Digestion is impaired during a solar eclipse

When the dragon swallows the sun

astronomy

Eclipses have moved people at all times - and even cemented a new worldview

March 18, 2015

A solar eclipse is an impressive spectacle. Thousands of years ago people were fascinated by it and came up with all sorts of myths. When the new moon moves over the solar disk, even wars are said to have been decided. In the 18th and 19th centuries, total eclipses came more and more into the focus of research. And in 1919 observations of a cosmic shadow play even underpinned Einstein's general theory of relativity.

Text: Helmut Hornung

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Cosmic shadow theater: A solar eclipse always occurs when the sun, new moon and earth line up ... [more]

Cosmic shadow theater: A solar eclipse always occurs when the sun, new moon and earth are on a line and the shadow of the moon falls on the surface of the earth.

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Cosmic shadow theater: A solar eclipse always occurs when the sun, new moon and earth are on a line and the shadow of the moon falls on the surface of the earth.

Hardly any other natural phenomenon moves people as strongly as a solar eclipse. Even the Babylonians, who observed the course of the stars in the second millennium BC, noticed an approximately 18-year cycle after which similar eclipses were repeated. The priest astrologers had no explanation for this. Neither could they predict these events, as they reflect the eternal ticking of the heavenly clockwork, the blueprint of which only began to be revealed in the 16th century.

The true course of the sun, moon and stars may also have remained a mystery to the Greek mathematician and philosopher Thales von Milet. Nevertheless, he is said to have prophesied a total solar eclipse in 585 BC. If one believes the historian Herodotus, this darkness over Asia Minor even decided a war between the Lydians and Medes. Just on the day of the great battle, the moon moved in front of the sun. Thales had warned the Lydians, but the Medes knew nothing of the shadow play. In fear, they stopped fighting and made peace.

A cosmic shadow play instilled fear and horror not only in the people of earlier times. When the moon darkened the sun over Kenya on February 16, 1980, many residents of the country fled to their huts or tried to drive away the demon of darkness with deafening noise. The Chinese of earlier centuries may have reacted in the same way when they saw with their own eyes how a terrible dragon with sparkling eyes devoured the sun. (Fortunately he spat them out every time.) In astrology, the star German, a solar eclipse traditionally heralds disaster.

But what is behind the phenomenon? The scientific answer to this question is far less exciting than mythology with dragons and demons, but it is based on an almost magical coincidence. After all, the moon, which is just under 3,500 kilometers in diameter, fits the sun, which measures 1.4 million kilometers, almost exactly. Because the star of the day is 400 times larger than the Earth's satellite - and 400 times further away. In the earthly firmament, the two celestial bodies therefore appear with the same apparent diameter of about half a degree.

This freak of nature alone does not make darkness. The moon must meet the sun in the sky and wander over its shining disc. That can only happen at the new moon. Only then - viewed from a point far out in space - will the moon stand between the earth and the sun. This constellation occurs every month, more precisely every 29 days, 12 hours and 44 minutes. Still, total solar eclipses are not all that common. Because the lunar orbit plane is inclined at an angle of five degrees to the plane in which the earth revolves around the sun once a year; this plane is called the ecliptic. In most cases the new moon passes unnoticed above or below the solar disk.

Occasionally, however, the new moon is very close or directly in one of the two nodes, as experts call the intersections between the lunar orbit and the ecliptic. The term dragon points has also been preserved to this day - in memory of the sun-hungry monster of the ancient Chinese. How the eclipse ultimately turns out depends, among other things, on the distance between the moon and the earth and the distance between our planet and the sun. The new moon is most favorable when it is close to the earth, which is far from the sun. But even then, the cone of the umbra on the earth's surface covers no more than a 300-kilometer-wide area, since it always only touches our planet with the tip.

Because the umbra cone travels 2,000 kilometers an hour in an easterly direction, total eclipse is a fleeting affair for a location along this narrow path. The totality can last a maximum of 7 minutes 31 seconds. The eclipse on March 20 lasts a maximum of 2 minutes 47 seconds.

Often the umbra does not reach the earth. If you are lucky enough to be exactly in the extension of the cone, you will see a ring-shaped solar eclipse: the moon leaves a tiny border around the solar disk. There are eclipses that begin in a ring, transform into total eclipses along the trace of the umbra and end again in a ring.

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Mind work: Theodor von Oppolzer and his colleagues created a directory of 8000 sun and 5200 ... [more]

Mind work: Theodor von Oppolzer and his colleagues created a list of 8000 solar and 5200 lunar eclipses - each meticulously calculated with paper, pencil and slide rule. The work Canon of Eclipses appeared in Vienna in 1887, one year after Oppolzer's death.

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Mind work: Theodor von Oppolzer and his colleagues created a list of 8,000 solar and 5200 lunar eclipses - each meticulously calculated with paper, pencil and slide rule. The work Canon of Eclipses appeared in Vienna in 1887, one year after Oppolzer's death.

Finally, there is a third variant: the partial solar eclipse. It can be observed outside the umbra wherever the penumbra, which is up to 7000 kilometers wide, sweeps over the surface of the earth. Such a spectacle has a larger audience because it occurs more often in a certain location than total darkness. A partial solar eclipse also occurs when the umbra cone does not hit the earth at all, i.e. there is no total eclipse anywhere on our planet.

With the computer and the appropriate programs, it is no problem today to calculate eclipses down to the second. When Theodor von Oppolzer and his colleagues set to work to record all eclipses (including those of the moon) between November 10, 1208 before and October 12, 2163 AD, the computer did not yet exist. In the 1880s, paper, pencil and slide rule were the only tools. In years of backbreaking work, the scientists created a directory of 8,000 solar and 5,200 lunar eclipses - each meticulously calculated.

