Billy Cowden, age 12, of South Windham, Maine, for his question:

Are there tides on the land?

The same mighty forces that cause the ocean tides also heave the continents and create enormous bulges in the atmosphere. Earth scientists suspected that this must be true, but until quite recently it seemed impossible to estimate the rise and fall of the Earth's solid crust. After all, the altitude of the land is measured from mean sea level. And whatever tides heave the land must coincide with those that heave the oceans.

Measuring the rise and fall of tides is still a ticklish problem. But modern Earth scientists estimate that the solid continents rise and fall several inches, twice during a period of about 24 hours and 50 minutes. These land tides swing around the globe in step with the ocean tides and, like the tides of the ocean, they vary from place to place and at different times of the year.

The highest land tides are estimated to lift the continent of North America about six inches. One would expect such upheavals to cause widespread damage to buildings and geological formations. But there are reasons why they do not. The tidal action is global, and it heaves the entire Earth's crust as a single unit. Though the crust appears to be rigid, it has a somewhat elastic quality. As the enormous crustal tides swing around the globe, the land rises and falls so smoothly beneath our feet that we do not even feel it.

These land tides are caused by the gravitational pull of the Moon and the Sun, the same forces that govern the ocean tides: The moon pulls more than twice as hard as the Sun because, though much less massive, it is much closer. The tidal pull on the Earth varies because the Moon orbits around us, changing its position in relation to the dun.

The strongest land and ocean tides occur when the Moon is new and again when it is full. At such times, the Moon and Sun pull together with their combined strength. We feel the weakest tidal pulls when the Moon is in the quarter and three quarter phase. At such times the Sun and Moon pull from opposite directions, reducing their total strength. As in the ocean, the two daily highs and lows are governed by the Moon passing overhead, which causes them to chase each other around the globe.

The Moon exerts more pull on our land and ocean tides than does the Sun. But the evidence suggests that the Sun has more control over certain tides in the global atmosphere. Its radiation warms the gaseous air, causing it to expand and bulge aloft. This atmospheric tide is highest under the noonday Sun, and As the Earth rotates, it ebbs and flows around the globe.