APPARENT MOTIONS CONT. APPARENT MOTION #3: PRECESSION The third apparent motion is more subtle and is due to precession, the slow wobble of the earth. Over a span of 26000 years, the celestial equator slowly rotates against the ecliptic, causing a slow shift in the seasons (roughly 1 day/70 years) and an equivalent change in the North Celestial Pole. 13000 years from now, it will be winter on June 21st.

Note that the angle between the pole and ecliptic does NOT change. What changes is the direction to which the pole points.

In this diagram, at present time represented by the blue lines, the sun lies below the celestial equator. That means it is winter in the northern hemisphere.

Thirteen thousand years from now, represented by the red lines, on the same day, the ecliptic will be above the celestial equator, and it will be summer.