In the Book of Leviticus, the Lord tells Moses what women are worth: three-fifths as much as men.

"Your valuation of a male from 20 years old up to 60 years old shall be 50 shekels of silver . . . If the person is a female, your valuation shall be 30 shekels."

It is a melancholy fact that today in the U.S. economy, thousands of years later, women are still earning only three-fifths as much as men on the average for full-time, year-round employment.

In 1939, median earnings for women who were full-time, year-round workers in the experienced civilian labor force were 58 percent of the median for men, according to government figures.

In 1960 the median for full-time, year-round women workers was 60.8 percent of the figure for men; in 1961, 59.4 percent.

And in 1978, the figure was also 59.4percent, with these full-time women workers earning a median of $9,350 for the year, compared with $15,730 for men.

So the figure has held fairly steady despite the fact that more than half the nation's adult women are now in the labor force, compared with only one-fifth at the turn of the century.

Experts have identified several majorreasons for the wage disparity, according to Elizabeth Waldan, senior economist at the Labor Department's Bureau of Labor Statistics.

One is discontinuity of employment-- the fact that women traditionally have tended to interrupt their work careers, sometimes for years, to have children. Others may work only part time. Because of household obligations, others may work only intermittently during the year.

This traditional pattern of women's work histories leads to lower pay. Women fail to build up seniority and accumulate the raises and promotions that go with it. And they may shift toa new job when they return to the labor force, and therefore fail to develope a high level of specific skills that brings more pay.

A second factor was that in the past,fewer women completed college and graduate school than men. Therefore, fewer qualified for high-paying professional jobs, like medicine, dentistry, law and the like. Their work tended to be concentrated in low-paying fields like retail sales. And nearly two-thirds of women classified asprofessional tended to be in teaching and nursing, which are relatively low-paying in the professional category.

Finally, as a recent Census Bureau publication notes, flat-out discrimination against women -- unequal pay for the same work, keeping women out of higher-paid and managerial jobs, restricting them to traditional "women's jobs" (clerical, sales, secretarial, nursing) -- "cannot be ruled out a contributor" to the differential.

Even as late as 1963, the tradition of paying women less for the same work was considered such a serious problem that Congress enacted the Equal Pay for Women Act.

Now, Waldman sees some signs all these conditions are changing and that in a few years, the ratio of female earnings to men's may go up. Many of the changes are products of the women's movement of the past decade.

One factor is that more women in the prime child-bearing age group (mid -20s to mid-30s) are working continuously instead of intermittently. Twenty years ago only about a third of this group worked; now it's three-fifths. These women are having fewer children, sometimes none at all, and are returning to work more rapidly after having them.

Therefore, they are building up more seniority, intensifying their skills -- and presumably will accumulatethe rewards in terms of better jobs and higher pay.

A second change is that more women are getting trained in the professional and technical skills that bring big money in the labor market.

Although the overall ratio of women'searnings to men's is only 59.4 percent now, in the occupations classified as professional and technical, requiring a high degree of skill, it is around 65 percent. And in some occupations, like computer operations readers, systems analysts, computer engineers, the earnings ratio is 100 percent or close to it.

So as a higher proportion of women receive training in professional and technical fields, and then get jobs in them, the earnings gap between the overall women's labor force and the men's should narrow.

According to Waldman, statistics clearly show that more women are getting into these types of jobs. Overall only16 percent of women were in professioal and technical jobs in 1978, according toCensus Bureau figures. But among women 25 to 34 -- younger women who havecompleted their schooling -- the figure was about 23 percent.

Moreover, according to Waldman, the proportion of women professionals in the relatively low-paid teaching and nursing fields is beginning to shrink (from nearly two-thirds to 55 percent) as more women go into the high-paid professional fields.

Finally, changes in public attitudes are opening up fields previously closed to women -- and laws like the equal-pay act plus attitudinal changes are wiping out the tradition of unequal pay for the same work.

All these changes correspond to the growth of the women's rights movement over the past decade, and some started in the 1960s or even earlier. But according to Waldman, it is only now that the changes appear to be accelerating enough to promise the possibility of a revision in the 1980s in the ratio of women's average earnings tomen's.

For the moment, however, the three-fifths figure still holds. Women who work full-time the year round are still only averageing 30 shekels to every 50 shekels earned by men.