Sidebar: Why a Total Eclipse?

In day-to-day life, Newtonian gravity is enough to predict how objects will behave. In order to see the effects of General Relativity, extraordinary conditions are needed – either high speeds, close to the speed of light, or strong gravity. In the early 1900s, the most accessible test for General Relativity was to watch the behavior of starlight as it passes very near the Sun.

According to General Relativity, light passing near any object with mass will be deflected, but only the Sun has enough mass for that deflection to be detectable from Earth, using the technology available in 1919. However, in order to observe starlight bending near the Sun, astronomers had to wait for a total solar eclipse. The best way to visualize why is to consider what the daytime sky looks like. Clearly there are stars still shining over the entire sky, but none are visible during the daytime. This is because the light from the Sun is scattered by our atmosphere, giving us our familiar blue skies. For the same reason, astronomers cannot observe starlight near the Sun on just any day – the Sun is too bright, and the blue sky obscures any attempted observations.

Therefore, to observe stars near the Sun, astronomers needed to wait for a total solar eclipse. Only during a total solar eclipse is the light of the Sun blocked out, making stars visible in the daytime.

However, observations during the eclipse alone, would not tell astronomers if the starlight had been bent. They also needed images of those same exact stars without the Sun in the way. Then, by comparing the two images – one taken without the Sun between the stars and the Earth, and one taken during the solar eclipse – they could determine if the Sun had any effect on the starlight. In addition, if the starlight had been bent by the Sun, they could determine by how much.