OKAY, you're out there on the high seas halfway to Bermuda and it's dusk and you've just laid your $500 Plath sextant down on the chart table and you've got your three star sights and you've pulled out the "Nautical Almanac" and now all you have to do is spend, UGH, half an hour figuring out exactly where you are.

No more.

Thanks to the wonders of modern science - and a checkbook that will allow a $335 investment - a six-and-one-half by three-and-one-half inch battery-operated pocket calculator will solve the problem in about ninety seconds.

This same calculator, the Texas Instruments Programmable 59, will also compute golf handicaps, keep your checkbook balanced (with individual drafts stored in the memory bank), play blackjack against you, calculate biorhythms and convert inches into millimeters.

The heart of all this is a twofold innovation in calculators - or microprocessors, to be more precise: plug-in croms (constant read only memories) that automatically insert 5000 fixed program steps into the calculator; and slide-in cards that can store sixty bits of variable information or 480 steps of programming instructions on a magnetic stripe.

If that sounds complicated, it simply means that the user can personally program his TI 59 to very specific needs.

Fly your Cessna 182 to New York often? Program the in-route VOR (navigation radio) info and weight-and-balance data onto several of the slide-in cards and stick the $35 Aviation Library crom into the unit.Now you can tell at the push of a few buttons: whether the plane will take off; how much fuel you have left; where you are; whether you're heading in the right direction; and how fast your ground speed is.

The TI 59 has the capabilities of an IBM 1600, according to its creators, which means it takes the place of a computer that utilized a full room of vacuum tubes twenty years ago. All this is done with thirteen silicon chips, the size of fingernails, that store and process information. The unit itself costs $299.95; seven different croms - each containing a single chip - are available at $35 each, including ones for Applied Statistics, Real Estate/Investment and Surveying. There's even a Leisure Program crom that will compute football odds and - wonder of wonders - keep score for up to ninety bowlers who are playing simultaneously.

Less demanding users may find the TI 58 a better investment. This handy device contains ten chips and accepts croms but not programming cards. It has half the memory and half the programming ability of the TI 59, for a mere $124.95, plus crom costs.

Happy sailing.