Thanks to the accident of timing, we now have learned the difference between a royal visit and a summit conference. A royal visit sends official Washington into a state of nervous anticipation and excitement. With a summit, it's just plain nervousness.

I shall leave it to my betters to explain why the visit of the Prince and Princess of Wales caused such tremors of delight in the top circles of Washington. Among the leaf-rakers, weekend tennis bums and disappointed Redskins fans with whom I consort, it was no big deal.

But even where I hang out, the approaching Reagan-Gorbachev summit is a topic of great interest and, generally speaking, apprehension.

The tone of disquiet radiating outward from the White House itself is causing a great many people to ask of President Reagan's visit to Geneva: is this trip really necessary?

Part of the queasiness has been caused by the bizarre events leading up to the summit: the defection and re-defection of KGB official Vitaly Yurchenko and the ship-jumping and subsequent departure of Miroslav Medvid. The cases of the inside- out turncoat and the sailor who bolted but refused asylum left the impression that you can't trust your senses when anything Russian is involved.

At a higher level, Secretary of State George Shultz and National Security Adviser Robert C. McFarlane appeared to be shaken by their presummit encounter in Moscow with Soviet leader Mikhail Gorbachev. Shultz described him as "combative" and came away from their meeting saying that no one should expect substantive agreements when Reagan and Gorbachev sit down in Geneva next week.

For his part, the president provided further evidence that he is going to the summit with his mind still scrambled on what he wants to say and do there. What the White House called "imprecision" in his remarks to the Soviet journalists on his thoughts about eventual deployment of a space-based strategic defense system clearly shook both our own negotiators and theirs.

Given what Shultz found about Gorbachev's suspicions of American society, it is easy to imagine the Russians concluding from the president's wavering utterances that the negotiation is just a charade to cover America's rearmament plans.

I don't think that is the case. I believe, on the contrary, that Reagan is perfectly sincere in wanting to cap the arms race, to reduce existing nuclear weapons stocks and to stabilize the system at lower levels of terror and destruction. I think he and his wife want history to write that Reagan was a peacemaker -- not just a weapons-builder.

But it is as evident as anything can be that Reagan has not learned arms-control issues well enough to analyze competing proposals and strategies. It is equally evident that there are powerful forces inside his administration who oppose arms control and will do their utmost to ensure the negotiating process does not produce any results.

They have sold him on "Star Wars" as the ultimate defense against nuclear weapons -- which the scientific evidence suggests it is unlikely ever to be. They have diverted his attention from the fact the Soviets well understand: that Star Wars technology could produce a new wave of offensive weapons that would jeopardize the existing nuclear parity of the superpowers.

For all these reasons, I have thought for several months that the Reagan-Gorbachev summit is likely to be a disappointment and could be a fiasco. It is altogether probable that Gorbachev will take umbrage at what he may interpret as Reagan's studied "imprecision" and conclude that the weapons-builders in the Pentagon have Reagan in their pocket. If Gorbachev challenges Reagan's sincerity, or pushes too hard for precise promises, Reagan is altogether likely to get his back up and revert to the ideological stereotypes he has long voiced about Soviet wickedness.

That kind of summit could set both nations back on the course of competitive arms-building and raise the risk of confrontation even higher.

But I don't think that is what Reagan wants to leave as a legacy. And because this president is often better on his second tries (remember, the New Hampshire primary victory followed the Iowa caucus loss in 1980 and the Kansas City debate wiped out the memory of the flubs in Louisville in 1984), I thought a possible Geneva II, in 1986 or 1987, would produce the results that are likely to elude him in 1985.

But people who know the Kremlin much better than I do tell me that Reagan may not get a second chance, if Geneva 1985 goes as badly as now seems possible. They say Gorbachev must decide now whether this is an administration with which he can usefully negotiate. He needs the answer before he submits his long-term plans to the Soviet party congress this winter. If Gorbachev goes home from Geneva complaining either of Reagan's obtuseness or of his obduracy, it may be a long time -- and another administration -- before we have a real chance for substantial improvements in U.S.-Soviet relations.

I hope that view is wrong. I hope Reagan and Gorbachev allay my apprehensions about what will happen next week in Geneva. But that explains the nervousness.