Diurnal and Nocturnal Triplicities
Sounds exotic, doesn't it? It just means day (diurnal) and night (nocturnal). There are four elements (Air, Earth, Fire and Water). Each element governs three signs. Each element has 3 planetary rulers for day, and 3 for night.
Here's how they break down, in order of precedence:
- Fire (Aries, Leo, Sagittarius): day rulers are the Sun, Jupiter and Saturn/night rulers are Jupiter, the Sun and Saturn
- Air (Gemini, Libra, Aquarius): day rulers are Saturn, Mercury, Jupiter/night rulers are Mercury, Saturn, Jupiter
- Water (Cancer, Scorpio, Pisces): day rulers are Venus, Mars, the Moon/night rulers are Mars, Venus, the Moon
- Earth (Virgo, Capricorn, Taurus): day rulers are Venus, the Moon, Mars/night rulers are the Moon, Venus, Mars
Note how the planets change order (priority) depending on whether or not they are day or night rulers. That order is important; first is strongest. A day chart has the Sun in houses 7-12 (above the Ascendant/Descendant axis) and is ruled by the Sun (no matter what the rising sign is). A night chart has the Sun in houses 1-6 (below the Ascendant/Descendant axis) and is ruled by the Moon (no matter what your rising sign is). Note that this refers to when the Sun was actually visible above the horizon (when you were born). You could be born at 2:00 AM, but still have a night chart. Diurnal energy is Yang/extroverted and nocturnal energy is Yin/introverted.
Overall, planets that are happiest(express their energy more easily) in day charts are the Sun, Jupiter, Saturn, Mercury. Planets that do better at night are the Moon, Venus, Mars.
Does triplicity make a difference in the natal chart? It's an interesting way to drill down into the strengths of your planets. After you look at rulership and exaltation, it's another layer that gives you more information. But it can get very convoluted, so I wouldn't get too hung up on it.
Here's an example of how it might work:
You have a night chart (Sun in the 2nd House). Look at the condition of your Moon (which is in Libra). Using the correspondences above, we see that the night rulers for an Air sign (Libra) are Mercury, Saturn and Jupiter. What's happening with those planets? Are they in their rulerships? Their detriments? They are going to take on added emphasis in your chart because they are the triplicity rulers, based on the sign your Moon is in.
Maybe Mercury is in Aquarius, which would put it in its triplicity. Good for Mercury; it's healthy. Except it does better during the day, so some minor shade is cast over it (notice in the night triplicity for Air signs, Mercury is bumped to the head of the line to give it some strength). Saturn is in Leo, the sign of its detriment. Saturn also does better during the day. Bad for Saturn, except that if you check out the Fire signs you'll see that Saturn is still in its triplicity (although it's last). This doesn't cancel out the fact that it's in its detriment (and in a nocturnal chart) but it helps a bit. And let's put Jupiter in Cancer (where it's exalted) so it's strong. But Jupiter is not comfortable in a nocturnal chart, which is why (in the Fire sign nocturnal triplicities) it was bumped to the head of the line (to give it more oomph).
When the planetary rulers were assigned to various triplicities, they were done so with the aim to strengthen or moderate their qualities. Kind of like fiddling with the ingredients in a recipe. Mars was hot and angry, so it was assigned rulership for Earth and Water (to cool it down).
A planet in its triplicity can moderate (but not erase) the ill effects of being in its fall or detriment. Venus is in its fall in Virgo. But if this is a day chart, Venus is in its triplicity (in Virgo). That helps. Plus, in the Earth triplicity, Venus is at the head of the line; even better.
For those who want the technical, definitive version, here is an excellent article (by Deb Houlding over at Skyscript) on the whole triplicities issue.