Interpolation

Definition

Interpolation is a mathematical technique used to estimate a value that falls between two known values. In astrology, it traditionally helps calculate a planet’s approximate position, house cusp, or other chart factor when reference tables list data only for set times or intervals. The calculation assumes a reasonably consistent rate of change between the listed points.

Astrological Meaning

Before astrological software, practitioners commonly interpolated values found in ephemerides, tables of houses, and other astronomical references. For example, if an ephemeris gives a planet’s longitude at midnight on consecutive days, interpolation can estimate its position at the exact birth time. The method may also be used when adjusting sidereal time or locating intermediate house-cusp values. Extra care is needed when a body changes speed, stations, or turns retrograde, because simple interpolation may then be less accurate.

Why It Matters

Interpolation supports chart accuracy. Even a modest difference in a calculated position can affect a fast-moving Moon, a house cusp, an angle, or the closeness of an aspect. Understanding the technique also helps astrologers evaluate hand-calculated historical charts and recognize where approximation may be involved. It is a technical tool rather than an interpretive symbol: its purpose is to establish reliable chart data before astrological meaning is considered.

Related Encyclopedia Entries

• Traditional Astrology
• Astrological Terminology
• Ephemeris
• Natal Chart