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A solar eclipse is an astronomical phenomenon in which the Sun, Moon and Earth create a straight line as they move, and the Moon's shadow falls on the Earth's surface. Depending on whether or not the lunar disk covers the entire Sun, we distinguish between total, annular, hybrid, and partial solar eclipses. During a total solar eclipse, it is possible to observe very interesting phenomena – the solar corona, the effects of Bailey's pearls, a diamond ring, etc. Reports of solar eclipses have been known since ancient times, and eclipses have been predicted for many centuries to come. Safety must be ensured during any observation of the Sun to avoid damage to the eyes.

Solar Eclipse

Basic information

A solar eclipse is an astronomical phenomenon in which the Sun, Moon and Earth create a straight line as they move. At the same time, the shadow of the Moon cast into space falls on the surface of the Earth. The solar eclipse can only occur due to the fact that when viewed from the Earth's surface, the Sun and the Moon have approximately the same angular magnitude of 0,5°.Although the Moon is 400 times smaller in diameter than the Sun, it is 400 times closer to Earth. Therefore, the apparent size of the solar and lunar disks in the sky is the same, and the lunar disk can accurately cover the entire Sun. Because celestial bodies do not move in circles but in ellipses, the mutual distance of the Sun from the Earth and also the Earth from the Moon periodically increases and decreases. Therefore, the lunar disk is sometimes larger and covers the entire Sun, other times it is smaller. Accordingly, total or annular solar eclipses occur.

If the Moon orbited the Earth in exactly the same plane as the ecliptic plane (the plane of the Earth's orbit around the Sun), the Sun would be eclipsed with each new moon, when the Moon is between the Sun and the Earth. In reality, however, the plane of the Moon's orbit around the Earth is inclined with respect to the ecliptic plane by about 5°, and therefore all three bodies get into one line only exceptionally. Therefore, solar eclipses are a relatively rare celestial phenomenon. The rarity of the solar eclipse is also due to the fact that each of them is observable only from a very small area. The Moon's shadow on the Earth's surface is at a maximum of only 270 kilometres wide, often much less, about 100 kilometres wide at the optimal mutual position of all three bodies. The length of the belt in which the shadow moves on the surface is a few thousand kilometres. Historical records and predictions of future eclipses show that in one place, a total solar eclipse is observable on average once every 400 years.

The following figure shows the relative position of the Sun, Moon and Earth during a solar eclipse. The distances and sizes of the bodies do not correspond to reality, the Sun is much larger and much further from the Earth, the Moon is in fact also farther from the Earth.

A total eclipse is observable only in a small area where the full shadow of the Moon marked with a dark grey colour falls. In contrast, a significantly larger area of the half-shadow is marked by the light grey colour. Light falls into this area only from a part of the sun's disk, it does not come from another part. On the surface of the Earth, we observe a sun disk only partially covered by the Moon, it is a partial eclipse of the Sun.

Types of solar eclipses

Minor differences in the angular size of the Sun and the Moon, which are caused by changes in the distance between the Sun and the Earth, or the Earth and the Moon have the effect that we can observe different types of solar eclipses.

Total solar eclipse

A total solar eclipse occurs when the angular size of the lunar disk is larger than the angular size of the solar disk. Then the area of complete shadow reaches the surface of the Earth, and the lunar disk covers the entire Sun in the sky here. The phase of a total solar eclipse is, of course, preceded by a phase of a partial eclipse, during which the lunar disk slowly slides in front of the Sun and gradually overshadows it. Likewise, a total eclipse is again followed by a phase of partial eclipse, when the shadow recedes. During a total solar eclipse, the entire solar disk is obscured and the most interesting astronomical details, such as the solar corona or Bailey's pearls, are observable. A brief moment of total eclipse is also the only and unique occasion when it is possible to observe the surroundings of the obscured Sun with an unprotected eye. During a total eclipse, it is practically completely dark in the landscape, as at dusk. The area in which a total solar eclipse is observable is called the belt of totality.

Annular solar eclipse

If the angular size of the lunar disk is smaller than the angular size of the sun disk, then the area of full shadow will not reach the surface of the Earth and no total solar eclipse will be observed at any point. The observer, who is on the earth's surface at the junction of the Sun and the Moon, sees the circular shadow of the Moon on the solar disk. At that time, the Sun has the form of a shining ring. At a more pronounced annular eclipse, when the size of the moon's shadow approaches the size of the solar disk, a noticeable gloom is observable in the landscape. The area in which the annular solar eclipse is observable is called the belt of annularity.

Hybrid solar eclipse

A hybrid solar eclipse is a situation where the angular diameters of the Sun and the Moon in the sky are identical. Such an eclipse is observable only in a very narrow belt of several tens of kilometres. The eclipse begins as an annular one and in a small area of a few kilometres inside the belt transforms for a short time to a total eclipse. Again, the solar corona is observable and the phenomenon of Bailey pearls can be observed around the entire circumference of the Sun.

