# 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.

Can anyone help please?   • $\Delta i_{\lambda}$ 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
• Thank you. It sounds obvious but I was having a difficult time picturing it. But now it is better. All the three vectors, i, i + delta i and delta i, are all in a plane which is parallel to the equatorial plane. All the three vectors hence make the same angle phi with the Radius vector as is the definition of phi. And on resolving delta i into its components, the cos component is along Re and in turn along Re cos phi. – ThoughtTraffic May 7 at 2:50