The Euler's equation obtained above is completely general and hence quite complex; we therefore have to rely on several approximations. The approximations we hereafter adopt are very strong, making it possible to explicitly integrate the equation and give a simple constant of motion. These approximations are also very convenient, or almost necessary, to highlight the particular physical situation of our interest.
First, we observe in the Euler's equation above that in a general situation the time derivative of θ must have both slow and fast components. The slow component represents long time-scale rising motion of the egg, while the fast component corresponds to the rotation motion of the egg with the time-scale comparable to Ω and n. However, what we experimentally do in order to observe the rising motion of a boiled egg is to rotate the egg such that the long time-scale rising motion is readily observable; this condition allows us to argue that the time derivative of θ does not have the fast component in the situation of our interest. We therefore assume that