Two planets sit at 10ยฐ Aries and 20ยฐ Gemini. The point halfway between them is 0ยฐ Taurus. That's a midpoint. Nothing more complicated than arithmetic.
What makes midpoints interesting is what happens when a third planet parks on that halfway point. The two planets start behaving like a pair, and the third one becomes the way that pair expresses itself.
The notation
Astrologers write it as A/B = C. Sun/Moon = Venus means Venus sits on the midpoint of the Sun and the Moon.
You'll see the same idea written as a "midpoint tree" or "planetary picture". Reinhold Ebertin's The Combination of Stellar Influences, first published in 1940, is still the standard reference, and most modern midpoint delineation traces back to it.
Direct and indirect midpoints
Every pair has two midpoints, 180 degrees apart. The near one and the far one. Both count.
Then it goes further. A planet doesn't have to sit exactly on the midpoint to activate it. Squares work too, and so do the 45 degree and 135 degree points. The reason is the 90 degree dial: fold the zodiac into quarters and conjunction, square, and opposition all land on top of each other. A planet 45 degrees from a midpoint is doing the same job as one sitting right on it, just a little quieter.
That gives you five live angles for every pair: 0, 45, 90, 135, and 180. Astrolas scans all five, which is why a full midpoint scan turns up more contacts than you might expect from a chart that looks quiet by aspect.
Orbs, and why they have to be small
This is where midpoint work goes wrong for most beginners.
A chart with ten planets has 45 pairs. Each pair has five live angles. That's 225 sensitive points scattered across 360 degrees, so on average there's one every degree and a half. Use a three degree orb and every planet in your chart will be sitting on something.
Ebertin worked to about 1ยฐ30'. Many astrologers tighten that to one degree, and some go to 30 minutes for anything they plan to say out loud. Astrolas defaults to a tight orb for exactly this reason, and you can widen it if you want to see the noise.
If a midpoint picture is real, it will still be there at one degree. If it only shows up at three, it isn't a picture, it's arithmetic.
The pairs worth learning first
You don't need all 45. Four of them carry most of the weight in practice.
Sun/Moon. The core pair. Whatever sits here describes how the two halves of a person work together, and it shows up strongly in relationship charts. Sun/Moon = Saturn tends to read as a serious inner tone, sometimes an early sense of responsibility. Sun/Moon = Venus is the easy charm the person doesn't know they have.
Mars/Saturn. Effort under restriction. A planet here shows where the person grinds. Ebertin's delineations for this pair are bleak, and worth reading as pressure rather than prophecy.
Venus/Mars. Desire and how it's expressed. Obvious in synastry, useful in a natal chart for anything about attraction and creative drive.
Saturn/Pluto. The long structural pressure pair. When a scan shows several planets clustered on Saturn/Pluto points, you're usually looking at a chart that has had to rebuild something from the ground up.
Where midpoints earn their keep
Midpoints answer the question aspects leave hanging: why does this chart clearly do something that the aspect grid doesn't show?
A person with no Sun-Saturn aspect but Sun on the Moon/Saturn midpoint will still read as someone carrying weight. The connection is there, it just isn't a two-planet aspect, so a standard grid misses it entirely.
They're also good for angles. Midpoints involving the Ascendant and Midheaven are among the most reliable in the technique, and they're the first thing to check when a chart's aspects look mild but the life clearly isn't. Same caveat as always: those need an accurate birth time.
Running a scan
The midpoint tool lists every pair, marks which ones have a planet inside orb, and lets you sort by which planet is doing the activating. Start by looking only at the pairs that involve the Sun, Moon, Ascendant or Midheaven. That's usually four or five pictures, and they'll tell you more than the full 225-point sweep.
One practical warning. Midpoint delineations from the 1940s are written in a flat, fatalistic register that reads harshly today. Take the mechanism, which is sound, and drop the tone.
Reading a midpoint picture
The mechanics are easy. Getting a sentence out of them takes a little structure.
Take Sun/Saturn = Mars. Read the pair first: Sun with Saturn is effort, duty, the weight of being responsible for yourself. Then read the third planet as the outlet: Mars is action, force, sometimes anger. Put them together and you get something like drive that runs on obligation. Work that gets done because it has to be, with a temper underneath it when the load doesn't lift.
Ebertin's books give three-line delineations for hundreds of these. They're worth having, but they were written in a different century for a different audience and they read like a diagnosis. Use them to learn the grammar, then say it in your own words.
Two rules keep the readings sane.
The pair is the subject, the third planet is the verb. Venus/Mars = Saturn and Venus/Saturn = Mars share three planets and mean different things. Order matters.
A picture with a personal point in it is louder. Mars/Jupiter = Neptune is abstract. Mars/Jupiter = Ascendant is a person you'd notice walking into a room.
Midpoints in synastry
This is where the technique gets uncomfortable and useful at the same time.
Take one person's Sun/Moon midpoint and see what the other person's planets do to it. A partner's Venus on your Sun/Moon reads as easy warmth. A partner's Saturn on it reads as a relationship where you feel measured. Neither is a verdict, but both are recognisable to people who have lived them.
The composite chart, which is built entirely from midpoints between two charts, comes from the same idea taken to its conclusion. Every planet in a composite chart is the midpoint of that planet in the two natal charts. It's a whole chart made of halfway points, which is why it reads as a description of the relationship as its own thing rather than as either person.
Where the technique gets abused
Three ways, all of them common.
Wide orbs. Covered above and worth repeating, because it's the single failure that makes midpoint work look like nonsense to outsiders.
Cherry-picking. With 225 sensitive points you can find support for any story. Decide what you're looking for before you scan, or scan the whole list and report the strongest few by orb rather than the ones that fit your theory.
Midpoints instead of the chart. A midpoint picture is a footnote on a chart, not a replacement for reading it. If you've moved on to midpoints before you can say what the Sun, Moon and Ascendant are doing, you've skipped the part that carries most of the meaning.
Used tightly, midpoints explain the charts that don't add up: the ones where a person is clearly doing something the aspect grid has no evidence for. That's a narrow job, and the technique is very good at it.