MarsMap: What Features of Mars are Visible? (Shallow Thoughts)

Akkana's Musings on Open Source Computing and Technology, Science, and Nature.

Wed, 07 Oct 2020

MarsMap: What Features of Mars are Visible?

[MarsMap screenshot] I've been working on my upcoming PEEC talk, Observing Mars at Opposition on October 16.

Mars' closest approach was yesterday, October 6, and the actual opposition will be next Tuesday, October 13.

So, wait, we've already missed closest approach, and the opposition will be over before the actual talk happens? Then why bother?

Fortunately, opposition is actually an "opposition season", not a single date. And for most people, the best part is a little past opposition. At opposition, Mars is highest at midnight. Since most people probably don't want to stay up til midnight to observe, it's actually better to wait a few weeks, until Mars is higher a little earlier in the evening, and it's nearly as big as it was at opposition.

A lot of my talk is about how to observe Mars through a telescope, and one of the important questions I need to answer is, "How can I figure out which Mars features are showing right now?" Mars features are somewhat subtle; except for Syrtis Major and Hellas, you probably won't be able to take a glance at Mars and immediately recognize what you're looking at.

So part of my talk preparation required answering that question.

I've been using an ancient Unix program called XEphem for many years. It's very accurate and reliable, but the program is no longer supported, and so it needs to be compiled. Not really an option for most people.

Sadly, the more accessible open source program, Stellarium, does not show Mars features. I'm told that the paid (and well respected) paid program Starry Night does, for Mac and Windows. On my Android tablet and phone, I use a paid program called Sky Safari (it's also available for iOS and Mac). I bought the middle, "Plus", level since I use it for basic observing, not finding ultra-faint galaxies, and it shows Mars features pretty well. I suspect the cheaper basic version also shows Mars features, but you might want to check to make sure before purchasing.

So, several paid programs plus a program you have to compile from source. What free options are there?

Sky & Telescope magazine has a Javascript app called Mars Profiler -- but it seems to be wrong! At least right now, XEphem and Sky Safari (and my MarsMap, which I'll talk about in a sec), are both showing Solis Lacus front and center; Mars Profiler shows it way off to one side. So if you use it, be wary.


My app. Ahem. A long time ago, solving this same problem, I wrote a little C app called MarsMap for Linux/Unix and PalmOS (remember Palm Pilots?). Later I translated it into a Java applet (remember Java applets?) Neither of those is viable now (though the source is linked from my software page.

After failing to find a reasonable, free, web app to show Mars features accurately, I realized I ought to translate MarsMap to JavaScript so people (including me) could use it on modern browsers and phones.

The Java version of MarsMap used a color Mars map I made by adding labels to an unlabeled JPL Mars map (there are also east-west flipped and north-south rotated versions courtesy of Bill Arnett). I translated the Mars meridian calculation code from C to JavaScript. Then the only remaining issue was the code that takes the rectangular map and projects it so it looks like a globe. In the C and Java versions, I just did the trigonometry, but I suspected that in 2020, there was probably a library that would do that for me, using nifty WebGL accelerations and other stuff I'd rather not have to learn.

Sure enough. Three.js works wonderfully and can do all kinds of amazing things, way beyond what I needed for MarsMap. So not only was I able to show a globe and offer buttons to change the time, but I could hook up mouse events that let you drag the globe around (use shift-Drag for that; sorry, touchscreen users).

So here is MarsMap. It's a bit feature poor at the moment (for instance, it doesn't offer those nice flipped and reversed maps) but hey, it's a free Mars map, in time for the 2020 opposition. And it's set up to "Install to Home Screen" on phones and tablets, so you can pretend it's a regular app.

And if you're interested in Mars observing, you might want to come to my talk! Virtually, that is, since it'll be on Zoom. friday, October 16 at 7pm: Observing Mars at Opposition.

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[ 18:10 Oct 07, 2020    More science/astro | permalink to this entry | ]

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