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.

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.

¿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.

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.

Precautions

• he Sun should never be observed directly, with the naked eye or with sunglasses. During a partial eclipse, the Sun is never completely covered by the Moon and therefore looking at it without safe and adequate protection can damage your eyes, just as it would on any other day when there is no eclipse.
  If you need to purchase glasses for observing the eclipse, the CNIG sales points offer glasses that will allow you to observe, without risk, eclipses and sunspots. Purchase glasses.

  

• The Sun should not be observed with devices (cameras, videos) or instruments (telescopes, binoculars) that are not prepared for it and have the corresponding solar filters. Nor should it be observed with filters not approved for the safe observation of the Sun.

• The Sun can be observed without any danger by viewing its image projected on some type of screen located in the shade. For example, the sun can be projected on a wall or ceiling with a flat mirror covered entirely with paper except for a hole of less than 1 cm in diameter. Do not look at the image of the Sun in the mirror, look only at the projected image.

• The Sun can be safely observed with filters commonly known as eclipse glasses. They must be approved by the European Community for solar observation (opacity index 5 or higher) and must be used according to the instructions printed on them. They must be in perfect condition. Do not walk while wearing them, preferably remain seated. Do not take them off until you have looked away from the sun. They should not be used with optical devices, although they can be superimposed on prescription glasses.

• The use of optical instruments is not recommended except by professionals or experts with recognized experience in solar observation. Solar filters that screw onto the eyepiece should not be used, as they reach a high temperature and may break. If necessary, filters should be placed in front of the lens.

Observing the Sun with eclipse glasses or projection eclipse glasses

Observar el Sol siempre entraña riesgo, pues la gran cantidad de radiación que emite a diversas longitudes de onda (principalmente infrarrojo, visible y ultravioleta) puede dañar la vista, produciendo incluso ceguera si la observación es prolongada. Como regla general, y como en un día cualquiera, no debe observarse el Sol directamente: ni con aparatos, ni filtros o a simple vista. Esto es así tanto al observar el Sol sin eclipsar, como con el Sol parcialmente eclipsado o durante un eclipse anular.

Observing the Sun always involves some risk, since the large amount of radiation it emits at various wavelengths (mainly infrared, visible and ultraviolet) can damage eyesight, even causing blindness if the observation is sustained long enough. As a general rule, and as any other day, the Sun should not be observed directly: neither with devices, nor filters or with the naked eye. This is true both when observing the Sun without eclipse, and with the Sun partially eclipsed or during an annular eclipse.

To avoid any accident, it is advisable to observe the Sun by projecting its image on a cardboard, screen, wall or ceiling. Only if you have a professional filter, approved for visual observation of the Sun, can it be used for this purpose. Homemade filters are stronglydiscouraged. They are not suitable for viewing the Sun: sunglasses, smoked glasses, X-rays, reflections on water, home-made filters... Some of these systems filter visible light but not invisible light (infrared and ultraviolet radiation), and may cause temporary or permanent damage to eyesight.

We strongly recommend viewing the Sun by projection, as described below, or using an approved filter, for example those popularly known as eclipse glasses, which reduce sunlight by a factor of more than 30,000 times. They must comply with the corresponding European Community certification. Observation with eclipse glasses should be done for short periods of time (half a minute) followed by breaks of longer duration, and never use them in conjunction with binoculars or a telescope, instruments that require their own filters placed in front of the lens.

The simplest method for projecting the image of the sun is to use two pieces of cardboard, one of which should have a small hole drilled in it (a few millimeters, it does not have to be circular). Standing with your back to the sun, hold this cardboard so that its light passes through the hole and appears on the other piece of cardboard, located two or three feet away and in the shadow of the first. (It becomes a camera obscura.) Instead of a perforated cardboard, it is also possible to use a kitchen skimmer, obtaining in this case multiple images of the Sun.

To project the image of the Sun and obtain more luminosity, binoculars (or a small telescope) can be used, although for very short periods of time so that they are not damaged by the heat of the Sun. The focus of the image is achieved by adjusting the dioptric correction of the eyepiece. The image below left shows this method. Caution: do not look through the device!

Another safe and simple system consists of projecting onto a shaded wall or ceiling the image of the Sun obtained with a flat hand-held mirror covered with a piece of paper with a hole of 5 to 10 millimeters in diameter. This system is shown schematically in the upper right image.

Upcoming solar eclipses

Although it is common to be able to observe a partial solar eclipse every several years from a given location, it is not so common to witness a total or annular eclipse. In Spain, the last visible annular eclipse occurred in 2005, and the last total eclipse was seen in 1959, and only from the Canary Islands; the Iberian Peninsula has not seen a total solar eclipse since 1912. This drought of eclipses will end soon, because in the years 2026-2028 there will be two total eclipses and one annular eclipse that can be seen from some points of our country.

Upcoming solar eclipses visible from Spain

• Total solar eclipse, August 12, 2026
• Total solar eclipse, August 2, 2027
• Eclipse anular de Annular solar exlipse, January 26, 2028

Total eclipse, August 12, 2026

The first total solar eclipse visible from the Iberian Peninsula in more than a century will take place on August 12, 2026. The totality band of this eclipse will cross Spain from west to east and will pass through numerous province capitals from La Coruña to Palma, including Leon, Bilbao, Zaragoza and Valencia. Spain is located at the end of the totality band, so it will occur when the Sun is setting very close to the horizon. This will make it necessary to observe the eclipse from a place with good visibility to the west. Since it happens in summer, the chances of having a clear sky are high in a large part of the country.

Total eclipse, August 2, 2027

Almost a year after the 2026 eclipse, another total eclipse will cross the Iberian Peninsula. The totality band will cross the Strait of Gibraltar from west to east and will cover the southern tip of the Peninsula, including cities such as Cadiz, Malaga, Ceuta and Melilla. The eclipse will take place during the morning (10:50am approximately), and the maximum duration of totality in Spain will correspond to Ceuta, with 4 minutes and 48 seconds.

Annular eclipse, January 26, 2028

The band of annularity of this eclipse will cross the Peninsula from southwest to northeast just before sunset. The band will include cities such as Seville, Malaga, Murcia and Valencia, where the annular phase will be seen in its entirety. In Palma and Barcelona only the beginning of the annular phase will be seen as the Sun will set before the annular phase ends. Due to the low elevation of the Sun above the horizon, the observation of this eclipse will require excellent visibility in the direction of sunset.