Observations of total solar eclipses over the last three centuries have contributed greatly to our understanding of the Sun, Earth and the Moon. Comparison of eclipse timing to predictions has helped astronomers to refine their knowledge of the orbits of the Earth and the Moon. Total solar eclipses have allowed examination of the makeup and characteristics of the Sun and its atmosphere without having to contend with the bright surface of the Sun. In the early 20th century observations of stars during eclipses helped confirm aspects of the theory of relativity (see the article Eclipses in History).
Study of the Sun and understanding of its activity have been recognised as important to life on Earth. As the source of almost all energy on Earth we need to understand how it works and how its energy output varies over time. The Sun is the source of the solar wind which, when the Sun is active, can send highly energetic particles and hot electrified material towards Earth and this can disrupt satellites and other communications devices as well as electrical systems on Earth. In addition study of the Sun's nuclear fusion process may assist with research into eventually producing and controlling nuclear fusion as an energy source on Earth. Study of the Sun also helps us to understand other stars.
Because an understanding of the Sun is so important a number of satellites have been launched in recent times to study the Sun. These include SOHO (Solar and Heliospheric Observatory), TRACE (Transition Region and Coronal Explorer), STEREO (Solar TErresterial Relations Observatory, SDO (Solar Dynamics Observatory) and Hinode (Sunrise). Satellites are above the Earth's atmosphere and this provides the benefits of being able to block out most of the bright photosphere for study of the corona. They can examine the Sun in a range of wavelengths some of which are filtered by the Earth's atmosphere and they have the benefit of not having the distortion caused by the Earth's atmosphere with which to contend.
Satellite based observations have contributed enormously to our understanding of the Sun and have generally taken the place of observations that were previously undertaken during total solar eclipses. However, there still remain scientific experiments that are best done at the time of a total solar eclipse. Following are examples of some of these experiments.
The inner corona is difficult to observe using satellites due to the need to limit the scattering of sunlight in the satellite instrumentation, so this part of the Sun can still best be examined during a total solar eclipse.
As the Moon moves across the Sun and across the Sun's atmosphere it progressively covers or uncovers areas and these areas can be precisely located. This gives a method for accurately determining the position of emission sources of various wavelengths of light revealing information on the position of various compounds and the location and strength of magnetic fields.
Identification of the exact edge of the path of totality using a number of observers allows calculations to be made of the exact size of the Sun so that the change in the Sun's diameter can be monitored to reveal any changes over time.
Investigations of the Sun from the ground during a total solar eclipse have the advantages over satellite observations of being less costly, are able to be set up and adjusted at the last minute and can be continuously monitored by humans.