# Derivative of the Unit Vector in Spherical Coordinate system

I have a doubt regarding a step in the derivation of the momentum equation in spherical coordinates. I am referring to the textbook "Fundamentals of Atmospheric Modeling" by Mark Jacobson, 2nd Edition, Chapter 4, Page 86. The change in unit vector in the x direction, delta i, is described using geometry. The text describes the angle between delta i and Re (The radius of Earth) as phi, the latitude. I do not understand how this can be so. Only after understanding this can I proceed to the expression for delta i in vector form.

• $\Delta i_{\lambda}$ is defined, along with $i_{\lambda}$, to lie in a plane which is parallel towards the equatorial plane. The rest is fundamental trigonometry? – AtmosphericPrisonEscape May 6 at 21:31