For a final year engineering project a student was asked to build a `sun-tracker'. This is a radio receiver connected to an aerial which follows the sun across the sky. After first contact the moon eclipses the sun and the signal level will gradually decrease. We hope to see the signal drop in steps as the sun moves behind the moon. This is because some areas of the sun (those associated with sunspots, plages and prominences) will emit radio waves more powerfully than others. When one of these areas is hidden the radio signal will drop more sharply (and when uncovered it will rise more suddenly).
The line where the moon and sun appear to touch is an arc (drawn in red). When we see a step in the radio signal we will know there is an active area somewhere along the arc.
When the sun moves out from behind the moon the arcs are drawn the other way round. By looking at the intersection of the arcs we can identify two possible positions for the active area. Instruments located at a different site will see the arcs drawn at different angles. If we can combine our data with that recorded by these other instruments we will be able to pinpoint the exact location.
During totality the radio signal will not drop to zero because the corona will still be visible and emitting radio waves. After third contact the radio signal will increase. From the time between the first and fourth contacts it will be possible to determine the size of the sun appears to be at 10GHz.