Anne Bristow, age 11, of Richmond, Virginia, for her question:

Is it true that trees cool the air in summer?

It is quite true that trees add coolness, plus several other pleasant qualities to the hot summer air. All these things are actually by products of various activities that go on in the living greenery. A sunny summer day is the busiest time for the vital process of photosynthesis. This miraculous activity goes on mainly in the leaves, where green chlorophyll uses the energy of sunlight to manufacture basic plant sugar from water and the carbon dioxide its the air. The water, rich with dissolved chemicals is absorbed by the roots and hoisted aloft to busy greenery.

Part of this stupendous hoisting fob is the pushing power of osmotic pressure. Part is the pulling power of evaporation. Vacant spaces are left as enormous quantities of moisture evaporate from the leaves    and moisture from below seeps up to fill them. On a hot day, a tree loses tons of moisture through evaporation. And this gaseous water vapor tends to cool off the air. At the same time, the tree's spreading branches shade the ground from the direct rays of the scorching sun. In the woods, evaporation and shade cool down the temperature. What's more, while the sun shines, the busy photosynthesis recipe keeps adding refreshing supplies of oxygen to the surrounding air.

Every 24 hours, all our stars but one appear to move as the earth rotates around its axis. They move in ever widening circles around the fixed North Star. Every calendar day, the rest of the Little Dipper swings once around the bright gem star at the tip of its handle. Draco and the Big Dipper, Cepheus and the Cassiopeia swing around larger circles. North of the equator, at least half of these stars are visible summer and winter.

Outside the magic circle, the stars move in wider and still wider circles. At the equator, Polaris is near the horizon and we see only half the north circumpolar constellations at a time. Farther south, another set of circumpolar constellations swings around the South Celestial Pole. However, there is no faxed star there to mark the central spot. Every 24 hours, summer and winter, these southern constellations circle around a vacant patch of the sky.