Kim Jones, age 12, of Delta, Ont., Canada, for her question:
WHY ARE SOME HIGH TIDES HIGHER THAN OTHERS?
Every 24 hours two high tides and two low tides wash up and down on the beaches and go chasing each other around the globe. However, the times of arrival and departure differ from day to day. What's more, high tides are higher during the periods of the new moon and the full moon.
A seaport can count on one high tide and one low tide every 12 hours or so. But tomorrow's timetable will differ ¬and it does not help to consult the tidal predictions for beaches farther along the shore. Some of these variations are caused by the shape of the coastline, but most are caused by the rotating earth and its complex relation to the orbiting moon.
Our tides are pulled up by the combined gravitational pull of the sun and the moon. If both were at the same distance, the sun's pull would be 27 times stronger because it is 27 times more massive. But the moon is so close to us that it pulls more than twice as hard.
Now picture the lunar orbit. When the moon is new, it is between the earth and the sun. The sun and moon pull together from the same direction, so we get the highest tides of the lunar month. We get more high tides during the phase of full moon. This time the earth is in the center of the lineup, but sun and moon pull together.
As the moon continues its endless orbit, this revolving picture changes from moment to moment. During the quarter moon phase, the sun, moon and earth form a right angle, with the earth in the corner. This means the sun and moon pull at us from different directions, which cancels some of their combined strength.
A similar situation occurs during the three quarter lunar phase. At such times the combined pull of gravity from the sun and the moon is weaker and so are the earthly tides. Then the high tides are somewhat lower than usual. The highest high tides of the lunar month are called a spring tide, and the lowest high tides of the lunar month are called the neap tides.
We might expect the lunar orbit to be exactly level with our equator. Instead, it is slightly tilted, which adds to the complications. Today it may swing south, tomorrow north of the equator. Since the tidal bulge is directly below the overhead moon, this causes slight variations in the high tides from day to day.