JUDGE Thomas Penfield Jackson imposed a tough sentence on Mayor Marion Barry yesterday, but a fair one. In spite of the fact that the mayor faced a maximum of a year in jail, many observers were surprised that a first offender convicted of the single misdemeanor of drug possession was given any hard time at all. In the aftermath of last summer's federal court trial there had been speculation that Mr. Barry would be put on probation, perhaps ordered to pay a fine and probably compelled to provide some kind of community service. But Judge Jackson's sentencing statement provided ample justification for his decision to impose a six-month jail term, followed by a year's probation and a $5,000 fine.

A penalty of this magnitude for drug possession is well within the sentencing guidelines and is not unusual in federal court, where defendants charged with more substantial offenses often plead to possession. In Superior Court, where the bulk of the city's drug offenders are tried, jail time for first-offender possession is uncommon. But so is the mayor uncommon. He is not a teenager without a family or a job, a youngster experimenting with dangerous drugs or a person whose conduct has no impact on the community at large. As Judge Jackson pointed out, he is the elected leader of the city that is the nation's capital and in that capacity he was responsible for enforcing the drug laws. Instead, he not only broke those laws but by his conduct made himself vulnerable to the dealers and traffickers who run the terrible drug trade in this city. Judge Jackson found that Mr. Barry had violated the public trust, broken the law repeatedly over a long period and attempted to obstruct justice in order to avoid exposure and prosecution. These findings all influenced the judge's decision not to settle for probation but to set a penalty midway between the minimum and maximum terms available for this offense.

No one rejoices to see a man who has achieved so much over a lifetime brought so low. It appears, as Judge Jackson noted with approval, that the mayor has finally accepted responsibility for his addiction and has taken steps toward recovery. The whole city must hope that progress continues and is sustained through the difficult months ahead for the mayor and his family. But the sentence is a just one in light of all the circumstances surrounding the offense, and the city is ready to move on.