The correct answer is A) sidereal hour angle. The sidereal hour angle of a star is a celestial coordinate that remains relatively constant in value. It is the angular distance measured westward along the celestial equator from the vernal equinox to the hour circle passing through the star. This coordinate does not change as the Earth rotates, making it a reliable reference point for celestial navigation. The other options are incorrect because they represent hour angles that are not constant. The Greenwich hour angle and local hour angle vary as the Earth rotates, while the meridian angle is a different type of celestial coordinate not related to the hour angle.