If you space folk can read this, honk

First there was the slightly embarrassed opening day ceremony, starring Carl Sagan, with his four-month-old daughter Sasha in his arms, saying: ''Let the search begin.''

Like a lot of people, astronomers have their problems with rituals.

Then the radio telescope on a hill in the town of Harvard, Mass., tilted its 84-inch concave ear and got down to work, listening for messages from outer space.

''Can it be that the earth is the only inhabited planet? Is that possible?'' Sagan asked.

Paul Horowitz, the Harvard University physicist in charge of project SETI - search for extra-terrestrial intelligence - left the Sagan question carefully rhetorical, putting the chances of success at one in a thousand. But even if the odds were a hundred thousand to one, could the human race - more curious than any cat - resist pointing this great electronic question mark at the galaxy, silently miming the words of a William Saroyan play, ''Hello out there''?

Communication, finally, is not only an act but an intent - a longing. People light signal fires, put notes in bottles, and tap-tap Morse code into the night not just out of immediate necessity but out of primal loneliness. And so, for the next four years, the radio telescope on this hill in laconic New England will monitor 128,000 channels simultaneously, listening for E.T.

The supposition seems to go like this:

It is not intolerable to be the only beings in the universe intelligent enough to receive a message. But if anybody were out there, intelligent enough to send a message, it would be intolerable for us not to know.

Horowitz and his associates are guessing that any message will be transmitted on the frequency of the hydrogen atom since this is the most plentiful element in the universe. There is a sort of assumption that space aliens - if they exist - are more intelligent than we, but kindly disposed toward accommodating to our crude state.

If only we could feel so charitable about those we consider aliens on earth!

A receptive posture is a vulnerable posture, and this SETI project runs certain classic risks common to those who wait and listen: first, of course, the disappointment of silence; then, the possibility of messages one does not wish to hear.

As it cocks its ear, the radio telescope, for all its sophistication, bears a family resemblance to ancient cosmic devices like Stonehenge, which also said in effect: ''We're here, if you're there.''

Waiting, listening has always been a popular community pastime. It is also one of the great private dramas of life.

Poets with yellow pads of paper wait - listen - for the perfect word.

Monks in the desert wait - listen - for the indisputable still, small voice.

Juliet waits for Romeo - and listens in her heart a hundred times to his speech before he even knows he's going to deliver it.

We all wait and listen, even when we haven't the faintest idea what we're waiting or listening for.

The anthropologist Loren Eiseley thought maybe we were given instructions before we were born and journey through our days, trying to remember them - listening with that strange, faraway expression on our faces.

What will the searchers on the Harvard hilltop signal back if, against all odds, they receive a message? There will be plenty of time - light-years - to frame an appropriate response. For once, it is being a listener that counts. And perhaps Eiseley was right - we already have the message, and the answer.

If this is true, the SETI searchers might well tape Kafka's remark to their computer: ''You do not need to leave your room. Remain sitting at your table and listen. Do not even listen, simply wait. Do not even wait, be quite still and solitary. The world will freely offer itself to you . . . it has no choice, it will roll in ecstasy at your feet.''