- Moitié is a French word derived from the Latin medietas, meaning "medium measure", "mean value" or "half". Its general use in traditional astrological texts is either to define a half-measure of any planet's orb (thereby identifying the distance that the orb extends on either side of the planet), or to denote the mean value obtained when two planetary orbs are added together and then halved to find the distance at which their rays unite.
To understand how the meaning of the word can alter according to context, it helps to realise that older authors usually expected a planet's influence to become noticeable on another planet whenever its conjunction or aspect with the other planet fell within the range of its own planetary orb. A standard set of planetary orbs had become widely reported by the 8th century (see table below), with most authors reporting that the Sun's orb extends over 30° of the zodiac, 15° either side of the centre of its body. This 15° limit was therefore often described as the moiety of the Sun's orb, being only half of its full radius; and any planet within 15° of a conjunction with the Sun is said to enter its beams (or go under its rays) regardless of the length of its own planetary orb. The same approach applied to other planets, so that the Moon was considered to be in orb of an aspect with any planet that was separated from aspectual contact by less than 12° (the Moon's orb on either side of its body), even in if the other planet's orb was smaller and not making contact with the Moon. Saturn, for example, has a smaller orb which extends 9° on either side of its body, so at a separation of 10°, the Moon is able to aspect Saturn, but Saturn is not able to aspect the Moon.
Many later authors, such as the 16th century French astrologer Claude Dariot, took a different approach in which the two planetary orbs were added together and then halved to find a mean measure at which both were in aspectual contact with each other. For the Moon and Saturn this would be 10°30 (12° for the Moon, plus 9° for Saturn = 21°/2 = 10°30). This is widely referred to as the "moiety technique" for calculating when the aspect between two planets begins to become effective - the name deriving from the fact that the two orbs are added together, and then halved to obtain a mean value that applies to both.
The table below shows the standard set of orbs that has been most consistently reported by historical texts. An early source of these values is Pophyry's Introduction to the Tetrabiblos, (CCAG, 5, part 4; p.228), where the text reads:
The rays of the Sun come to 30 degrees: 15 in front and 15 behind. The Moon has 24 degrees: 12 in front and 12 behind. Saturn and Jupiter 18: 9 in front and 9 behind. Mars 16: 8 in front and 8 behind, Venus and Mercury 14: 7 in front and 7 behind.
For a full explanation of the moiety technique for calculating orbs refer to the section 'Moiety of the Orb'
in the article The Classical Origin and Traditional Use of Aspects.
For a quick reference on when planets are within moiety of orb according to this technique refer to The Table to Show Orbs for Aspectual Contact.
See also: the glossary entry on orbs.
© Deborah Houlding