Obliquity of the ecliptic
Cross-traditionAstronomical factEditorial synthesis
Definition
The tilt between the ecliptic and the celestial equator: about 23.4 degrees. It is the same tilt that gives Earth its seasons, and it enters the calculation of the Ascendant and Midheaven.
Also called: Obliquity · Axial tilt · Epsilon
In more depth
The Earth's rotation axis is not perpendicular to its orbital plane. Seen from the sky, this appears as a tilt of about 23.4 degrees between the ecliptic and the celestial equator, and the two great circles intersect at the equinoxes. This single angle is why the Sun climbs higher in summer than in winter.[1]
Standard practice separates the mean obliquity, which changes slowly and smoothly, from the true obliquity, which adds the small periodic wobble called nutation. Converting between equatorial and ecliptic coordinates uses this angle, so anything that depends on the relationship between the horizon, the equator and the ecliptic (the chart angles in particular) depends on it too.[2]
References
Numbered citations in the text link to the records below; each record links to the full source registry.
- NASA Science. Basics of Space Flight, Chapter 2: Reference Systems. current (living reference page). Astronomical fact View source record
- Sean E. Urban (editor); P. Kenneth Seidelmann (editor). Explanatory Supplement to the Astronomical Almanac. Sean E. Urban and P. Kenneth Seidelmann. University Science Books. 2013. Astronomical fact View source record