Secular variation refers to changes in what?

Secular variation specifically pertains to changes in the Earth's magnetic field over long periods of time. This phenomenon is observed as fluctuations in magnetic bearings, which are the directional measurements relative to magnetic north. As the magnetic field can shift due to various geophysical processes, understanding secular variation is essential for accurate navigation and surveying.

The other options, while they may pertain to changes in their respective fields, do not capture the essence of secular variation. Geographic coordinates refer to specific locations on the Earth, celestial movements involve the positions and motions of celestial bodies, and time zones relate to divisions of the Earth for the purpose of timekeeping. None of these options deal with the long-term changes in the Earth's magnetic field that define secular variation. Thus, magnetic bearings is the most appropriate choice.