Total or partial solar eclipses are spectacular events. How did we go about predicting them? Being able to anticipate these alignments is crucial for studying the Sun, even today.
The first solar eclipse of 2024 is approaching. It is scheduled for Monday, April 8. Even if the eclipse is not visible in mainland France, only in places in overseas France, it is eagerly awaited. It is thus possible to follow the solar eclipse online and live. This is a total solar eclipse.
But how did we go about successfully predicting solar eclipses? And why is it useful for scientists to know how to predict them? Remember that the existence of eclipses is the result of chance. “ The fact that the Sun and the Moon have the same apparent diameter as seen from Earth is a stroke of luck. Moreover, we know that the Moon is moving away from the Earth. One day there will no longer be a total solar eclipse », explains Léa Griton, doctor in astrophysics and researcher at IRAP (Institute for Research in Astrophysics and Planetology) in Toulouse.
When did we start predicting eclipses?
“ The first predictions of eclipses date back to Antiquity », Tells Numerama Nicolas Lefaudeux, optical engineer and amateur photographer, passionate about eclipses. As the Paris Observatory explains, one of the oldest known eclipse predictions dates from 731 BC: it was then a lunar eclipse. The prediction, which is attributed to the Babylonians, even mentioned that the Moon was not visible at the time, because it was not up.
Other Babylonian historical traces show elements that could have made it possible to predict eclipses, such as tablets containing saros. We speak of saros to designate a period of 18 years and 11 days, which is used to approximately predict the return of eclipses (both Sun and Moon). In China too, elements making it possible to determine the date of lunar eclipses have been found. It is unknown whether the saros also made it possible to predict solar eclipses at this time. The saros technique is limited: it allows you to predict the date of a solar eclipse, but not to indicate where it is visible from.
The solar eclipse is more complicated to predict than the lunar eclipse
“ Lunar eclipses are easier to predict than Solar eclipsescontinues Nicolas Lefaudeux. Lunar eclipses are visible from any point on Earth where the Moon is visible. Predicting the Sun-Earth-Moon alignment is sufficient to predict the eclipse. » What changes with a solar eclipse? “ Solar eclipses are only visible over a limited area of the Earth. It is necessary to predict the alignment of the stars and the position of the eclipse on the earth’s surface to predict the eclipse », Indicates the specialist.
Everything changes with Kepler’s laws
What was missing to start predicting eclipses more accurately? A more in-depth knowledge of the movement of the stars, explains Nicolas Lefaudeux. “ Once we moved away from geocentric to heliocentric, we figured out how to predict eclipses more accurately. », says the amateur photographer. With Kepler’s laws, established at the beginning of the 17th century, it became possible to calculate orbits more precisely to predict eclipses. Newton’s laws, stated at the end of the century, then allow us to better understand the orbits themselves (why they are like that).
An astronomer used this new knowledge to successfully predict an eclipse: it was Edmond Halley. The total solar eclipse of May 3, 1715 remains associated with this British scientist. It was from the same principle that he predicted the return of Halley’s comet, the parent body of several swarms of shooting stars, in 1758.
Do we know how to perfectly predict eclipses today?
There are still areas for improvement. “ We know the Earth well in three dimensions, we know the distance between the Earth and the Moon, as well as the shape of the Moon. The parameter that we know less well is the size of the Sun. It could be a hundred kilometers larger than what is estimated today. », explains Nicolas Lefaudeux.
Being able to predict eclipses is therefore not only interesting for their spectacular aspect: these events provide researchers with information about the characteristics of the Sun. “ Eclipses have allowed scientists to discover what is called the solar corona, that is to say the atmosphere of the Sunsummarizes Léa Griton. During an eclipse, the main source of light is hidden, allowing us to see the light emitted by the gases around the Sun, which is very faint compared to the light emitted by the Sun itself.. »
This plasma appears clearly in photographs of eclipses, such as this photo taken during the eclipse of August 21, 2017. Eclipses allow us to better understand this matter, which represents 99% of the known matter in the universe. We can thus learn more about the solar wind which propagates towards the Earth, and more broadly study the interactions between the planets and the Sun. », concludes Léa Griton. Anticipating solar eclipses also means giving ourselves the means to unlock the secrets of our star.
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