WE HAVEN'T seen -- and won't until Sunday night -- the last episode in public television's remarkable series "Cosmos." So we can't tell you yet just who, according to Carl Sagan, "speaks for Earth." But we can tell you that the first 12 programs in this series have been television at its long-awaited best -- entertaining, informative and fascinating.

In those 12 hours, Mr. Sagan and his associates have escorted their viewers through the universe, exploring the past, present and future in startling, mind-stretching ways. Last Sunday, for instance, they moved among unidentified flying objects, ancient Egyptian hieroglyphics and radio telescopes in trying to explain how the people of Earth may someday discover that there is intelligent life elsewhere in the universe.

The series has all the flair and excitement of those ever-popular movie and TV space spectaculars. There are imaginary flights to the stars and real trips all over the world. The titles of some of the episodes convey the idea -- "The Shores of the Cosmos Ocean," "The Backbone of Night," The Lives of the Stars."

What makes "Cosmos" different from all those other space shows -- and, God knows, there are enough of them on the air these days -- is that it is dealing with the facts and theories of science. Its real contribution is that it has used the techniques and special effects of science fiction effortlessly to communicate a body of knowledge that often seems too difficult or too tedious to comprehend when presented in other ways.

WETA reports that "Cosmos" is drawing a larger audience here than any other 13-week series in public television's history. But that audience is still small when compared with those attracted routinely to the commercial networks. Yet "Cosmos is fulfilling the promise proponents of television have always said was there: that the techniques of the medium could be used to enrich viewers without boring them and to give them more than just fun and games as part of their daily or weekly fare. See it when it comes around again next fall. It will give you a new standard by which to judge the rest of television programming.