Three or more planets gathered in a single sign or a single house. A chart has ten planets to distribute, so a stellium puts a large share of one chart into one register — that department of life tends to run loud, for better and worse, and the rest of the chart is quieter for it. Practice varies on the threshold: some astrologers require four, and some count the Ascendant, Midheaven or nodes as members. This app counts three of the ten planets, and says which sign or house holds them.
A stellium is what a transit lands ON, not something a transit does. One slow contact to a stellium is several contacts in sequence: the transiting planet crosses each member in turn, so what would be a single passage over a lone planet becomes a long season with repeated peaks, and the same theme arrives from three angles instead of one.
nothing by itself — it concentrates whatever reaches it.
Four planets in Aries is a stellium: Saturn crossing that sign spends two years making the same argument four separate times.
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