The origin of the concept of a void of course moon can be traced back to the 1st-2nd century CE through influential astrologer Antiochus of Athens. The term originates from the Greek compound word ‘Kenodromia’, the translation of which is akin to running in the void or emptiness. It’s a great term which superbly illustrates the fundamental concept which is, in the most general sense, that of nothing happening. The Moon’s lack of connection with another planet indicates not being able to manifesting anything, no progress or news, aimlessness, decrease, feelings of isolation or lack of support. Ancient astrologer Firmicus Maternus mentions that this can be mitigated by having a benefic planet in one of the angles. The Void of Course Moon is most commonly referred to in Horary astrology.
This foundational concept has become defined in slightly different ways over time, so that we now have at least three definitions which define the condition of the moon making no aspect to another planet within a certain number of degrees, also taking into account (or not) sign boundaries.
1. Hellenistic Definition
The earliest astrologers writing about the concept (Antiochus and Porphyry of Tyre (3rd Century CE) ) define it as the Moon not making an exact applying degree based aspect (including conjunctions) to any other planet within the next 30 degrees, irrespective of sign boundaries. The aspects considered in this definition are the 5 major, or Ptolemaic, aspects of conjunction, sextile, square, trine and opposition. So the moon makes no contact with any other planet for approximately 2-2.5 days. This definition is found all the way through to the 6th or 7th century CE.
It is called running in the void whenever the Moon is joined to nothing, neither zodiacally nor by degree, not according to figure nor according to bodily adherence nor is it about to make an engagement or assembly within the nearest thirty degrees.Porphyry, Introduction 
What is notable about this definition, compared to the modern one we’ll see later, is that it is relatively rare for this to occur. This type of void of course moon only happens once or twice a year. It therefore makes sense that it was noted by ancient astrologers as having special significance.
For example in 2021 a void of course moon according to this definition only occurs twice – on October 2nd and October 5th (using either Placidus or Whole Sign houses and using just the 7 traditional planets).
The specific symbolic signification attached to the Hellenistic definition was difficulty for the native to be able to accomplish things in the future, so it was not seen positively. There doesn’t seem to have been any indication that the concept was applied to other planets.
2. Medieval Definition (1)
By the time we get to the Medieval tradition (17th century onward) there has been a change in the definition. Now, rather than being defined as lack of aspects for the next 30 degrees regardless of sign boundaries, the Moon was seen as being void of course if it does not complete any further applying aspects before it leaves the sign it is currently in (i.e. before the next sign boundary).
Emptiness of Course is that a planet separates from the connection of a planet (by assembly or looking), and will not connect with a planet so long as it is in its [current] sign.Abu Ma’shar, The Great Introduction 
For some reason sign boundaries became a relevant part of the concept. Furthermore, the concept was extended to the other planets, not just the Moon.
This lunar condition occurs quite frequently compared to the Hellenistic void of course Moon, occurring every couple of days, and has become the most popular definition in modern astrology.
Astrologer Chris Brennan theorises that the change in definition from the original Hellenistic one may have occurred due to the development and increase in the practise of interrogational astrology, where a planet completing an aspect before it crosses a sign boundary has more significance.
3. Medieval Definition (2)
A third definition has been proposed by Sue Ward who re-analysed William Lilies text ‘Christian Astrology’ in the 1990’s, and argues that Liliy’s void of course Moon (which was previously assumed to be as definition 2 above) has been misunderstood due to astrologers overlooking Liliy’s use of the word ‘forthwith’ and the original definition of the term ‘application’  . Lily writes:
A Planet is void of course when he is separated from a Planet, nor doth forthwith, during his being in that sign apply to any other…William Lily, ‘Christian Astrology’ 
Sue Ward argues that in modern usage orbs belong to apsects, but in Lily’s time the orbs belonged to the planets instead. Furthermore, the meaning of ‘application’ needs to be considered in this context also, since it didn’t just mean a planet moving towards another, but was more specific, meaning the movement of one planet to another whilst in the ‘joint moieties’ of the orbs of the planets. A moiety is half of a planet’s orb, so the joint moiety is the sum of these.
The other consideration is the use of the word ‘forthwith’. Forthwith means immediately/promptly/without delay, therefore the Moon is void of course only when it is not applying to another planet (regardless of sign boundaries) and that this condition is in operation at the time of the chart: we are not looking at whether or not this condition will occur later. So if the Moon is at the end of a sign but is making an applying aspect within joint moieties, regardless of sign boundary, then it is not void of course.
Having looked at Lily’s chart examples Sue found that the majority of them do use this definition of a void of course Moon.
So there you have it – 3 definitions for a void of course Moon. How has the void of course moon shown up for you astrology, and which definition do you use? I’d love to hear your experiences, share them below!
- Image by Erwin Nowak
 Brennan, C. 2017, Hellensistic Astrology, the Study of Fate and Fortune, Amor Fati Publications
 Abu Ma’shar The Great Introduction to the Science of the Judgements of the Stars Translated by Dykes, B.N. 2020, The Cazimi Press
 Ward, S. 1992, ‘A Question of Horary’, The Horary Practitioner, Issue 15.
 Brennan, C., The Astrology Podcast, Episode 296, ‘Considerations Before Judgement with Sue Ward’. Available at https://theastrologypodcast.com/2021/03/24/the-considerations-before-judgement-with-sue-ward/