Sade Sati is not in the Brihat Parashara Hora Shastra. We searched all 4,197 verses we have entered from the text, in Sanskrit and in English, and neither the name nor the idea of a fixed seven-and-a-half-year stretch appears once.
If you have been told you are "in Sade Sati" and have been bracing yourself, sit down for a moment. The text does care where Saturn is moving. But it reads that movement a completely different way, one that depends on your own chart, and for some people the same transit is a good season.
How we searched for Sade Sati
Sade Sati, as it is usually described, is the seven and a half years Saturn takes to cross the sign before your Moon's sign, the Moon's sign itself, and the sign after it. The name is Hindi for "seven and a half". We looked for it three ways.
First, our own entries. AstroEngine holds 4,197 verses of the text, checked against every printing available to us, each in Devanagari with our English reading. We searched them for the name in Hindi and English, and for "seven and a half". Nothing came back.
Second, two printings from cover to cover: the public-domain Bombay edition of 1899 with Shridhar Jatashankar's Sanskrit commentary, and a modern printing with a Hindi commentary. We searched their scanned text for every spelling, and for the Sanskrit phrase for "seven and a half". Neither has it, in the verses or the notes.
Third, we tested the test. The same search does find the words the text really uses for this subject. Ashtakavarga, the method below, turns up dozens of times in both printings, and gochara, the word for a planet's transit, appears in both too. The search can see the language of Saturn's movement. It does not find Sade Sati.
To be fair about the limits: printings differ in length, and some carry later chapters ours do not. We searched these two in full, not every printing that exists. Within them, Sade Sati is not there. Kaal Sarp Dosha came out the same way, by the same kind of search.
Saturn's own scorecard: the ashtakavarga
So how does the text read Saturn's movement? Not by the Moon alone, and not by a fixed count of years. It uses the ashtakavarga: a table of points the book builds for each planet, sign by sign, out of your whole birth chart. Think of it as Saturn's own scorecard for each of the twelve signs in your chart. The points, called bindus, decide what Saturn's passage through a sign will bring.
The clearest rule is about the empty squares. "In whichever houses Saturn's Ashtakavarga shows a zero" count, the text says, when Saturn or the Sun passes through them, the body is strained46.44. Read today, that is a season to look after yourself, sleep properly, not push too hard. It is the text's reading, not a diagnosis; a real health worry belongs with a doctor.
Now notice the condition. The rule asks what your Saturn scorecard says about that sign. A sign full of points is not a sign of empty points. Two people with the Moon in the same sign can have completely different scorecards, and so completely different years. A single rule for everyone born under one Moon sign cannot hold that difference.
The same transit can bring good years
Here is the part almost nobody hears. The text uses Saturn's movement to time good things too. In the year Saturn reaches a sign worked out from the points, a weak house can lose ground; and by the same method, for a house that is strong, "the wise should state the time of attaining that house's auspicious result"33.61.
Read that again. The same kind of transit one chart dreads, another chart welcomes. If your Saturn table is rich where Saturn is now walking, the text is reading a time when something in your life comes to fruit. That is a long way from seven and a half years of fear.
The nearest the book comes to a Moon-and-Saturn warning is a rule about choosing dates. It says to avoid a certain sign for starting good things, "especially when Saturn's sign, or its trikona, coincides with the 8th house from the Moon"46.22. That is advice about when to begin something, worked out from your chart's own points, not years of trouble. A few rules in these chapters also time a parent's loss by Saturn's transit; we leave those out, as on every page.
Where the Sade Sati idea sits
We cannot say who first described Sade Sati, or when, and we will not guess. What we can say is narrower and firmer: the fixed seven-and-a-half-year period is not part of the Brihat Parashara Hora Shastra as these printings give it. The text's own tool for Saturn's movement is the ashtakavarga, and it reads each chart by its own points.
So if you have been told you are in Sade Sati, ask the question the text would ask instead. What does Saturn's table show for the sign it is crossing now? A sign with points gives one kind of year; a sign without them gives another. And if you have been told Saturn itself is the problem, Is Saturn always bad? shows how the text judges Saturn by its strength, not its name. How a rule like this is checked against a real chart is in how AstroEngine reads the BPHS.
Your Saturn, read by its own table
People ask about Sade Sati by name, so your reading shows when Saturn crosses those three signs in your chart, says plainly that the seven-and-a-half-year frame belongs to the later tradition, and then reads each sign the text's way: by its points in Saturn's table, and by whichever planet's years are running underneath. Every paragraph ends with its verse, and every verse leads to its page in print.