Grade 9 Academic Science – Unit 3 Space
Retrograde Motion
Section 8.3
Early star-watchers quickly saw a difference between planets and stars in the night sky…due to
the apparent movement of the planets relative to the fixed position of the stars. Thus, the planets
became known as the “wanderers.”
Logically, we think that a wandering planet would move slowly in one “forward” direction
(eastward) across the night sky against the background of the fixed-location stars…and for most
of the year, this is accurate. Yet to our surprise, a planet will change its direction of travel, go in
the exact opposite direction (westward), and after a while, stop again and begin traveling again in
a forward (eastward) direction. This is called RETROGRADE MOTION…so much for our simple
logic.
Why Retrograde Motion
 Nicholas Copernicus devised the heliocentric model (Sun-centred) of our Solar System.
In his model, the planets orbit the Sun. Thus, the Earth orbits the Sun (i.e., we are not
stationary).
 The planets orbit the Sun at different rates. The Inner Planets move faster than Earth
and complete one Sun-orbit in less than one Earth year. At the same time, the Outer
Planets move slower than Earth and one Sun-orbit takes longer than one Earth year.
 As Earth moves, the other planets also move.
 The planets orbit the Sun in an elliptic pattern. Moreover, the Sun is not at the centre of
the ellipse.
To watch an illustration of Retrograde Motion for Mars and Venus, go to http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::800::600::/sites/dl/free/0072482621/78780/Retro_Nav.s
wf::Retrograde%20Motion
Task
Purpose: To demonstrate the cause of the apparent retrograde motion.
Hypothesis: The Earth overtakes and passes a planet as the two planets orbit the Sun at
different rates causes the apparent retrograde motion of a planet.
Earth
Mars
150,000,000 km
228,000,000 km
Approximate Period of
Revolution (Earth Days)
365
687
Circumference of Orbits
942,000,000 km
1,431,000,000 km
Radius of Orbit
Calculate the distance Earth and Mars each travels along its orbit for one Earth day.
Earth _____________________ Mars ___________________
Do the Earth and Mars travel at the same speed?
Which planet moves faster?
You are looking at a heliocentric model showing the orbits of Earth and Mars. As you know, Mars
moves more slowly than Earth. For example, the distance Earth travels in Time 1 to Time 2 is
greater than the distance traveled by Mars in the same time period.
The curves on the right are the background lines. This is where you will see background stars.
With straight lines, connect all the same-numbered points along the two orbits, extending each
line to intercept the same-numbered background line located on the right, and mark and label
each point where your line crosses the background line. For example, draw a straight line from
Earth 4 to Mars 4, extending it to the right until the line meets the curve labeled 4, and mark the
position where they meet with a dot and a “4”. Repeat for all numbers. Draw a smooth curve
through the dots you marked to show the path of Mars against the background sky.
Let’s see it from Earth….our relative position
Table 1 gives the data of Mars positions observed at the 1st and 15th of each month from May to
August. The positions are recorded in AZIMUTH and ALTITUDE.
 The azimuth is measured along the horizon in degrees, starting at north, going through
east, south, and west in a circle.
 The altitude is measured upward from the horizon in degrees.
We will use the data to see how Mars has moved over the course of four months and examine
how we can use a heliocentric model to explain its retrograde motion.
Table 1. Positions of Mars
Observation Date
Azimuth (Horizontal Direction)
Altitude (Vertical Direction)
May 1
240
45
May 15
210
50
June 1
170
50
June 15
150
45
Task



July 1
170
40
July 15
180
45
August 1
140
50
August 15
120
55
Using the Table 1 data, plot the motion of Mars on the graph provided.
Write down the dates next to each position you plot.
Draw a smooth line connecting your data points to illustrate the path of the planet through
the sky.
According to your figure, when was Mars located farthest to the west?
What was the azimuth value of Mars on this date?
On what date was Mars located farthest to the east?
What was the azimuth value of Mars on this date?
After completing Figure 1, you see that Mars was moving in different directions at different times.
List the dates Mars moved from east to west, and the dates Mars moved from west to east,
respectively.
During which dates does Mars appear to move with normal (prograde) motion as compared to the
background stars? In what direction (east-to-west or west-to-east) does Mars appear to be
moving relative to the background stars during this time?
