Mars is sooooo "Retro", Dude!

All of the planets in our solar system rotate around the Sun in the same direction. The outer planets move slower than we do. Most of the time, the planets appear to be moving in the "correct" direction around the field of stars in the background (the constellations that are relatively "fixed" in space). But, occasionally, the planets appear to be going "backward" in the sky relative to the background stars.

Now, this is not a motion that you would see in a given night ... you would have to mark the position of the planet over the course of several weeks to really get a sense of the different motion.

We call this motion "retrograde" (going against the normal motion). Also, this retrograde motion always occurs when the planet is in "opposition" with the Earth. (This means that the planet lines up along the Earth-Sun line ... it goes Sun - Earth - planet - thus, the Sun and the planet are on "opposite" sides of us.) Since we move faster than the outer planets, we "catch up and pass" the planets when we are near opposition. Since our relative motion around that opposition is much faster than the outer planet, we will be "ahead" of the planet, and looking back .. thus we will see it against a different star field.

Mars is a much more interesting planet to investigate, because it has a relatively large tilt to its orbital plane (relative to the Earth Sun orbit plane), and it the closest outer planet. (The far outer planets don't exhibit as large a retrograde motion, and it is confined to the curve of their orbit - Mars exhibits loops based on its orbit tilt and relative speed.)

I'll try to put some more information here, but for the moment .. I wrote a program to illustrate this motion for certain opposition dates in the past (and one next year).

First, some background:

• If we look "down" on the solar system from "above" - all the planets appear to be going counter-clockwise in their orbits.
• We can measure how far they are around in their orbit from an angle relative to a fixed direction in space.
• That fixed direction is called the "point of Aries", and is the direction of the line that goes from the Earth to the Sun on the first day of Spring.
• As we look down on the solar system, we can put the "zero" angle as East (this would be "Fall" in the Earth's orbit).
  • Thus the 90 degree mark (North) would be "Winter" .. 180 degrees (West) = "Spring" and 270 degrees (South) = "Summer"
• If Mars and the Earth are in opposition, they would have the same angle relative to the Sun.
  • Thus, a line drawn between them would pass through the Sun.
  • So, if we plot the Earth and Mars for several weeks before and after opposition, and connect the planets, we can see that line change.
• The background stars can seen as a "grid" with two coordinates, Right Ascension (RA) and Declination (DEC).
  • Right Ascension is the angle measured around from the point of Aries - on the Earth-Sun plane (the "ecliptic")
  • Declination is the angle above or below that ecliptic
    • Thus, the RA/DEC is a fixed system .. the stars {relatively} always have the same RA and Dec
• Plotting the position of Mars on a RA vs DEC graph will illustrate the retrograde motion (backward motion relative to the starfield).
• We could also plot the RA and DEC on a graph that illustrates a whole year of constellations (to see where it is in the sky depending on when it happens).

Ok, so I have a program that illustrates this above information. It is designed to run on any Windows platform (3.x, 9x, NT, etc.). There are about 9 dates chosen to give a set of different views. The data used to create the program is an approximation, but a pretty good one, I think. There are going to be some improvements to the program in the relatively near future - but I welcome any comments also!

Download the program, and give it a try. Let me know what you think!

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