Placidus

Placidus is the system which is most often used by astrologers. That does not necessarily mean that it is the best (astrologers, apart from rare exceptions, do not have much understanding of the mathematical realities behind the figures they handle daily). Placidus is certainly a system that makes sense, and it is devoid of some aberrations found in some other systems. But it should always be kept in mind that if it is omnipresent, it could be more due to habit than to its intrinsic value.

The Placidus system is based on time. The cusps (limits between houses) are determined so that it takes the same time for a planet to move through each house above the Ascendant (on the day side of the chart). And each house below the Ascendant (on the night side) is crossed within a fixed duration too, but this time is not equal to that needed to move through the houses above the Ascendant.

The Placidus system, in relation to its time-based definition, fitted well with primary directions, a prediction tool based on relatively heavy mathematical calculations. Astrologers, however, have long given up primary directions, but they have retained Placidus!

A major weakness of Placidus is that it does not work for high latitudes. Past 66° (North or South), it becomes completely meaningless; the formulae can no longer be applied. Campanus does not have this limitation. (Try to enter a chart with 70 degrees of latitude North or South while Placidus is selected as the default house system, and see what happens.)