Download Planetary motion

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)
Figure 1
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).
Aquarius
Venus
28th July 2003
Figure 2
Mars
26th September 2003
Figure 3 shows an enlarged section of the same area of sky.
1
22hr 40m
22hr 50m
22hr 30m
22hr 20m
Uranus
12o
26th October 2003
8th July 2003
28th June 2003
28th July 2003
16th October 2003
14o
18th June 2003
MARS
27th August 2003
16o
26th September 2003
th
6 September 2003
Figure 3
Figure 4 shows the movement of Jupiter through the constellation Leo during the last few
months of 2003 and the first half of 2004.
30o
15o
10hr
11hr
12hr
Jupiter
Mercury
Figure 4
0o
Venus
2
Figure 5 shows an enlarged section of the area of sky.
20th September 2003
JUPITER
10o
2nd May 2004
21st June 2004
17th February 2004
29th December 2003
5o
10th August 2004
11hr 10m
11hr
10hr 50m
10hr 40m
Figure 5
Now 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 open the file called "Theories of the Solar System"
3