How is "zenith" defined in celestial navigation?

In celestial navigation, "zenith" is defined as the point that is directly above the observer at an angle of 90 degrees to the horizon. This means that if a person were to extend a line straight up from where they are standing, they would reach the zenith. It represents the highest point in the sky from the observer's perspective and is critical in various calculations, such as determining the altitude of celestial bodies. Understanding the concept of the zenith is essential for accurate navigation and for interpreting celestial positions, which are crucial when using sextants and star charts. Other definitions provided in the choices relate to different celestial phenomena and positions, but they do not accurately capture the specific meaning of "zenith" in navigational context.