What is the hour angle expressed in relation to?

The hour angle is defined as the measure of time since a celestial object, such as a star or the sun, last crossed the observer's meridian, expressed in angular measurement (degrees). It represents the angle between the observer's meridian and the meridian of the celestial object, where the observer's meridian is an imaginary line that runs from the North to South Pole and passes directly overhead at the observer's location.

This relationship is essential for determining the position of celestial objects in the sky relative to the observer. As the Earth rotates, the hour angle changes, allowing astronomers and navigators to track the motion of celestial bodies throughout the day.

The other options are related to different aspects of celestial navigation and astronomy. The observer's latitude is important for determining the angle above the horizon at which celestial objects appear, but it does not define the hour angle. The celestial equator serves as a reference for declination in the celestial coordinate system, but it does not determine hour angle. The Earth's rotation axis is relevant for understanding the overall motion of celestial bodies, but it does not directly relate to the hour angle itself.