Solar Eclipses

Solar Eclipses

A solar eclipse is a phenomenon in which the light of the Sun is totally or partially obscured by the interposition of a star between the Sun and the observer. In solar eclipses seen from the Earth, the star that hides the Sun is the Moon.

Esquema de eclipse de sol

Types of eclipses

From the observer's point of view, solar eclipses are classified as total, annular and partial. Such an observer will say that he or she has seen a total eclipse when he or she sees the Moon entirely cover the disk of the Sun. However, another observer located hundreds of kilometers further north or south than the first one will see the Moon cover only a part of the Sun, so for him or her the eclipse will be partial. There are times when the Moon does not entirely cover the Sun from any point on Earth, so for all observers the eclipse is partial.

Another common type of eclipse is the annular eclipse. These occur when the observer sees that the disk of the Moon does not totally cover the disk of the Sun, even though their centers are well aligned. This is because the Moon is farther away from the Earth that day than in the case of a total eclipse, so that its disk appears smaller than that of the Sun. In such a case a bright ring is seen surrounding the lunar disk.

Eclipses de Sol

¿When do eclipses occur?

The plane in which the Moon orbits the Earth (shown in blue in the figure below) is tilted 5º with respect to the plane in which the Earth (and Moon) orbits the Sun (shown in yellow). Since eclipses require the near-perfect alignment of the three celestial bodies, eclipses occur very few times during the year. The Moon takes about one month to complete one revolution around the Earth, so if the two planes coincided we would have 12 eclipses of the Sun and 12 eclipses of the Moon each year. In practice, the number of eclipses that occur each year is between 4 and 7, including those of the Sun and Moon. In many cases the eclipses are partial (or even penumbral in the case of some lunar eclipses), and visible from a fraction of the Earth's surface. When the Moon is close to the Sun in the sky, in the new Moon phase, there is a possibility of a solar eclipse. When the Moon is in the opposite direction of the Sun, visible all night in full Moon phase, there is a possibility of a lunar eclipse.

Orbitas de Tierra y Luna

For more information on solar eclipses see the article "Algunas precisiones y curiosidades sobre los eclipses de Sol" (pdf) published in the Almanac of the Astronomical Observatory for 1999, or the book  "Eclipses de Sol. El eclipse anular de Sol del 3 de octubre de 2005 en España", published by CNIG.