Human beings haven't been as ingenious as some creatures in taking advantage of the sun's power. Plants make their own food using some dirt, a little water and solar energy.

Still, we used to be a lot beter at solar housing than we are now. All those once-cheap (and now scarce and therefore increasingly expensive) fossil fuels spoiled us and stripped us of our collective knowledge of how to get the sun to heat our homes.

Indians of the Southwest, for example, cut their homes into the north face of stone cliffs, so that the sun heated up the stone floor. The floor released the warmth later into the night air and kept residents warm at night.

Principles of passive solar heat are wonderfully simple: When the sun is out, its heat is absorbed and stored in a "thermal mass" (such as rock or water). At night or when it is cold, energy stored in the thermal mass is released into the cooler air.

Passive solar heating can't achieve the same high temperatures an active system can, but that amount of heat isn't needed in a passive home. Passive homes tend to radiate heat throughout the home from one warm structural surface to another, and radiant heat feels warmer than forced-air heat does at identical temperatures.

The passive solar home is the heating device, the kind of solar device known as a heat trap.

Several factors are required to make the trap work:

Good insulation and a tight house. That also is true of active systems.

Southern exposure of the major window constellations. In this latitude, the sun is always south. In the winter here, it is fairly week and low in the sky. But enough of the sun's light can slither into a home with good window exposure to provide plenty of heat -- perhaps all that's required even in this climate.

Some kind of good "thermal mass" to absorb the heat once it gets inside the home, perhaps stone walls and floors. Within the dictates of cheerful esthetics, they should be dark-colored, since the darker they are the better they will absorb the light. The storage mass will also take excess heat from the air, since windows tend to overheat open space, particularly in the summer.

Some device to trap the heat gained through the windows after the temperature drop at night, such as insulated drapes or shutters. Windows are great at getting the sun's heat into the house, but they also radiate about 10 times as much heat per square foot to the outside as do walls and ceilings.