Q. HOW IS NORTH DETERMINED FOR OTHER PLANETS? WHAT ABOUT THE REST OF THE UNIVERSE?

A. Maps are made with the convention that Earth’s north and south are mirrored on the other planets in the solar system. They are orbiting around the sun in the same plane as Earth, called the plane of the ecliptic.

As the planets rotate, the ends of their rotational axes are their north and south poles, though some of the planets have a rotational axis that is much more severely tilted than Earth’s.

Like modern terrestrial maps, the maps of bodies in space are oriented with north on top. This somewhat arbitrary choice was almost certainly made because of Earth’s magnetic North Pole and because of sailors’ age-old reliance on the North Star for navigation in the Northern Hemisphere.

On other planets, however, magnetic fields are weak or nonexistent.

Probably because humankind tends to see itself as being the center of the universe, when scientists started to map the solar system beyond the home planet, the same north/south divisions were kept, with the dividing line being the Earth’s equatorial axis. Eventually the definitions were extended to the hemispheres of the galaxy and the hemispheres of the universe itself. C. CLAIBORNE RAY question@nytimes.com