 U sub e is the eastward velocity of the earth and is pointed into the page. We know that U sub e equals r, which is the shortest distance vector between Earth's rotation axis and the point on the surface, times omega, which is Earth's rotation vector. The units of omega are seconds to the minus one, which makes it a frequency. Note that U sub e equals r times omega, which is also equal to omega cross r. We can see this if you take your right hand with your fingers pointed in the omega direction and the palm in the r direction, and you fold your fingers into the palm. Your thumb will point into the page, which is the direction of U sub e and is in the positive x direction.