Astronomers can make certain studies only during solar eclipses. Years ago, for example, some astronomers thought that a small planet might exist between Mercury and the sun, but they needed an eclipse to test their beliefs. Investigations during an eclipse showed that there was no such planet.
Albert Einstein developed an important theory about the effect of gravitation on light. He predicted that gravitational interaction caused light from stars beyond the sun to deviate or swerve from a straight path as it passed the sun. A deviation would cause an apparent change in the position of the stars that are observable near the sun at eclipse. Einstein's theory was tested and proved during an eclipse.
At eclipses, astronomers can also observe the spectra of the corona and the chromosome, or outer layers, of the sun. This provides important information about the chemical and physical make up of these areas. For example, such studies reveal that the corona has a temperature over 1,000,000 C. The eclipses of Jupiter's satellites by Jupiter's shadow can be seen with a small telescope. In 1675, Olens Roemer, a Danish astronomer, discovered the approximate speed of light by studying Jupiter's eclipses.
