THE NATURAL WORLD is the subject of three recent nonfiction books for children. The first, Sky Watchers of Ages Past, takes a look at early astronomy:

"Shortly before noon, she was near the top, at a place where three slabs of rock stood upright near a wall. On the wall, in the shadow of the rocks, a large spiralwas carved in the stone.

"She bent forward to look at it. Suddenly, out of the corner of her eye, she saw something bright. A narrow band of light was shining on the wall. . . . It passed to the right of the spiral's center."

Malcolm Weiss begins his account of primitive sky- watching with the discovery in 1977 by artist Anna Sofaer that this Pueblo carving at Chaco Canyon in New Mexico was a kind of calendar. At noon on the day of the summer solstice, a ray of light shone through the spaces between the rocks and fell on the center of the spiral, marking a fixed point in the continuous flow of time.

In later chapters Weiss examines other early observers of the heavens. For them, gazing upward, "The great bowl of the sky was a stage upon which sun, moon, stars, and planets moved and played out their parts." Each culture composed its own myth about the sun, but often the stories were alike. In the dark days of the winter solstice the sun is born in a cavern, and then, spending more and more time above ground, it nourishes the spring sowing, growing stronger and stronger until the summer solstice. Then it begins to weaken. In the fall, as the scythes reap the harvest, it is wounded and dies, only to be born again, to repeat the cycles of the agricultural year.

For Polynesian sailors myths took the place of navigational tables. They guided their canoes out of sight of land with the help of fables that fixed in their minds the motions of the constellations. Most extraordinary of all were the discoveries of the Mayas of Central America, who could predict eclipses of the sun and moon, and knew how long it takes for the planet Venus to complete its orbit. The Mayas wrote down their predictions in books that were part of a great library. Few of these precious books remain. "We found a larger number of books," wrote a Spanish bishop, "and as they contained nothing but superstitions and lies of the devil, we burned them all, which the Indians regretted to an amazing degree, and which caused them great anguish."

That 16th-century book-burning is one of the few emotional moments in this simple and factual book. Weiss writes clearly, and the drawings are helpful. His examples are well chosen, revealing how universal was the fascination with the sky, and how widespread were the attempts to understand the relation of sun, moon and stars to the passage of time and the changing of the seasons.

In Dreams of a Perfect Earth, the sentences are sometimes awkward, but the subject matter is compelling. The Milnes have done a good job of showing the interrelatedness of living things. In their "dream of a perfect earth," they warn of the peril of unlinking any part of the chain.

They are at their best in chapters that sort the earth into categories: forests and fields, rivers, plains, mountains, deserts and cities. In a miscellaneous rich jumble we learn good things and bad things about each environment in trn. A paper mill uses as much fresh water as a city. A bristlecone pine in California may be the oldest living thing. Mayflies are proof of the cleanliness of a stream. Salt marshes produce more valuable protein than solid land. New Zealanders drop fertilizer on high grasslands from biplanes. Brown bats populate the attics of Manhattan. Over and over, the watchword is the same: "Unless the earth is valued for the wondrous variety of life it contains, it will be destroyed."

Both of these books suffer a little from an earnestness of tone that often afflicts nonfiction written for children. Only once do the Milnes turn frisky: "Any desert is like a discotheque. It gets busy after the sun sets."

There is nothing solemn or prosy about the last of the three books, The Land I Lost: Adventures of a Boy in Vietnam.

This remembered land is not the Vietnam of bomb craters and search-and-destroy missions. Although the author was wounded in the South Vietnamese army, his memories are not of war. Instead he writes of the animals of his childhood village and the surrounding countryside. His tales are described on the book jacket as true stories, but they read like lively folk tales, or fables by Aesop. The beasts seem as terrible as dragons, and they are outwitted, like most dragons, by guile and tough cleverness rather than by superior strength.

This is a scary book. There are giant bats and rats. There are killer bees. A water buffalo kills a tiger. A monkey dismembers a child. A wild hog gores a farmer. A crocodile snatches a bride. Most terrible of all is the horse snake, which can kill with its poisonous bite or crush a man to death.

But there is a cheerful swing to these stories. Again and again, courage and wit triumph over danger, as in this conclusion to the story of a cousin who kills a horse snake with a knife:

"The snake was so heavy that it took eight men to carry it home. The next day it was on display in front of our house. . . .

"My cousin put some coconut oil on his hair to make it shinier and he stayed near the snake. The young girls smiled at him a littleebit more than usual, and the young men seemed jealous.

"My cousin was a hero!"

Perhaps, after all, a Vietnamese village is no more dangerous than an American suburb. We too manage to keep our equanimity even though we fear the venomous nuclear power plant down the road or the deadly encounter with the drunk driver on the jungle of the highway.