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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:
http://www.fourmilab.ch/cgi-bin/uncgi/Solar
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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