It’s about polarity. As we are living in an ever changing rhythm of day and night, we perceive things differently depending on day or night. During the dark, noises seem louder, our senses seem to sharpen. As we cannot see, we have to trust in our instincts. That part of our brain is stimulated, which just acts-and things later.
The same principle is applied in astrology. Planets act differently, depending on whether a person is born during the day or during the night.
We talk about a day birth, when a person is born with his/her Sun over the horizon (the AC-DC line), and thus after sunrise and before sunset. We do have a nighttime chart, when somebody is born with the Sun below the horizon, and thus after sunset and before sunrise.
In the same way, we distinguish between diurnal planets and nocturnal planets.
The female planets, Moon and Venus are both nocturnal. Also Mars, even being a male planet, is nocturnal. These planets are said to be “in sect” in a nighttime chart.
The other male planets, Sun , Jupiter and Saturn, are diurnal. They perform better in a day chart. They are “in sect “ in a diurnal chart (and “out of sect” in a nightime nativity).
Mercury is not really considered to be either a diurnal or nocturnal planet. He can be both. If he rises after the Sun, he is considered as nocturnal. If he preceeds the Sun and rises earlier, he is a diurnal planet.
What does this mean? A benefic is stronger in its proper sect: Venus in a nocturnal chart, Jupiter in a diurnal chart. This also means, that Venus in a diurnal chart is less benefic than Jupiter, and Jupiter in a nighttime chart less benefic than Venus.
Malefics are more dangerous, when they are “out of sect”. Mars is nocturnal and thus more malefic in a daytime chart. In his own sect, he will “behave” better. The same is true for Saturn: in a nighttime chart he will be more difficult than in a daytime chart.
These rules are very much applied in traditional astrology.They were considered as part of the prelimnary analysis of any nativity. Many of the ancient masters also considered the position within the chart important: in a nighttime chart, the nocturnal planets were ideally placed above the horizon (as the commanding planets) while the diurnal planets performed better below the horizon, and thus in the houses 1-6.
In a daytime chart, the diurnal planets were the commanding planets and had to be in the upper part of the chart, while the nocturnal planets were better hidden in the lower part of the horoscope.
As we see, the technique is quite simple, but permits at a glance to make out, which of the malefics represents the major problem for a native and which of the benefics promises a more reliable gift.
The fine tuning of a chart comes after this simple sect analysis.