Solar eclipses: the coincidence that switches off the day
Total, annular and hybrid: how a solar eclipse happens, what can be seen during totality and which scientific discoveries it has made possible in history.
The Sun's diameter is about four hundred times larger than the Moon's, but it is also about four hundred times farther away. From this coincidence, which answers to no physical necessity, arises one of nature's rarest spectacles: for a few minutes the lunar disc exactly covers the solar one, the day darkens and the corona, the outer atmosphere of our star, appears in the sky.
Total, annular, partial
A solar eclipse occurs at new moon, when the Moon passes in front of the Sun near one of the nodes. If the Moon is close enough to the Earth to cover the solar disc completely, the eclipse is total; if it lies towards apogee and appears too small, it leaves a ring of light and is called annular. There are also hybrid eclipses, total along one stretch of the path and annular along another. Anyone outside the central band sees only a partial eclipse.
The Moon's shadow traces a strip across the Earth that is usually no more than a few hundred kilometres wide, and for this reason any given place witnesses a total eclipse on average once every three or four centuries. Totality never exceeds about seven and a half minutes. On 12 August 2026 the band of totality crossed Spain; on 2 August 2027 it will touch southern Spain, North Africa and Egypt, with more than six minutes of darkness near Luxor.
What can be seen
In the last moments before totality the light fades in an unnatural way, the temperature drops and many animals behave as they do at dusk. The last rays filter through the valleys on the Moon's edge, forming Baily's beads, described by the English astronomer Francis Baily in 1836, and then a single brilliant point, the so-called diamond ring. During totality the brightest stars, the planets and the pinkish prominences of the chromosphere appear.
Looking at the partially eclipsed Sun without suitable filters is dangerous for the eyes, even when the light seems weak. Only during the brief phase of totality is the disc completely hidden, and it is then that the corona is revealed, with its silvery streamers stretching for several solar diameters and changing shape according to the star's cycle of activity.
Eclipses and discoveries
Solar eclipses have played an important role in the history of science. In 1868, observing the spectrum of the prominences during an eclipse visible from India, Jules Janssen noticed a yellow line that Norman Lockyer attributed to an unknown element, later called helium after the Greek name of the Sun. In 1919 the British expeditions promoted by Frank Dyson and Arthur Eddington, to the island of Príncipe and to Sobral in Brazil, measured the bending of starlight near the Sun predicted by general relativity.
The day that dies
In ancient cultures the solar eclipse was the most feared omen. In China court astronomers had the duty of announcing it, and a legendary tale handed down in the Classic of Documents tells of two astronomers, Xi and He, punished for failing to predict one. In the Greek world the poet Archilochus, in the seventh century BC, describes in a fragment the dismay at the Sun that Zeus darkened at midday, turning day into night.
Today we can calculate the path of every eclipse centuries in advance, but the coincidence that makes it possible will not last for ever. The Moon is slowly moving away from the Earth, and in a very remote future total eclipses will give way to annular ones alone. We are, in this sense, privileged spectators of one epoch in the long history of the solar system.