Caption: The ecliptic plane, the ecliptic axis (AKA the ecliptic pole) the celestial axis (indicated by the line to the north celestial pole (NCP)), and the Earth's axial tilt.

Features:

1. The ecliptic plane has a perpendicular which we can call the ecliptic axis (or ecliptic pole).

2. Everyone knows there Earth's axis has a tilt, mostly they don't know what it is tilted with respect to. It's tilted with respect to the ecliptic axis.

3. The axis of the Earth is CURRENTLY tilted from the ecliptic axis by ∼ 23.4° (see Wikipedia: Axial tilt: Earth's axial tilt)---the value 23.5° is commonly cited, but that is just a rounded approximation.

   Note the Earth's axial tilt in the image is approximate.

4. To be more precise, at the epoch J2000.0 (i.e., nearly the start of year 2000), the Earth's axial tilt was 23°26'21.4119'' = 23.43928108° ≅ 23.4° (see Earth Orientation Center: Useful Constants). Currently, the Earth's axial tilt is decreasing by ∼ 47'' per century (see Wikipedia: Axial tilt: Earth). This means the Earth's axial tilt will be ∼ 23.2° in 4000 CE.

   The time variation of the Earth's axial tilt is due to astronomical perturbations.

5. The Earth's axis direction over human lifetime is approximately constant. However, there is a slow precession relative to the observable universe due to astronomical perturbations. A precession is the sweeping out of a cone or a double-cone by an axis. The Earth's axial precession is just called the axial precession (AKA precession of the equinoxes). It has a period of order 26000 years, but there is NO exact period since the astronomical perturbations are always slightly changing rate of the precession.