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The Question

(Submitted February 17, 1997)

Hi. My friend and I are working on a small project about the oppositions of Mars and we had a few questions. We calculated the synodic period, and we were wondering why some intervals are longer than others? Also, we know that favorable oppositions of Mars occur in August, but why is this so? Is the month of unfavorable oppositions in March? And last of all: Mars will be at opposition on March 17, 1997. On March 21, the Sun is viewed from the earth in the direction of the Vernal Equinox. What would the Mars-Earth separation be (the distance) in March 1997, and would this be a favorable opposition? We hope you can help us! Thanks for your time.

The Answer

I'm not sure exactly why some opposition intervals are longer than others. I can tell you why some oppositions are "favorable" and some are "unfavorable", and why the favorable ones occur in August. It is likely that the same reason also explains why the opposition intervals differ.

The orbit of Mars has a larger eccentricity than the Earth's. So its maximum and minimum distances from the Sun varies much more than the Earth. Hence, the distance between the Earth's orbit and Mars' orbit varies as well. Picture now the two orbits in space (say, looking down on the solar system). The locations where the minimum and maximum distances between the orbits occur are fixed in space. Earth arrives at where the two orbits are closest sometime in mid-August through mid-September. Of course, Mars usually isn't there. But when it is, there is a favorable opposition of Mars. The unfavorable oppositions occur on the opposite side of the orbit, hence in mid-Feb or mid-March. The fact that this is near the time of the Vernal equinox is unrelated.

The Mars-Earth Distance on March 17, 1997 will be 98 million kilometers (or 0.66 Astronomical Units). During favorable oppositions, the distance is 57 million kilometers. The next favorable opposition will occur in Aug, 2003.

To "see" the oppositions take place, try using an orrerary or a desktop planetarium for your computer (or possibly on the web). You might also be able to figure out why the intervals between oppositions differ.) For more on the oppositions of Mars, I'll also suggest you take a look at Sky and Telescope magazine. They usually have an article or two a few months in advance of an opposition.

Or you could take a look at "Solar System Live" at:

This is an electronic orrerary you can play with to see the planets in their orbits. Using the "Inner system" display mode, it shows the orbits of Earth and Mars quite nicely.

Jim Lochner
for Imagine the Universe!

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