Obliquity of the ecliptic

Obliquity (symbol ε) is the tilt between Earth’s rotational axis and the normal to its orbital plane — equivalently, the angle between the celestial equator and the ecliptic on the sky sphere.

Current value and slow change

Earth’s obliquity is about 23.4° (often quoted as 23°26′). It is not fixed:

• It varies roughly between 22.1° and 24.5° on a ~41,000-year cycle (Milankovitch obliquity cycle), affecting seasonal extremes over geological time.
• Short-term change is small (on the order of arcseconds per century) as the ecliptic pole shifts slightly due to planetary perturbations.

For everyday charting, ε is taken from a standard model at the epoch (e.g. mean obliquity at J2000 in IAU precession models). House and angle routines in the ephemeris stack use this geometry when transforming between equator and ecliptic.

Role in chart calculation

1. Equator ↔ ecliptic — converting positions between equatorial RA/δ and ecliptic λ/β requires ε and the vernal equinox direction (coordinate-systems, ecliptic-and-equator).
2. Seasons — obliquity is why sunlight angle changes through the year; the tropical zodiac anchors 0° Aries to the March equinox, not to a star boundary (tropical-zodiac).
3. Houses and angles — local horizon/meridian work uses Earth rotation (equatorial framework); cusps are expressed as ecliptic longitudes for the wheel display (chart-angles).

Not the same as ayanamsa

Obliquity is an Earth–orbit geometry angle. Ayanamsa is a separate sidereal offset subtracted from tropical longitude for Jyotish-style charts (calculate_sidereal). See sidereal-vs-tropical.

References

• Axial tilt — Wikipedia (obliquity of the ecliptic)
• Milankovitch cycles — long-term obliquity variation
• NASA: Milankovitch cycles and climate — obliquity range and timescale

See also ecliptic-longitude, natal-chart.