Cancer Third Term: Mercury

This is the third of the five terms, or bounds, into which the Egyptian table divides Cancer. It runs from 13 degrees of the sign to 19, a stretch of 6 degrees held by Mercury. It opens where Venus's term closes at 13 degrees, and it gives way to Jupiter's term at 19 degrees, so a planet moving forward through Cancer leaves Venus's bound and enters Mercury's at 13 degrees exactly.

Set out in degree order, the five terms of Cancer run Mars (0-7), Venus (7-13), Mercury (13-19), Jupiter (19-26) and Saturn (26-30). This term of 6 degrees sits between the two extremes of the sign, where the widest are Mars's and Jupiter's at 7 each and the narrowest is Saturn's at 4. The unevenness is not an accident of copying. No sign in the Egyptian table divides its five terms equally, and no two signs divide them the same way, which is why the table has to be consulted rather than derived.

The five rulers are the five planets visible to the naked eye. The sun and moon take no terms at all in this system, so each sign is shared out among Saturn, Jupiter, Mars, Venus and Mercury, every one of them holding exactly one stretch of it. A planet standing anywhere between 13 and 19 degrees of Cancer is standing in Mercury's bound, and Hellenistic technique reads the lord of the bound from the degree occupied rather than from the sign as a whole. The table this atlas follows is the Egyptian one, transmitted by Vettius Valens in his Anthologies and recorded by Claudius Ptolemy in the Tetrabiblos, which stayed in ordinary use after Ptolemy set out his objections to it and offered boundaries of his own.

Every one of the five term rulers holds exactly one term in each of the twelve signs, so Mercury's stake in the Egyptian table is twelve stretches of unequal width, and this 6-degree term in Cancer is one of them. Added together those twelve come to 76 degrees of the zodiac's 360, the third largest share any of the five planets holds.

The whole division runs Venus 82, Jupiter 79, Mercury 76, Mars 66 and Saturn 57, and the five figures sum to exactly 360 with nothing left over. That ordering belongs to the table itself rather than to any modern gloss on it: the two planets Hellenistic astrology called benefic hold the most ground, the two it called malefic hold the least, and Mercury sits between them. The same five figures circulate in the Hellenistic sources as the planets' years of the terms, so the arithmetic that divides the zodiac and the arithmetic that measures out a span of years are the same arithmetic.

Measured against its own twelve, this 6-degree term sits between the 3 degrees it holds in Pisces at the narrow end and the 8 degrees it holds in each of Aries, Libra and Scorpio at the wide end. A ruler's share is therefore not spread evenly around the zodiac, and reading a chart by the bounds means reading a different width of ground in every sign.

What the surviving sources do not supply is the reason for any of it. The table reaches us as a table: the order of the rulers and the size of each stretch are transmitted without the principle that fixed them, and no reconstruction has settled the question. The atlas records the figures as they are transmitted and keeps the question of their rationale open rather than supplying an explanation the sources do not carry.