What does a compass measure?

A compass measures direction relative to the Earth's magnetic field. This tool utilizes a magnetized needle that aligns itself with the magnetic field lines of the Earth, indicating the cardinal directions (north, south, east, and west).

This capability is crucial for navigation, as it allows sailors and pilots to determine their heading and make necessary adjustments to their course. Compasses are fundamental navigational instruments that aid in steering a vessel and maintaining accurate bearings, particularly when out of sight of land or other reference points.

The other options refer to entirely different measurements that are not related to the function of a compass. For example, measuring speed relates to the vessel's speed through the water, depth gauges focus on how deep the water is below a vessel, and thermometers measure the temperature of the water. Each of these actions requires separate instruments specifically designed for those purposes.