun in 12 years, Saturn in 30 years, Venus in a fraction of an Earth year