House Cusps and Chart Angles
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The house cusps, and chart angles (Ascendant, Midheaven, Equatorial Ascendant and Vertex) are accurate for all dates limited only by the uncertainty of changes in the earth’s rotation rate (sometimes called DeltaT). Solar Fire uses the best available Swiss Ephemeris calculations to determine these changes. However, there is still an inherent uncertainty of about 15 minutes of time around 1500 BC, and possibly several hours around 3000 BC.