As the book Canon of Eclipses Published in Vienna in 1887, its editor had already died a year. Oppolzer had long known the explanation for the 18-year Saros cycle that the Babylonians had already found. The sun passes a certain node of the lunar orbit every 346.62 mean solar days; this period is called the eclipse year and is about 20 days shorter than our usual year for calendar calculations. 19 eclipse years correspond to 6585.78 days.

A synodic month, the time between two new moons, lasts 29.5306 days. Coincidentally, 223 synodic months are almost exactly as long as 19 eclipse or 18 calendar years, namely 6585.32 days. This has consequences: after each of these 18-year periods, the game plan repeats itself in the sky because the eclipse conditions are almost identical. The above-mentioned Thales of Miletus probably predicted its solar eclipse with the help of this Saros cycle.

An eclipse begins with the first contact of the new moon on the eastern edge of the sun. This dent continues to grow gradually. With complete coverage, i.e. during totality, the sun is enveloped by a white-bluish shimmering halo. The prongs of this crown protrude into the sky up to twice the diameter of the solar disk. At this moment the observer looks at the outer sun's atmosphere, the corona, which is about one million degrees hot. When the sun is minimally active, it appears asymmetrical, with short "feathers" at the poles and long rays in the equatorial region. During the maximum of solar activity, it looks uniform.

A solar eclipse, whether partial or total, is not very productive for science today. In the 18th and 19th centuries, however, the moments of darkness brought many enlightening insights. In particular, the prospect of observing the corona and protuberances - gas flows that can be observed as filigree arcs at the edge of the sun during totality - prompted the researchers to go on expeditions to remote areas.

For example, Jules Janssen traveled to India for the eclipse on August 18, 1868, divided the light from the protuberances into a spectrum and found that these tongues of flame consist mainly of hydrogen. At that time, the researchers already knew that the sun was a gigantic ball of gas, meaning that it had no solid surface. The corona seemed to be the branch of the atmosphere. Charles A. Young and William Harkness wanted to take a close look at the darkness of August 7, 1869. To do this, they split the dull light from this crown into a spectrum. This showed a bright line that glowed in the green area. It could not be assigned to any known element on earth.

Apparently, Young and Harkness had discovered a new element that occurs only within the solar corona. It was therefore given the name Coronium. It was not until 1942 that the Swedish scientist Bengt Edlén identified the mysterious corona line: It comes from iron, whose atomic nuclei have lost half of their 26 electrons each. This is only possible with very little dilution and under extremely high temperatures of the gas. The corona had to be a million degrees. In contrast, the 300-kilometer-thin photosphere - the visible gas shell of the solar ball - brings it to just 5500 degrees.

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Tongues of flame: During a total eclipse, bright hydrogen gas streams appear near the edge of the sun ... [more]

Tongues of flame: During total darkness, bright hydrogen gas streams appear near the edge of the sun, which can usually be observed as filigree arcs. Astronomers used to be particularly interested in these prominences. The photo was taken during the solar eclipse on May 28, 1900.

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Tongues of flame: During total darkness, bright hydrogen gas streams appear near the edge of the sun, which can usually be observed as filigree arcs. Astronomers used to be particularly interested in these prominences. The photo was taken during the solar eclipse on May 28, 1900.

Total solar eclipses have not only increased our knowledge of our daytime stars. The difference between the calculated and actual contact times reflects disturbances in the lunar orbit and irregularities in the rotation of the earth. Deviations have been found in ancient and Arabic records. From this, the experts conclude that the moon moves four centimeters away from the earth every year. In addition, our planet seems to be turning slower and slower; the length of the day increases by 0.0016 seconds per century.

Perhaps the most important discovery during a solar eclipse was made on May 29, 1919. At that time, a new physical structure of thought was confirmed. In 1907 Albert Einstein addressed the question of how gravity influences the path of light. The astronomer Johann Georg von Soldner had been interested in precisely this problem more than a century earlier. If light were made up of particles, he thought, it would have to obey gravity just as much as a rock that is thrown up and falls to the earth.

Einstein calculated that a ray of light that grazes the sun should be deflected by its gravity by 0.875 arc seconds. This prediction should be checked during a total solar eclipse. Only then can the sun and stars be observed in the sky at the same time. Scientists had to measure the positions of stars near the Sun's edge and compare them with those in their catalogs to determine the deviation.

At the beginning of the 20th century, such an observation made great demands on the measuring accuracy of the instruments. 0.875 arcseconds are very few and correspond to about two thousandths of the full moon diameter. However, Einstein doubled this value in 1915. This demanded his general theory of relativity, according to which mass literally bends space - like a sleeper denting his mattress. This curvature of space should bring the light on the inclined path and shift the location of a star at the edge of the sun by 1.75 arc seconds.

On March 8, 1919, two astronomical expeditions started from England. One led to the island of Principe off the coast of Spanish Guinea, the other to the city of Sobral in northern Brazil. The aim of the researchers was to observe the total solar eclipse on May 29, 1919. Sir Arthur Stanley Eddington obtained 16 images on Principe, only two of which were usable. They each showed five stars - and the effect predicted by Einstein. Andrew Grommelin in Northern Brazil was also successful. On the eight photo plates, the stars appeared shifted by roughly the predicted value.

Today we know that the observations resulted to a large extent from measurement errors. Nevertheless, Einstein's general theory of relativity had in principle passed the first test. "Revolution in Science" was the headline of the Londoners Times on November 7, 1919. Once again, astronomical observations had changed our view of the world.