Every Sunday morning I enjoy a walk along a two-mile stretch of the world's most sensitive gravimeter. The sun's and moon's gravitational tugs on my body amount to only a few millionths of the earth's, and yet that tiny force acting over the Pacific Ocean regularly raises the sea level so high as to batter my path with waves. But isolating these feeble forces inside a laboratory tasks even professional instruments. That's why I was astonished to learn that Roger Baker of Austin, Tex., has developed a device that can gravitationally track the position of the sun and moon--for about $100.

Image: DANIELS & DANIELS SENSE AND CONTROL CIRCUIT(top) detects tiny movements of the flag and adjusts the current in the feedback coil to compensate. To use the instrument as a seismograph, add the filters as shown. The temperature control circuit (above) can maintain the sensor's temperature to within one hundredth of a degree Celsius.

One of my favorite truisms of science asserts that yesterday's discovery is today's calibration and tomorrow's noise. That is particularly applicable here. Last January I described another Baker invention that can detect micropulsations in the earth's magnetic field. But even these minute magnetic effects would swamp the gravitational forces that we are trying to measure here. In fact, eliminating all the spurious influences is by far the most challenging part of this project, and success depends on experience. Only expert gadgeteers should attempt it.

In Baker's apparatus, a small but powerful magnet delicately floats between two permanent magnets. A clever optical device senses small movements caused by gravitational shifts, seismic activity, thermal expansion, a stomping toddler and so on. A control circuit counters the motion by fine-tuning the current through an electromagnet. Changes in this current thus track forces acting on the floating magnet. The float is weighted to make it insensitive to high-frequency motion. But it does capture slowly oscillating signals from earthquakes (with undulations of a few tens of hertz) and the changing position of the moon as the earth turns on its axis. In fact, Baker originally designed the instrument as a vertical-motion seismograph.

The optical sensor accounts for much of the instrument's sensitivity. To monitor the float's position, it uses an opaque flag to block some of the light from an LED and keep it from reaching a phototransistor a few millimeters away. When the float and flag move, the light signal changes quickly; shifts in position on the order of a nanometer have a discernible effect.

Radio Shack sells ceramic magnets in sets of five for about $2 (part no. 64-1888). You'll need two sets for a total of 10 ring magnets. Stack six of them into two groups of three and epoxy them to bolts. The seventh magnet will serve as the float; the other three are unneeded. Baker keeps the float from moving sideways by attaching it to a steel razor blade. The sharp edge of the razor will abut a brass plate on the side of the instrument support. Two miniature rare-earth magnets (Radio Shack part no. 64-1895) behind the plate fix the razor's edge to the brass. A smear of oil along the blade creates an almost frictionless hinge that allows the float to swing up and down but not side to side. Finally, Baker attaches a one-ounce lead fishing weight to lower its natural frequency and the small opaque flag to sense its position