The phenomena being described is a solar eclipse. Some key details that indicate this:
- It occurred on a specific date (May 29th, 1919)
- Two teams were sent to two separate sites for observation, implying it was a widespread event visible from multiple locations.
- It was one of the most awaited ones, suggesting it was a significant astronomical event.
- The eclipse was the only chance to "turn off" the light from the Sun, allowing certain observations.
So in summary, the total solar eclipse of May 29th, 1919 was being observed to test Einstein's theory of general relativity by measuring the deflection of starlight by the Sun's gravitational field during the eclipse.