During which dates does Mars appear to move with backward (retrograde) motion as compared
to the background stars? In what direction (east-to-west or west-to-east) does Mars appear to be
moving relative to the background stars during this time?
If a planet were moving with retrograde motion, how would the planet appear to move across the
sky in a single night? Where would it rise? Where would it set?
Suppose I said, “Mars is moving with retrograde motion tonight and will rise at midnight.” A fellow
student makes the following statement: “Mars is moving with retrograde motion. During the
night, it will be moving west-to-east rather than east-to-west. Thus at midnight, it will rise in the
west and move across the sky and then later set in the east.” Do you agree or disagree with the
student? Explain your reasoning.
Compare your two figures. Provide two observations (e.g., At what positions in your first figure do
the dates of prograde motion occur?)
Retrograde Motion Laboratory
Due Date for Laboratory Report:
Purpose: To learn about the retrograde motion of the planets
Hypothesis
Materials
 Length of string
 Thumb tack or masking tape
 Tape
 Tape measure or ruler
 Coloured markers
 Two pieces of coloured paper (...each a different colour)
 15 pieces of white paper
 Protractor
Methods
 Cut each piece of coloured paper into 12 smaller Marker Squares and label each square
starting at 1 and ending at 12
 Tape 15 pieces of white paper on the wall side-by-side. This is the Night Sky.
 Fasten your string to a point on one edge of the desk / counter top. This is the Sun.
 Measure a distance along the string at 40 cm and 60 cm. The first measurement
represents the distance from the Sun to Earth, while the second measurement is the
distance from the Sun to Mars
 At an angle of 15O, place Marker Square #1 for both Earth and Mars
 For Earth, measure 15O around the circle (e.g., 30O, 45O, 60O...165O) placing Marker
Square #2 at 30O, Marker Square #3 at 45O and so on.
 Repeat this step for Mars EXCEPT measure only 6O between each marker (e.g., 21O,
27O, 33O, 39O) until all 12 Marker Squares for Mars are placed.
 Using your string, line up Earth Marker Square #1 to Mars Marker Square #1 and extend
the string to the paper on the wall. With a marker, mark and label the spot #1
 Repeat this step for all other markers from 2 to 12
 Draw a line on the white paper joining each spot (e.g., 1 - 2 - 3 - 4 - )
 Record your observations
 Repeat the lab for Mercury. Mercury distance along the string is 16 cm and the distance
moved for each marker is 30O).
 Record your observations
 Prepare a complete lab report of your findings.
Retrograde Motion
Background: Planets tend to move across the sky in an easterly direction.
Occasionally, something strange occurs. A planet appears to slow down and begin
moving backward to the west. In this activity you are going to find out why this
happens. The diagram below represents a part of our solar system. Earth (inner
circle) and Mars (outer circle) are shown at several positions in their orbits around
the Sun (central small circle). Each point on the planet’s orbit shows where it is at
that month.
Procedure
1. Look at the diagram below, where a straight line has been drawn from Earth’s position in
January to Mars’ position during the same month. The straight line extends past the
dotted line on the right of the page, where a third dot is plotted. Mark this point “JAN”
2. Draw a line connecting the positions for the months Feb, Mar, April, May, June, July,
Aug, and Sept. Make sure to extend each line approximately 1 cm past the dashed line.
Place a dot at the end of the line and label the dots in order. Note: If paths cross, draw
the lines slightly long and place the dots slightly farther away than you did for the other
lines.
3. Using a pencil, start with the dot labeled "1" and carefully connect the dots in order (This
line represents the path the planet Mars follows in its orbit as seen from Earth.)
1
The dots that you put at the ends of the lines represent the positions where an observer on Earth
would see Mars for the month indicated on the diagram. The line you drew connecting the dots
represents the path Mars appears to follow.
Critical Thinking and Application
1. During which months does Mars appear to be moving backward in its orbit,as seen from
Earth?
2. Carefully observe what is happening to Earth and Mars in their orbits when Mars seems
to loop "backward." Why does it look as if Mars moves backward in its orbit? (Hint: what
is Earth doing when Mars seems to move backwards)?
3. To an observer on Earth, would ALL the planets visible in the night sky appear at some
point to go backward?
4. Name two other planets that WOULD go backwards: _______________ and
_______________
5. Name two planets that would NOT go backwards: _______________ and
_______________
6. Explain why the planets from #5 would not follow Mars’ same pattern.