MOON'S DRIFT MAKING DAYS LONGER, STUDY SAYS.

Byline: Paul Recer Associated Press 

If your day seems to be getting longer and longer, it is.

Tidal forces from the gravitational pull of the moon have caused the rotation of the Earth to slow over millions of years, a change that gradually is making each day a fractional second longer than the one before, according to a new study measuring changes in the motion of the planet.

The change can't be measured by an ordinary clock, but over eons, the lengthening day does make a difference, said Charles P. Sonett, co-author of the study being published today in the journal Science. ``About 900 million years ago, the Earth's day was only 18 hours long.''

Sonett, regents professor of planetary sciences at the University of Arizona in Tucson, led a team that measured the historic changes in tides by microscopically examining sediments laid down over millions of years at sea beds in the United States and in Australia.

The sediments, called tidalites, were deposited in ancient channels and estuaries in patterns that reflected the tidal influence of the moon and the sun on the ocean.

Tides change throughout a month, ranging from neap, or lowest tides, to the spring tides, which occur when the sun, Earth and moon are nearly aligned. These changes are reflected in the thickness of the sediments.

By measuring the sediment layers, said Sonett, it is possible to calculate the frequency of the neap and spring tides. This, in turn, allows the researchers to calculate the length of the day - the time it takes the Earth to rotate on its axis. The same data also reflect the distance between the Earth and the moon.

``The slowing of the Earth is primarily the effect of the moon's tidal force,'' said Sonett. The gravitational pull of the moon, he said, ``acts like a brake on the Earth. It is like putting a wrench against a spinning wheel.''

A tidalite sediment laid down 900 million years ago in what is now Utah shows that the Earth's rotation was about 25 percent faster than it is now, giving the planet an 18-hour day, said Sonett.

The moon, now 240,000 miles away, was then closer to the Earth, he said, and the lunar month was only 25 days, compared to the present 29.5 days. Sonett said the moon is receding from the Earth at the rate of about 1-1/2 inches a year.

This affects the lunar month, which is the time it takes the moon to orbit the Earth and the period from one full moon to the next. As the moon's orbit grows, the lunar month lengthens.

Sonett said that in theory, the moon will continue to recede from the Earth and the Earth will continue to slow its spin for at least an additional 15 billion years. At that point, the moon and Earth would be in synchronous orbit, about 336,000 miles apart. The moon would be locked into a fixed point above the Earth and the planet would show the same side to the moon constantly, he said.