Partial solar eclipse

In contrast to a total, annular or hybrid solar eclipse, a partial eclipse can be observed over a large area of the Earth's surface. In the event of a partial eclipse, the lunar disk does not cover the entire Sun, which is observable as a larger or smaller sun crescent. Each total, annular or hybrid eclipse begins and ends with a phase of partial eclipse, in which the lunar disk slides in front of the Sun, or it recedes again. All these eclipses are then observable at all times as partial, outside the belt of totality or annularity. Eclipses can also occur that are visible from anywhere on the Earth's surface only as partial ones. Partial eclipses may not be noticed at all during normal activities. Only when covering a large part of the solar disk (above 95%) is it possible to perceive dimming of the surroundings.

Phenomena observable during a total solar eclipse

Only during a total solar eclipse can they be observed with a naked eye. Because the glow of the sun's own disk is shielded, phenomena that are normally over-illuminated by sunlight are visible. The most prominent observable phenomenon is the solar corona. It is formed by radiant glowing gases escaping from the surface of the Sun and passes smoothly into interplanetary space. The shape and size of the corona depends on the current activity of the Sun. In the corona, jets of plasma from the surface of the Sun called protuberances can be observed. At the beginning and at the end of a total solar eclipse, it is possible to observe the so-called Bailey's pearls around the perimeter of the Sun. This phenomenon, similar to a chain of radiant pearls, arises from the passing of sunlight through various inequalities at the edge of the Moon. The effect observable at the moment just before the beginning of the total eclipse, or after its completion, when the last radiant part of the solar surface is still visible and the ring of the solar corona is already visible, is called the diamond ring. During a total solar eclipse, the brightest stars and planets are visible in the sky, the sky is dark blue as after dusk. The solar corona shines like a full moon.

Solar eclipse prediction

The solar eclipse, as one of the most impressive phenomena in the sky, has not escaped people's attention since ancient times and encouraged their imagination. According to some civilizations, it was the wrath of the gods, elsewhere they believed that the sun was being eaten by a dragon or that the end of the world was coming. One of the first alleged records of a solar eclipse is from China, when around 2136 BC the emperor executed his two astronomers for failing to properly predict the eclipse. A confirmed report on the solar eclipse comes, for example, from the year 762 BC from Mesopotamia or there is a record of 36 eclipses between 721 and 420 BC from China.

Due to the fact that eclipses occur periodically in different cycles, ancient astronomers have learned to predict them relatively well. In the 7th century BC, the period of Saros was discovered in Babylon, a period between the same eclipses that lasts about 18 years. During this period, there are 43 different solar eclipses. In addition to the most famous period of Saros, there are other periods of eclipses (Tritos and Inex), which were discovered later.

Table of the nearest solar eclipses (until 2025)

6. 1. 2019 partial North-east Asia, North Pacific Ocean
2. 7. 2019 total
South Pacific Ocean, Chile, Argentina
26. 12. 2019 annular
Saudi Arabia, India, Sumatra, Borneo
21. 6. 2020 annular Central Africa, South-east Asia, Pacific Ocean
14. 12. 2020 total South Pacific Ocean, Chile, Argentina, South Atlantic Ocean
10. 6. 2021 annular Northern Canada, Greenland, Russia
4. 12. 2021 total Antarctica
30. 4. 2022 partial south-eastern Pacific Ocean, south of South America
25. 10. 2022 partial Europe, north-east Africa, Middle East, West Asia
20. 4. 2023 hybridIndonesia, Australia, Papua New Guinea
14. 10. 2023 annular western USA, Central America, Colombia, Brazil
8. 4. 2024 total Mexico, central USA, eastern Canada
2. 10. 2024 annular southern Chile, southern Argentina
29. 3. 2025 partial north-west Africa, Europe, northern Russia
21. 9. 2025partial South Pacific Ocean, New Zealand, Antarctica

Data is taken from the NASA website: There, it can also be found that the future total solar eclipse visible from the territory of the Czech Republic will occur on October 7, 2135, and in the territory of Prague the total eclipse will not be observable until April 20, 2433.

Solar eclipse observation and safety

Safety rules must be observed when observing the sun! When looking directly at the Sun with an unprotected eye, there is a risk of serious, sometimes permanent, damage to the eyes. This danger is all the greater when viewed through binoculars, which concentrate sunlight even more. Eye damage is also a threat by a partial solar eclipse, even when a large part of the sun's disk is covered by a lunar disk.

For observation, it is ideal to use a special foil (a manufacturer is, for example, the company Baader Planetarium) designed directly for observing the Sun or special glasses fitted with this foil. It is also possible to reliably use higher-grade welding filters or spectacles or shades made therefrom. For occasional and short-term observation, it is possible to use a developed exposed black-and-white negative film or an exposed (black) part of the X-ray image, as well as a magnetic disk from a previously used diskette recording medium. It should be noted that sunlight contains not only visible light, but also ultraviolet and infrared components. We do not see either of them, but both can damage the eye with sufficient intensity. A suitable filter must also sufficiently filter out this invisible radiation.

It is definitely not advisable to use soot-stained slides, sunglasses and ordinary coloured foil. Although these films may shade a sufficient portion of the visible light and appear dark enough, it is not easy to determine whether they transmit a dangerous proportion of invisible radiation. For the same reason, it is advisable to be distrustful of various street vendors with protective filters or goggles.