In one of his plays Shakespeare wrote, "darkness has its uses." And so it does. What would it be like - what would we be like - if there were no darkness? In my most recent column we considered the wondrous nature of sunrise. In this one we find sunset to be just as significant.

A tide of darkness sweeps round Earth, turning us away from the daystar. Eventide is the time when many tend to pause in relaxation following a busy day. Colors in the sky make it easy to enjoy the hour of daylight waning into darkness. Brilliant colors, different every evening, wash away concerns of life as well as light of day. Cloud forms ignite imaginations and the play of sunlight in air never seems to run out of variations: pillars of light, solar halos and sundogs, sun transformed into Japanese lantern - all this and more. Colors of all hues rise to their peak, then diminish and fade to darkness.Too many people turn their backs upon the sky as the colors dim. Those who continue to watch see grandeur almost beyond belief. Sometimes a delicate lunar crescent follows the sun down. Evening planets appear in their cycles. Always the stars come out, unless clouds intervene. Nothing is more provocative than stars. Nothing!

What if we had never seen the stars? "It is a feeble light that comes to us from the stars, but without it what would be the present condition of Man's mind?" Thus wrote Jean Perrin. And Plato said, "Astronomy compels the soul to look upward and leads us from this world to another."

The great science essayist and novelist Isaac Asimov wrote a story titled "Nightfall." He described a planet orbiting in a multiple star system, resulting in a place where darkness was almost unknown. Every two thousand years or so, however, all the stars came into an arrangement so that only one was left in the sky, and that last one was eclipsed. On these extremely rare occasions the sky was dark, the most dreadful thing imaginable for the residents of this world. For them, seeing stars was part of mythology, and they believed it would result in madness. Asimov's story was based upon words penned by the poet Emerson: "If the stars should appear one night in a thousand years, how would men believe and adore, and preserve for many generations the remembrance of the City of God which had been shown!"

So, again, what might it be like if we had no sunset - no darkness? What, indeed, would be the condition of Man's mind? One thing we can say with certainty: Conceptionally we would live in a much smaller world. We would know nothing about stars, nebulae, black holes, galaxies or the vast expanding universe. We would measure our potentials in units that go no farther than the moon and sun, and it is not likely we would know the distances of even these celestial bodies. Would we have science at all? Mathematics? Perhaps so, but it would be much different. If we did have occasional night, as in Asimov's story, what would we make of the stars? Would we fear them? Would we want to learn about them? How much could we learn in the short time they would be visible? How much more slowly would our knowledge about many things develop? Would our desires to explore be as passionate? What kind of intellectual beings would we be? Life itself - plant as well as animal - would be different. How different, without the coolness of the night to relax from the intense energy of the day? It is not possible to answer these questions in detail. We would be different, and our knowledge would be greatly limited. Consider. We would know nothing about the immense cosmos.

Much of our learning - of our knowledge - results from contrasts: bitter helps us appreciate sweet; pain allows us to enjoy comfort; cold makes warmth meaningful; and darkness brings awareness of so much we could not possibly know if we were always immersed in light.

Contrasts are the foundation of measurement and of science.

Sunset! A tide of darkness sweeps over us every few hours. While we are in the light of the sun we work and play. For all of history we have tended to do the things that keep us physically active during the day. During the night many have tended to close their doors and minds to the darkness, but others boldly have gone into the night to look around. And what have they found? The universe! They have found that almost everything that exists becomes evident only in the night. A few hundred billion stars, including sun, compose the Milky Way Galaxy. A few hundred billion galaxies compose the universe we have been able to observe and measure. As yet, we have not been able to know how many planets orbit stars, but growing evidence indicates they likely are the rule rather than the exception. We have not yet discovered life beyond Earth, but because it gets dark we are aware of possibilities. Our thinking has been stimulated by darkness.

Is this what Shakespeare had in mind? Yes, darkness does, indeed, have its uses.