Greetings from Palmia Observatory
During a recent PBS show "Ancient Skies" the Ptolemaic view of the universe described the use of epicycles to correct for the retrograde motion of Mars. Can we go outside now and observe the motion?
Well, I didn't know but a quick internet search revealed that the retrograde motion of mars could be observed every 20-24 months as Mars moved into the right position. But the same search indicated that the period of retrograde motion had already passed and we would have to wait another two years for us to go outside and witness it ourselves.
Just so we start off on the same point of view, look at this screenshot from Wikipedia that descries how and why retrograde motion is observed. Of course Mars itself does not actually reverse directions, but it just appears to do so because of the relative motion of Mars and Earth.
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| Retrograde Motion of Mars as seen from Earth (Source: Wikipedia) |
Darn, so we missed our chance to go outside and witness and measure this motion for ourselves. But we can use modern ephemeris tools, like in this case here, "GoSkyWatch" to calculate the apparent azimuth and elevation for Mars as we w0uld see it from our location.
When I looked at the position of Mars I found a nearby star, Procyon, that we could use as a deep field reference point to measure the movement of Mars.
In the chart below, the relative position of Mars, with respect to Procyon, is plotted from December 16, 2024 to May 4, 2025. I chose to calculate the positions at four day intervals. My initial reading of when the retrograde motion could be seen indicated that it would all be over by April, but when the data was plotted it turned out that the motion could still be observed in May.
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| Chart showing Mars GoskyWatch position relative to Procyon (Source: Palmia Observatory) |
Wow, Mars moves all over the place when compared to Procyon. How would we ever make any sense of this? A better way of describing the retrograde motion is to use the modern Right Ascension and Declination method of plotting positions.
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| Retrograde motion of Mars using GoSkyWatch RA and Dec (Source: Palmia Observatory) |
This plot now begins to look more like the idealized motion illustrated in the earlier mentioned Wikipedia article.
So, we missed our chance to observe this with our own eyes and measuring devices, but with modern tools we can sort of create our own view of how it might have looked. And it seems that the end of this retrograde cycle is still ongoing as of May 4 calculation. Oh well we don't really need to go out and take a look right now.
But let's do one more chart using GoSkyWatch predicted locations just a couple of weeks later after retrograde motion has completed from the period between April 2 and May 20.
Wow, note now how the position of Mars just drifts along almost linearly as the predictions are made 4 days apart. So, this type of relative motion could have been observed by ancient astronomers and when the retrograde motion of Mars was observed they knew their theories had to be updated.
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| Mars position using GoSkyWatch with respect to Procyon after retrograde motion (Source: Palmia Obsservatory) |
Until next time,
Resident Astronomer George
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