Online Astronomy eText: Background Physics: Motion and Forces Gravitational Interactions of the Earth and Moon: Precession (if you are math-phobic, refer to The Use of Mathematics in My Lecture Classes) The gravitational interaction of the Earth and Moon produces a number of effects. The most obvious of these is the orbital motion of the Moon around the Earth, but there is also a motion of the Earth around the center of mass of the Earth-Moon system (discussed in Gravitational Interactions of the Earth and Moon: Barycentric Motion), tidal effects on the Earth and Moon (to be discussed in Gravitational Interactions of the Earth and Moon: Tides), and precessional effects on the Earth's axis of rotation (to be discussed here). Consider the motions of the Earth: (1) Orbital motion (around the Sun once a year) --> changes in the stars visible in different seasons, a difference of about 4 minutes per day between the length of the day and the rotation period, and small seasonal changes in that 4 minute difference causing sundials to run fast or slow by as much as a few seconds a day (leading to what we call the Equation of Time, as demonstrated by the analemma). (2) Rotation around our axis --> day/night, and because of the approximately 23.5 degree tilt of our axis, seasons. (3) Precession = change in the direction of the axis, but without any change in the tilt This changes which stars are near (or not near) the Pole (see Ursa Minor for an illustration of recent changes. But it does NOT affect the seasons as long as the angle of 23.5 degrees stays the same (4) Nutation, a wobbling around the precessional path caused by the tilt of the Moon's orbit and regular changes in that orbit. This is a change in the tilt of our axis (small, never more than about 1/2 degree one way or the other), slightly increasing or decreasing seasonal effects. Occuring over an 18 year period and is due to the MOON exclusively (Save for the brief notes above, this page is merely a placeholder at the moment) Only item (3) will be discussed in detail on this page. The other topics will be covered on other pages. For instance, the average difference of 4 minutes per day between the rotation period and the length of the day is discussed here.