Introduction to Earth Sciences I
1.1 The Earth's Size and Shape, continued
This oblateness, combined with the inclination of the rotational axis with respect to the plane of the ecliptic (the plane that contains the Sun and in which most of the planets revolve around the Sun), governs much of the planetary dynamics as we will see later on in Topic 2 of these lectures.
The shape of the Earth is now known in great detail primarily from satellite observations. The Earth departs from spherical in many important ways and this results in the mass of the Earth being unevenly distributed. This too governs the planetary motion. Imagine how a basketball would spin in space if you glued a small weight on one part of the exterior. It would wobble as it spun. If the mass were moved around in a more uniform manner the period of rotation would change. We will learn about how the Earth rotates in the lectures under the second topic.
In fact, the Earth's period of rotation does change regularly by a few milliseconds (thousandths of a second). Accurate measurements of the length of the day (the rate of rotation) show that the rotation period changes with the seasons as water is redistributed over the Earth and the relative amounts of ice, water and water vapor varies. The effect of an El Nino can be seen in changes of the length of day. Human activity also changes the rotation period -- when the Three Gorges Dam in China is filled the change in length of day will discernible to a skilled amateur astronomer.