Expert: Philip Stahl - 12/13/2004

Question
Dear Mr. Stahl,

   I have two questions. My first question is what causes the movement of our North and South Poles referred to as the precessional wobble? My second question is what is the reason our North and South poles are held at the approximately 23 degree angle that causes our seasons?

Answer
Hello.

First, the precession of the equinoxes - which is the technical term for the 'wobble' to which you refer. Let me also warn in advance that the motion is not simple, and the forces are not self-evident.

The differential gravitational force of the Sun on the Earth tends to pull the Earth's equatorial "bulge" into the plane of the ecliptic (the projected circle of the orbital path). Differential gravitational force refers to the fact that the force of gravity from the Sun is stronger on the near surface of the Earth, then the far surface (i.e. on the other side 180 degrees away).

Meanwhile, the differential g-force caused by the Moon tends to pull the "bulge" of Earth into the plane of the Moon's orbit which is nearly in the ecliptic. These combined forces, in other words, tend to pull the Earth's axis into a direction approximately perpendicular to the ecliptic.

This is why the motion is also known as: LUNI-SOLAR precession. Since it entails forces from BOTH the MOON and the SUN.  (About two-thirds of the effect from the Moon, the other one-third arises from the Sun)

Like a spinning top, however, the Earth's axis won't easily yield in the direction of these forces - but *precesses* instead. Thus, the obliquity (angle) of the ecliptic remains at roughly 23. 5 degrees. What happens then is that the Earth's axis seems to follow the path of an imaginary large cone in the sky, completing about 1 degree every 72 years.

This precessional motion is known as the "precession of the equinoxes" and lasts for nearly 26,000 years (take 360 degrees  x 72 years/ deg)

This precession, it must be emphasized, is a motion of the AXIS of the Earth and not to be confused with the variation of latitude (or simple polar motion) caused by a wandering of the terrestial poles themselves - a different explanation entirely.

Precession, thus, does not affect the cardinal directions on Earth or positions of geographical locations. It does, however, affect the positions of the stars in terms of which are seen and in particular, which are in the direction of the North (or South) celestial poles. (By way of example- owing to precession, Polaris is now called the "North star" - but will no longer be in 12,000 years. Then, VEGA will be the north star)

Second question, as to 'why' the North and South poles are held at 23.5 degrees. The only remote explanation so far is that this is how they originated when the primitive Earth formed from the collapsing solar nebula over 4.6 billion years ago. Thus, as angular momentum was transferred from the Sun to the planets, it engendered a particular spin direction for the Earth - which is preserved to this day.