A writing friend suggested I sensationalize the headline of this little but interesting and instructive tale. So assuming the headline got your attention, read on if you want to know how I noticed a scientific error in the caption on a NASA/JPL Photojournal page. I promise you’ll discover something interesting about the Martian sky if you do.
The page describes a time lapse image of the two moons of Mars taken by the Spirit Rover.
I hope to use that illustration in a middle-grade book that I am currently developing about humanity’s future in space. One of the tidbits I will include is that the larger and inner moon, Phobos, completes an orbit in less than a third of a sol (Martian day, about 24h37m). As a result, it rises in the west and sets in the east, appearing in the sky twice a night and progressing through its phases.
When I found the page, I noticed that the caption stated that the two Moons were traveling in the same direction, from the top to the bottom of the image. I contacted NASA and asked them to clarify, since I thought the moons were moving in opposite directions.
I was right, and the error was corrected. Even better, Jim Bell, the NASA scientist who verified my statement, suggested it could be a useful lesson about how science and scientists work.
I’ll quote him, since I couldn’t say it any better.
Mr. Bortz is right! I don’t know how that error escaped our attention. But then again, there’s a lot that I don’t know… In that time-lapse, Phobos is moving from top to bottom and Deimos (and Aldebaran) is moving from bottom to top.
My guess is that whoever wrote the caption was thinking about some of the movies that we’ve made that show Phobos and Deimos moving relative to the fixed stars (like this one posted at the bottom of this page). If we remove the motion of the sky overall, Phobos and Deimos appear to move in the same direction because, after all, they both orbit Mars in the same direction. It’s just that Phobos orbits faster than Mars spins so it appears to be moving backwards in the sky. It’s kind of like passing another car on the freeway–you’re both going forward over 60 mph (probably), but the slower car appears to you in your car to be moving backwards when you pass.
Please thank Mr. Bortz for pointing out this error–which I hope he uses as a teachable moment about the fallibility of scientists–as well as the importance of perspective when considering absolute and relative motions!
So there you have it. One small step for a ScienceBlogger, one giant leap for your imagination of what it would be like to stargaze on Mars.