What phenomenon can only be observed between the North and South Tropics?

Only between the North and South Tropics can the Sun be observed at its zenith, that is, directly overhead. In the Northern Hemisphere, the Sun rises in the southern sky and reaches its maximum height at noon. Depending on the season, it is higher (in summer) or lower (in winter) above the horizon, but never reaches the zenith. The further north the observer is from the Northern Tropic, the lower above the horizon he will observe our star. In the Southern Hemisphere, the Sun moves in the same way, only in the northern part of the sky.