A U.S. satellite put into earth orbit two years ago to look at the sun has compiled data indicating an almost steady drop in the sun's output of energy, a condition that could have been a cause of this year's harsh winter.

"The duration and magnitude of the change we've seen is certainly enough to have had an effect on our climate," said Dr. Richard C. Willson of California's Jet Propulsion Laboratory, where a sun-looking telescope was built and put aboard the Solar Maximum Mission Satellite. "At the very least, it's a suspicious coincidence," he said.

In the 18 months starting Feb. 16, 1980, the telescope found an almost continuous decline in the sun's radiant energy, the light leaving the sun--from the far ultraviolet through the visible and into the far infrared regions.

On plotting the information received, Willson found that the sun's output of light in the 18 months ending in last August was one-tenth of 1 percent less than it had been when the satellite started measuring solar luminosity.

"This is a small change in the total energy output of the sun," Willson said, "but it has great potential significance for the earth's fragile ecosystem."

Scientists believe that any systematic rise or fall in the sun's release of energy can change the earth's climate. A decrease of as little as one half of 1 percent in the 17th century--called the Maunder Minimum when sunspot activity almost vanished from the sun--is believed to have coincided with the cold extremes felt throughout Europe in what is now called the Little Ice Age.

What causes the sun to undergo changes in its release of energy is unknown, but Willson noted that even while his instrument was recording a steady decline in solar output it witnessed an abrupt reversal of the downward trend through part of the last five months of the period ending with August, 1981.

While hesitating to blame this year's colder-than-normal winter on a drop in solar radiance seen two years ago, Willson said there could be a strong relationship between the two if only because it takes at least a year for any changes undergone by the sun to be felt on earth.

"There is a long lag on earth because the atmosphere and the oceans are so slow to heat up or cool down," Willson said. "If you turned off the sun tomorrow, you wouldn't see its full effects on earth for three years."

Willson's findings confirm a somewhat cruder discovery made in 1980 by an instrument aboard the Nimbus 7 weather satellite that the sun was losing as much as one-twentieth of 1 percent of its energy through all of 1980.

"Our instrument can't see as small a change as Willson's can see," said John Hickey of Eppley Laboratory Inc. in Newport, R.I., where the Nimbus telescope was built, "but we saw the same trend that he's seen. Our instruments are in full accord."

The findings of both spacecraft are the strongest suggestions yet that the sun does not deliver an unchanging supply of energy.

The foremost proponent that the "solar constant," as it has long been called, is an inconstant number is Dr. John A. Eddy of the High Altitude Observatory in Boulder, Colo., where telescopes have recorded images of what appears to be a shrinking sun.

Boulder has found that the sun may have been losing as much as a meter and a half of its diameter every hour along the equator and may have been shrinking by as much as one-tenth of 1 percent every 100 years for the last four centuries.

"There's no reason to believe that the solar constant is constant," Eddy said by telephone yesterday from Boulder. "I think we have to expect anything within reason from the sun."

Eddy said the sun could start expanding again at any time, increasing its energy output. For this reason alone, he cautioned against blaming one particular winter on a decline in solar energy.

"Relating it to this winter is very chancy. I'll begin to believe that when these things happen time after time," he said.