Note: At Jennifer’s request. Thanks for asking, Jennifer!
First, what does “void of course” mean?
Void of course, or void in course, is a translation of the Latin “vacua cursus.” It might make more sense in English if we called it “empty course,” or “empty path.” It is sometimes abbreviated as VOC. It has nothing to do with our expression “of course,” i.e. “certainly.”
A planet is void of course when it has no further aspects or conjunctions to make in the sign it is in. Its business in that sign is complete; now it’s cruising toward the next sign with no particular events or interruptions along the way. Any planet can be VOC, not only the Moon.
By the way, I’m only using the Ptolemaic aspects here: sextile, square, trine, opposition, plus the conjunction. I’m not using any other divisions of the 360-degree circle.
Also, I’m only using the classical seven planets: Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon.
It is absolutely possible, though rare, for the Moon to travel all the way through a sign without making a single aspect. (It’s possible only if you’re sticking to the Ptolemaic aspects, that is.) The Moon in this condition is called feral. (It is also possible for another planet to be feral, though that would be rarer still.)
So how do I know when the moon is void of course?
Put up a chart and have a look at what the Moon is doing. Look at the sign it’s in and the degree. If it still has aspects to make before changing signs, it’s not void of course yet. If it’s completed its aspects and isn’t applying to anything else while in that sign, it’s VOC. (This assumes you know how to identify applying and separating aspects in a chart.)
If you prefer, you can instead obtain a good astrological calendar such as one from Llewellyn’s, which notes the exact time of VOC Moon every time it occurs. Make sure you know what time zone the calendar uses, and adjust to your own, and to Daylight Saving Time, as needed.
The Moon can be VOC for anything from a minute or two up to its full 2.5-day passage through a sign (see feral above).
Over the weekend I’ll put up a chart example to illustrate how to follow the Moon’s activity and determine when it will be VOC.