The sun would be directly overhead (90 degrees from all horizon directions; the zenith) at noon in the southern hemisphere (the winter solstice, December 21) as seen from a latitude of 23.5 degrees south at midday (the winter solstice). In the northern hemisphere (the summer solstice, June 21) it would be at its lowest altitude at noon (the nadir), located 23.5 degrees north at midday. In addition, there are two more points on the horizon where the sun is completely hidden by clouds or is not visible over land. These are called "nun" and "ilon". A location can only be at one place at a time - either it has the sun at its zenith or it doesn't. If it does then that's where you have to be to see it at its highest point in the sky. If it doesn't then you can't say that the sun is at its zenith even if it actually is because there is no way for you to know. Now, assuming that this location is within the tropics, i.e. between the equator and the poles, then when the sun is at its highest point in the sky (at noon on the winter solstice), it will be directly above the center of the earth.