Solar eclipse on 12 August: between light and shadow

On 12 August 2026, we will witness an orbital alignment in the timeless cosmic dance between the Earth and the Moon. Our natural satellite will position itself between the Sun and the Earth, partially obscuring our star from the perspective of observers on our planet.

In an extraordinary coincidence, the Sun is approximately 400 times larger than the Moon, but it is also, on average, around 400 times farther away from the Earth than the Moon. As a result, both the Sun and the Moon appear to have nearly the same angular size in our sky. This remarkable circumstance allows the Moon to completely cover the Sun, producing total solar eclipses.

The fact that the Moon’s orbit around the Earth is inclined by about five degrees relative to the Earth’s orbit around the Sun means that a perfect alignment of these three celestial bodies occurs only under specific conditions. The planets of the Solar System orbit the Sun within approximately the same plane, known as the ecliptic. When the Moon crosses this plane (at the so-called lunar nodes) and is in conjunction with the Sun or in opposition to it, we may observe either a solar eclipse or a lunar eclipse.

This orbital coincidence occurs approximately every six months, during a period known as an eclipse season, and the favourable alignment lasts for about a month. The forthcoming eclipse season will bring both a solar eclipse (on 12 August) and a lunar eclipse (on 27-28 August). While such phenomena are not particularly rare on a global scale, witnessing an eclipse from a specific geographical location is considerably less common. This is due to the relatively small size of the Moon’s umbral shadow, which is projected onto the Earth’s surface and measures only about 300 kilometres in diameter. The last total solar eclipse visible from mainland Portugal took place more than a century ago, in 1912, and the next one after the eclipse of 12 August is not expected until 2144, more than a hundred years from now.

In regions experiencing a partial eclipse, observers will be able to watch (while always protecting their eyesight with certified eclipse glasses) the Sun’s disc being gradually covered by the Moon, taking on the characteristic crescent shape from around 18:30 (mainland Portugal time, known as first contact). The eclipse will reach its maximum at approximately 19:30, after which the Sun’s disc will gradually re-emerge until fourth contact at around 20:30, very close to sunset.

In areas where the eclipse is deepest, a drop in temperature will be noticeable and the light will take on muted bluish-silver tones (although it will not become completely dark). Observers may also notice the onset of a breeze, sometimes referred to as the “eclipse wind”, caused by differences in temperature and air pressure between the region affected by the eclipse’s shadow and the surrounding areas. The shadows cast by objects will also change: as they are illuminated only by a thin sliver of the Sun, the transition zone between light and shadow becomes much narrower. Shadows will appear almost binary in nature, with either a dark region (umbra) or an illuminated region, while the transitional area (penumbra) is dramatically reduced.

When the eclipse approaches its peak, an unusual phenomenon will occur. Beneath trees with dense foliage, there are always small gaps through which sunlight passes. In these spots, the light behaves like it does in a pinhole camera, projecting an inverted image of the partially obscured solar disc onto the ground. These projections can multiply, covering the ground with a myriad of tiny crescent-shaped images of the eclipsed Sun. The solar eclipse is so inclusive that even ants could, in a sense, "observe" the eclipse.

In the narrow strip of Portuguese territory where the eclipse will be total, night will briefly fall for just 26 seconds. During this fleeting moment, stars and planets will become visible, and it will be possible to observe the solar corona. Such observations are only possible during the total phase of a solar eclipse or from space using an occulting disc, as is done by ESA’s PROBA-3 mission. Several scientific questions about the Sun remain unanswered, including the long-standing mystery of the corona’s temperature: measurements taken during eclipses indicate temperatures of between one and two million kelvin, whereas the solar photosphere, the Sun’s visible surface, is only about 5,800 kelvin.

Another fascinating phenomenon, occurring on the threshold of totality, is the appearance of “Baily’s beads”: bright points of light visible along the edge (limb) of the Moon, caused by sunlight still shining through lunar valleys and illuminating the Moon’s mountainous terrain. At the peak of this phenomenon, the last traces of sunlight create a concentrated flash known as the “diamond ring” effect.

The retina contains no pain receptors, meaning that permanent damage can occur without any immediate sensation of discomfort. Under no circumstances should binoculars or telescopes be used to observe a solar eclipse unless observation is being conducted under the supervision of an experienced astronomer who is familiar with the correct filters required for this type of viewing.

We wish you an excellent viewing experience of this forthcoming solar eclipse, which will undoubtedly leave us all with unforgettable memories.