The phrase 'applying to aspect', that is, applying to the perfect or exact aspect, is often misused to mean moving towards another planet by aspect, even though they have yet to make contact through their orbs. Claudius Dariot, much respected by 17th century astrologers, writing in the 16th century says this:

The Application happeneth when as the Circles or beames of the Planettes come to joyne togeather by a corporall Coniunction, or by aspecte of the one halfe of their Deamiters.

The further importance of using this phrase correctly will be demonstrated below in the explanation of the void of course condition.

Application shows the possibility of an event, therefore the two planets capable of bringing this about need to have some relationship - they have to be in contact through their orbs.

After perfection, that is 17' of arc after exactitude, separation begins. It ends when the planets have no further contact through their orbs. In fact, to use the word 'orb' like this is incorrect. The orb is the overall distance of the planet's influence, on a chart we look at this in two dimensions and see part of the orb in front of the planet and part behind. Those parts are halves of the overall orb and known as the moeities.

Void of Course

So, in order to be able to say that a planet is either applying to or separating from another, the moieties of both must be in contact. When there is no such contact the planet is void of course. Referring again to Dariot: ...voyd or without course is when a planet doth not apply to any other during the time that he tarrieth in that signe and then he is saide to have his course and motion voyd.

For example, Saturn is at 7? Pisces, the Moon is at 8? Gemini and Mercury is at 27? Libra. So the Moon has just separated from Saturn (Moon's moiety: about 6?. Saturn's moiety: about 4?. Joint moieties: about 10?.) The Moon's next major aspect - minor aspects cannot prevent a planet from being void of course - is a trine with Mercury. To see if this is actually an application the two moieties are added together. The Moon's moiety is about 6? and Mercury's moiety is 3?, so when they are about 9? apart application will begin. In this example they are about 19? apart and so there is no application; the Moon is void of course, it applies to no other planet from its present position.

As far as interpretation is concerned, the void of course Moon is often taken to mean that nothing will happen in regard to the matter under investigation. This is much different to the answer that might have been given had the trine to Mercury been taken into consideration. For example, suppose the question were one of marriage and Mercury represented the person enquired after; the Moon moving to trine Mercury might be taken to mean that the marriage would go ahead. Whereas accepting the Moon correctly as void of course would indicate that the marriage would not go ahead. The vast difference in the outcome demonstrates how useful horary can be for proving or disproving an astrological technique.