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The roughly seven and a half year stretch during which Saturn transits the sign before your natal Moon, the sign of the Moon itself, and the sign after it. It is the most frequently asked about period in this subject, but the term and that grouping of three houses do not appear in the Brihat Parashara Hora Shastra, which has no chapter on gochara at all, or in Phaladeepika chapter 26, which does. This calculator reports the window because leaving out the single most asked about transit would be worse than explaining it, but it is shown as a widely used modern convention rather than as a classical rule.

Phaladeepika 26.41 gives the rule directly: a planet transiting a sign that holds more of its own bhinnashtakavarga bindus is read as passing through the better ground. This calculator reuses the natal bindu tables and the 25 and 30 bands already sourced to Brihat Parashara Hora Shastra 72.3 to 72.6 and Phaladeepika 24.37, applied to the sign each planet currently occupies, rather than inventing a separate transit specific rule.

No. Checked across its full range of chapters, it has no chapter on gochara. Phaladeepika chapter 26, "Transits of Planets", does, and is the source this calculator cites for the parts of a transit reading it implements.