Above is an example of an interference pattern. As the earth turns, a celestial object appears to move relative to a stationary observer. The result is that waves arriving from two widely spaced antennas will interfere with each other. When the wave arrive in phase, they will add, creating peaks. As they arrive 180 degrees out of phase, they will cancel each other. The result will be valleys. The relative spacing of peaks and valleys is a function of the spacing of the elements. Use of this principle has allowed large monolithic dishes to be replaced by widely spaced collections of smaller antennas. The result is that radio astronomy routinely achieves resoltions measure in milli-arc-seconds. There is no free lunch in the observing game. The singal to noise ratio of interferometry is a function of the average dish size,not the sum of their areas. Thus, interferometers are not as sensitive as large monolithic dishes.