Why portable sundial need a compass?

A portable sundial needs a compass to ensure it is correctly aligned with true north. This alignment allows the sundial to accurately cast a shadow based on the position of the sun, which changes throughout the day and across seasons. Without a compass, the sundial could provide incorrect time readings, as the shadow would not correspond accurately to the sun's position in the sky. Thus, the compass is essential for precise timekeeping with a sundial.