HOW many hours are there in a day? You may think that it is a trick question; it isn't. The answer depends on how a day is defined. It takes the Earth 23 hours 56 minutes 4.1 seconds to rotate once around its axis. This is called the "sidereal" day. This is different from the "solar" day -- the time from one sunrise to another or the time it takes the Sun to come back to the meridian.

Now, let me rephrase the question: How many hours are there in a solar day on Earth? If your answer is 24 hours or 86,400 seconds, strictly speaking you are wrong. A solar day currently is 86,400.003 seconds, as compared to 86,400 seconds almost 200 years ago. It increases by approximately 40 nanoseconds per day due to the complex gravitational interplay of the Moon and Sun with the Earth. The number may look insignificant having little or no effect on our daily life, but it is part of a large-scale astronomical process with important long-term effects.

Whenever an object is gravitationally attracted toward another, various parts of the object feel gravitational forces differing in strength and direction because the force is inversely proportional to the square of the distance between the objects. The differences in gravity which occur in an object being attracted by another are called Tidal Forces or Differential Gravitational Forces.

The Earth is caught in the middle of a gravitational tug-of-war between Sun and the Moon. The gravitational force exerted by the Sun on Earth is much greater than that of the Moon, even though the Moon is much closer. This force is responsible for keeping the Earth on its nearly circular orbit around the Sun. However, Moon's tidal force on the Earth is greater than that of the Sun. This is because the tidal force, in contrast with the gravitational force, decreases as the cube of the distance between two bodies. Being closer, the Moon wins the tug-of-war as far as tidal forces are concerned causing tidal bulges in the oceans and distortion of the Earth's rocky surface. The latter is known as Solid Earth Tide.

The Earth rotates about its axis much faster than the Moon revolves around the Earth. The Earth's rapid rotation coupled with higher orbital speed drags the tidal bulge forward of the Moon's orbit. But the Moon's gravitational attraction is trying to put the brakes on our spinning planet and pull it backward. Consequently, Earth feels a resistance, similar to a resistance encountered by us when we try to walk through water. The net result is the tidal flow of ocean water together with the flexing of solid earth gives rise to tidal friction. It pulls the line between the tidal bulges ahead of the line between the centers of the Earth and Moon.

Tidal friction slows the Earth's rotation resulting in longer days and months. At present the length of a day increases by 0.0015 seconds per century. At this rate for example, the length of a day 100 million years from now will be 24 hours 25 minutes. To keep the day consistent with atomic clocks and astronomical observations, every 18 months a leap second is added to the Coordinated Universal Time.

The rate at which the length of a day increases changes from century to century. When Earth was created 4.55 billion years ago, the day-night cycle was only 6.5 hours. At the beginning of the Cambrian Period about 600 million years ago, days were 21 hours long, whereas in the early Carboniferous Period some 350 million years ago, a day lasted about 23 hours.

Today Earth's slowing rate is higher than what it was millions of years ago. This is due to resonance of the present rate of rotation with the back-and-forth movement of the oceans.

How long will it take Earth's rotation to slowdown to the point when the day will be a month long? Billions of years, may be. Before that happens, the Sun will probably evolve into a Red Giant and engulf both the Earth and Moon. Tidal friction is also making Earth recede from the Sun, but scientists are not sure whether the Earth-Sun distance billions of years later will be large enough to save us from destruction.

"What other body could pull an entire ocean from shore to shore? The moon is faithful to its nature and its power is never diminished."

The writer is a Professor in the Department of Physics & Engineering Physics, Fordham University, New York