They are, in order of decreasing importance, personal safety, energy conservation, esthetics, and astronomical research. By following four simple guidelines, it is easy to satisfy them all. Many light fixtures waste energy by directing light upwards, where it only blocks out the stars, or sideways, where it casts long shadows and creates glare to blind the people it is intended to protect. The use of modern, efficient fixtures and proper installation will make for much more effective lighting. Second, use the most efficient types of lighting sources available.
Incandescent bulbs consume far more energy than fluorescent, mercury, metal halide, or sodium lamps.
Professional astronomers prefer low pressure sodium light, which is the most efficient of all and can be blocked by filters for most types of research. Third, turn off lights when they're not needed. Purely ornamental lights serve no purpose at 2 a.m., for example. Fourth, don't use more light than necessary. The human eye can adapt to amazingly low levels of light, if given the chance.