Experiment 3: Retrograde motion of Planets

Objective :

To identify the retrograde motion of Planets with respect to the Background stars.

Introduction :

Retrograde motion refers to the change of direction of the planets as they wander through the fixed background of the stars. All planets show retrograde motion when viewed from Earth with respect to background stars.

Superior Planets show retrograde motion when they are in opposition, Inner planets Venus and Mercury appear to move in retrograde in a similar mechanism, but as they can never be in opposition to the Sun as seen from Earth, their retrograde cycles occur when they are in lower conjunctions with the Sun. For example, Mars appears to move monotonically in one direction but then suddenly during the time of opposition, appears to move backwards making a retrograde loop. Later it again shows motion in forward direction. Although it actually does not move in opposite direction, following figure illustrates the motion-

(source : http://todayastrology.info/retrograde-motion-of-mars.html)


To view the retrograde motion of mars using stellarium follow the steps-

  • Step 1: Start the software Stellarium. You may set it in such a way so that the entire sky is visible.

  • Step 2: Select a date around which Mars comes closest to Earth and then moves away. You will need to experiment on this by looking at different dates.Find the dates when planet is near opposition. Select a set of background stars with respect to which you will measure the angular position of Mars. Visually trace the angular position of Mars.

  • Step 3: Now view the motion of the Mars with respect to background stars by incrementing the units of sidereal day. Recall that in one sidereal day, all the stars come back to their original location. Note that, the motion of the mars with respect to stars (say Canopus).

  • Step 4: In order to do a virtual observation of this retrograde motion of Mars, you may set the date to September 2009, which is one of the closest approaches of Mars to Earth. Now follow the following steps to see the Mars motion :

    • Locate the Mars

    • Insert the planet's orbit. (Press 'O')

    • Insert the planet's label. (Press 'P')

    • Remove the Atmosphere and Ground. (Press A and G)

    • Make the horizon round to see the whole sky through a hole. (Press 'up' and 'down' key)

    • Zoom in and out to get the whole horizon and Mars' orbit. (Use 'Page up/down')

    • Now you increment the sidereal day by pressing 'Alt+='.

    Historically, these apparent epicycles of planets have been an intense topic of discussion in Astronomy.

    (Please click here for larger view : http://youtu.be/ZprrYdzrBhQ)

  • Step 5: On a plain paper make a latitude-longitude grid to label the coordinates RA and Dec. Plot the positions of the choosen stars on the paper. Now plot the position of Mars at different times. The positions of stars will remain fixed. Identify the retrograde loop of Mars on your plot. You must clearly indicate the date/time of the observation.

Alternate Method- To Observe the Retrograde Loop of Mars using stellarium follow the steps :

  • Start Stellarium.

  • Remove Ground, Atmosphere and Fog by Pressing G, A and F. Press , to view Ecliptic line.

  • Remove the Grid view if grid is visible (Use E and Z).

  • Go to Sky and Viewing options window (Press F4). In Marking section →constellation → check the two boxes next to 'Show Lines' and 'Show Labels'. you can also use shortcut C and V.

  • Press P to view Planets, select 'Mars' and press [space] to center it, Press F11 for full screen mode.

  • Now Press 'Shift + T' to view 'Planet Trails'.

  • To observe retrograde motion set the date to 11 Nov 2011, that is when Mars is viewed in the constellation of Leo near Regulus or any other date when retrograde motion of Mars is apparent (Use F5).

  • Now (Press = ) to increase 1 solar day, keep on increasing the days, you will observe the track of planet Mars making a loop as it moves retrograde in constellation of Leo.

  • You can also (Press O) to view its orbit and observe how it moves.

  • (Please click here for larger view : http://youtu.be/xtuf-FShJM0)

    Retrograde motion of other planets can also be viewed using similar procedure.