This video explains that the Earth is not a perfect sphere, but that the diameter of the equator is wider than the diameter from pole to pole.
Students will learn that the gravitational pull of the sun and the moon on the asymmetrical shape of the Earth causes precession, a change in the direction of Earth's axial tilt.
This short video explains precession using easy-to-understand illustrations.
A full transcript of the video is available.
This video is the sixth in a series of Khan Academy lessons on Earth's rotation and tilt.
Students would benefit from watching this video on how Earth's tilt causes the seasons and this video on Milankovitch cycles, precession, and obliquity.
There is an animation students can watch, linked on the left-side menu to help students visualize the process.
Physics classes could write hypotheses for why the equatorial bulge exists. Students could research the phenomenon and then write a paragraph about why their hypothesis is correct or incorrect.
Math or art classes could make scale models of Earth out of clay by calculating the percentage difference between the diameter of Earth at the equator and the diameter of Earth at the poles.
Other resources on this topic include this video on how the shape of Earth and its rotation affect wind patterns and this video on Earth's rotation and revolution.
While it is often described as a sphere, the Earth is not a perfect sphere. The slight departure from a perfect sphere and the resulting interaction with gravity causes these orbital change. This resource is recommended for teaching.
Next Generation Science Standards (NGSS)
ESS1: Earth's Place in the Universe
MS-ESS1-2 Develop and use a model to describe the role of gravity in the motions within galaxies and the solar system.
HS-ESS1-4 Use mathematical or computational representations to predict the motion of orbiting objects in the solar system.