A Mars Cycle

Eric David has politely forced me to return to the subject of oppositions. (See his several comments to my last post – the one about the locus of Jupiter’s oppositions.) He’s had the patience to try to find the locus curve for Mars by using photographs and calculations over a hundred-year span.

I improved my program after using it, and had meant then to leave it aside for some later date, but now I have to show what it can do for Mars.


(Please let me know whether this picture – after your click on it to enlarge it – seems to have sharp enough resolution on your screen. I’d like to be able to post wide charts of this kind in one piece.)

The orange curve is the locus of Mars’s oppositions. Can’t make it too deep a color or it would obscure the little black tracks for Mars at actual oppositions.

The orange curve is the locus of Mars’s oppositions. Can’t make it too deep a color or it would obscure the little black tracks for Mars at actual oppositions.

I’ve made these tracks only 100 days long, as against 200 for Jupiter, because Mars moves retrograde for only about 80 days. The tracks are more interesting than for Jupiter, in that they loop generously above, across, or below the ecliptic . This is because Mars is nearer to us and its orbit is more inclined. I think the relation of the loops’ shape to the ecliptic is something that will be even more fun to talk about when sometime drawing tracks for Mercury.

The oppositions shown are for the same dates as in my 3-D diagram in Astronomical Calendar 2015: that is, over Mars’s approximately 15-year cycle, from its perihelic opposition of 2003 around to its less-close-to-perihelion opposition of 2018.

The figure under each date is Mars’s magnitude. Mars is brighter when it’s nearer. The open dots are sized for the magnitude, but after other comments (by Anthony Barreiro) I’ve made the dots range less widely in size, hoping to avoid a confusing impression that this is like the usual use of dot-symbols in star charts. My formula this time is to take Mars’s magnitude at opposition as ranging between -2.9 and -2.5, and size the dots’ diameters accordingly in the range between 4 and 2 millimeters. (Or that is what they would be on paper.)

Besides brightness, and northness-southness, there’s another factor to how good a planet’s opposition is: moonlight. This was relevant to our recent opposition of Jupiter, with the still almost Full Moon only 30 degrees away. So I’ve added another line to the labels: the angular distance to the Moon (negative meaning westward – to the “right”).

Mars’s 2003 opposition in Aquarius, the nearest in thousands of years, was excellent also in that the Moon was on the opposite side of the sky, just a day past New. But at most of the other oppositions in this cycle, including the two coming ones (2016 in Scorpius and 2018 in Capricornus) the dazzling Moon will be uncomfortably close. Not fatal. Telescopes can gaze at the little orange planet in other parts of its 80-day swing in space toward us.


7 thoughts on “A Mars Cycle”

  1. The diagram looked very sharp and clear on my screen when I clicked on it, and I think your choice of dot size conveys a very realistic impression of the magnitude differences between March and August oppositions.

  2. Thank you, sir! Now, my next project is going to be to print your two locus plots, Mars and Jupiter, on thin paper, then overlay them and trace the path of Jupiter onto the Mars plot, then scan the resulting plot on my scanner and view it on my computer to finally arrive at my long-envisioned diagram showing at what two regions of the ecliptic zone Mars and Jupiter could theoretically have very close mutual oppositions. I promise to leave this subject alone now so that you may move on to other topics in peace. Best regards, Eric (in Fredericksburg, Virginia).
    To return to another topic you raised in a recent post, I am eagerly awaiting a shadow transit of Io that is scheduled to start in 30 minutes or so. The C8 is outside cooling down already!

  3. Yes, the resolution of the chart was fine.

    Regarding the proximity of the moon to Mars: Does it inhibit the ability to spot Deimos and Phobos, or is Mars’ glare severe enough to cause problems on its own?

    1. I would think the two things add – the nearer the Moon is (which would also mean the nearer it is to Full) the more it would make the whole sky in the eyepiece less black, and the btighter Mars is the more badly it would outshine its satellites – but I really don’t know and it could be a project for comparative reports, rather like comparing the different Danjon visibilities people report for lunar eclipses.

  4. The resolution of the full size image looks fine on my 12-inch laptop screen. And the sizes of the circles seem about right to my eye.

    By the way, on the chart Mars’ magnitude at opposition ranges from -1.2 to -2.9. I believe the text in the sixth paragraph is in error.

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