Sect is a very important feature of traditional astrology, especially in its classic Hellenistic form, that has been largely lost to moderns. It is a very powerful organizing and interpretive principle.
Sect is from a word meaning to cut or separate, and it divides the planets up into two factions or teams.
Sect is a matter of whether a chart is a day chart (diurnal), or a night chart (nocturnal). So the position of the sun determines the chart’s sect.
Sun, Jupiter and Saturn are considered diurnal planets, so they are happiest and function best in a day chart.
Moon, Venus, and Mars are considered nocturnal planets, so they function best in a night chart.
Mercury can go either way – if it rises before the sun (at an earlier degree) it is considered diurnal. If it rises after the sun (at a later degree), it is considered nocturnal.
Once you have found the sect of a chart, the first thing to do is to identify the two teams of planets. The in sect planets are likely to function in a more effective manner since they are the team in power. The out of sect planets are likely to be either less helpful or more harmful since they are not in power – they are more likely to be off-balance, excessive, negative.
Sect is especially important in looking at the two malefics, Saturn and Mars, and the two benefics, Jupiter and Venus. Saturn is much more likely to be harmful and troublesome in a night chart, and mars is much more likely to be harmful in a day chart. Similarly, Jupiter is more helpful in a day chart, Venus more helpful at night.
This simple principle of sect provides a powerful way to look over a chart and determine which planets are likely to be the most helpful, and which the most harmful.