The Current Phase of the Moon

Ladies and gentlemen, we've left Mars. Well, at least I'm done with the Martian lander from my past few articles. I hope you had chance to experiment with it and find out that it's not too easy to land a craft on any planet!

While researching the Martian lander project, I bumped into another interesting scripting problem that relates to space. How do you ascertain the phase of the moon for a given date? There are formulas, of course, and you can do the math knowing that the lunar rotation is precisely—um...well, it's not quite that simple, actually.

Sidereal versus Synodic Period

Sure, you can just say that the moon orbits the Earth every 27.3 days, but that's relative to the stars, the sidereal orbit. The period between moon phases (such as a full moon) is also known as a synodic orbit, and that's 29.5 days.

So the simple task of ascertaining whether it's a full moon already has some math involved. Then there's the issue of the moon's illumination level being relative to where you are on Earth too. That makes sense. A full moon in Punta Arenas, Chile, is different from that in Lapland (though not by much).

The long and short of it is that the math behind calculating the illumination level of the moon isn't quite as simple as it may seem. You could take a known date and time of a full moon (for example, November 14 at 8:52 am EST) and keep adding precisely 29.530 days or 42,523.20 minutes.

Or You Can Scrape a Website!

But seriously, you also can let someone else do the work too, right? I mean, this column is just about a shell script, after all. So, let's see how Google does it! If you check Google to see the current phase of the moon, it actually references a website as shown in Figure 1.

Figure 1. Google reports the current phase of the moon.


Dave Taylor has been hacking shell scripts for over thirty years. Really. He's the author of the popular "Wicked Cool Shell Scripts" and can be found on Twitter as @DaveTaylor and more generally at