Q:

Left aligning centered equations

I have set of equations which I would like to just assign one number to. The equations should be aligned at the center of the page. The problem is that I would like all equations to begin at the same point whereas now they end at the same point. I use the \usepackage{amsmath}. Using the following input:
\begin{equation}
\begin{aligned}
u_1 = \partial \phi_1 / \partial x + \partial \psi_1 / \partial z \\
w_1 = \partial \phi_1 / \partial z - \partial \psi_1 / \partial x \\
u_2 = \partial \phi_2 / \partial x \\
w_2 = \partial \phi_2 / \partial z
\label{eq2.23}
\end{aligned}
\end{equation}

I get the following output:

The problem is that I want the u2 and w2 to appear directly below u1 and w1. If anyone can help me with this, I would greatly appreciate it!

A:

Use the alignment tab & to justify the equations, i.e. substitute = by &=.
\documentclass{article}
\usepackage{mathtools}
\begin{document}
\begin{equation}
\begin{aligned}
u_1 &= \partial \phi_1 / \partial x + \partial \psi_1 / \partial z \\
w_1 &= \partial \phi_1 / \partial z - \partial \psi_1 / \partial x \\
u_2 &= \partial \phi_2 / \partial x \\
w_2 &= \partial \phi_2 / \partial z
\label{eq2.23}
\end{aligned}
\end{equation}
\end{document}

