Spark image

Planetary motion

If you look up into the night sky you will see patterns of stars that we call constellations. These star patterns remain the same from night to night except that they move across the sky, rising in the east and setting in the west. However one or two "stars" seem to move compared with the others. The Ancient Greeks called these moving stars planets (from the Greek word for wanderers).

The movement of the planets is how you can tell them from stars. Venus, Mars, Jupiter and Saturn are also very bright and this also helps you to pick them out from the patterns of the "fixed" stars.

The two photographs below were taken on 6th and 16th September 2003 and show the movement of Mars (the very bright object) against the background of the stars. Notice how Mars has moved compared with the circled star.
(View the images at 150% or more to see the movement clearly)

Even more strange was that as they watched some the planets moved one way compared with the "fixed" stars and then turned and moved the other way this is called retrograde motion. After moving backwards for a while they turned again and moved in their original direction. The planet had done a sort of looping motion in the sky. (To make a complete loop might take a few months as is shown in Figure 2 below).

Figure 3 shows an enlarged section of the same area of sky.

Figure 4 shows the movement of Jupiter through the constellation Leo during the last few months of 2003 and the first half of 2004.

Figure 5 shows an enlarged section of the area of sky.

Most of the Ancient Greeks, except a few scientists such as Aristarchus, believed that the Earth was the centre of the Universe and this belief was held well into the Middle Ages especially by the Christian Church. The church said that the fact that Christ was born on the Earth made it the most important place in the Universe and so the Sun and all the planets must go round the Earth.

Unfortunately this belief made the observations of the looping motion of the planets and their apparent backwards motion at times very difficult to explain. How could a planet that was orbiting the Earth start going backwards? It just didn't make sense!

It was left to scientists to try and explain these effects.

To follow their exploration see:Theories of the Solar System

© Keith Gibbs 2020