Birth Time Sensitivity: How Many Minutes Change a Rising Sign?

Key Numbers

In close to a quarter of charts, a birth time off by less than 15 minutes is already enough to change the rising sign; in nine out of ten, an hour of error is enough. See Birth Chart Without a Birth Time for what stays accurate when the birth time is missing entirely.

By Rising Sign

Median minutes of birth-time error before the rising sign changes, broken out by the sign the simulated chart landed on:

Method

2,000 simulated birth charts (seed 20260913, a fixed value so the run reproduces exactly). For each sample: a birth year drawn uniformly from 1960–2010, a uniform month and day within that year, a uniform minute of the day, and a birth city drawn from the site's own GeoNames city list (public/data/cities.json), weighted by population. Venusward's production calculation layer (findSignChangeWindows()in lib/astro/sensitivity.ts) then finds, for each sample, the nearest earlier and later birth-time offset (up to 180minutes each direction) that would put the Ascendant in a different zodiac sign; the smaller of the two offsets is this sample's sensitivity value.

Script: scripts/research-sensitivity-sim.mjs. Reproduce with npx tsx scripts/research-sensitivity-sim.mjs 2000. Full generated data: lib/research/sensitivity.generated.json.

Limits

This is a synthetic sample, not a real birth registry — it draws uniformly across 1960–2010 and weights cities by population, which is not the same as the actual distribution of Venusward users' birth dates or locations. Every one of the 2,000 samples found a sign change within the 180-minute search window on at least one side (0excluded for finding none), so this run doesn't surface how the distribution behaves for the rare charts where the Ascendant moves unusually slowly. This describes the Ascendant only — the Midheaven, Moon, and Sun sign each change on their own, much longer, timescales.

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