This is the fourth of the five terms, or bounds, into which the Egyptian table divides Aquarius. It runs from 20 degrees of the sign to 25, a stretch of 5 degrees held by Mars. It opens where Jupiter's term closes at 20 degrees, and it gives way to Saturn's term at 25 degrees, so a planet moving forward through Aquarius leaves Jupiter's bound and enters Mars's at 20 degrees exactly.
Set out in degree order, the five terms of Aquarius run Mercury (0-7), Venus (7-13), Jupiter (13-20), Mars (20-25) and Saturn (25-30). At 5 degrees it ties with Saturn as the narrowest of the five, against a widest of 7 held by Mercury and Jupiter. 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 20 and 25 degrees of Aquarius is standing in Mars'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.