Now all the southern points are selected. Go to Selected -> Align Vertices as shown in the screen capture, and the Align Vertices dialog will pop up as shown. Note how the X, Y, and Z labels follow the same color scheme. We want to constrain these vertices along the vertical (yellow, Y) axis, so they'll form a series of coincentric circles all on the XZ plane. This will be useful later.

Make sure the "Apply" box for whatever alignments you want to make (in this case Y) are checked, then put in the value you wanted them aligned at. In this case, we want everything to be aligned with the equator, which we included in our selection for one thing: the "Max" Y value from the data set listed in the buttons is 0, that of the equator. You could put in any arbitrary value you wished, but for now click the Y Max button. The number in the position box will go to that value (0). Click OK.