A stillness descends. Creatures large and small quiet down. Nocturnal animals poke their heads above ground. It is almost as if the world is holding its breath.
The sun darkens as the moon slowly consumes the orb and for a few brief moments, totality is achieved. Night descends and the corona becomes visible as the sun disappears from the sky. Then the moment is gone and the world stirs once more.
There is no question a solar eclipse is an event to behold. It certainly captures the imagination of people around the world and from all walks of life. Solar eclipses have been recorded for as long as there has been history. They have almost a magical allure.
They also involve a fair amount of science.
No one knows who first observed a solar eclipse because the information is lost in our pre-history. Early records of solar eclipses date back to China, Egypt and Mesopotamia. Clay tablets reveal Chinese and Mesopotamian scholars were predicting the occurrences of solar eclipses by 2500 B.C.
Chaldean scholars calculated the Saros cycle to predict eclipses. This is a period of 6,585.3 days and represents the period of time between an exact repeat of an eclipse. It works for both lunar and solar eclipses, although it was likely worked out originally for lunar eclipses as they are easier to track. (A lunar eclipse is visible everywhere it is night while a solar eclipse is only visible to viewers within the range of the penumbra.)
The ancient Greeks and Romans utilized references to eclipses as a mechanism to synchronize their calendars. They were also the first to note a number of phenomena related to eclipses, such as the corona.
By 1605, Johannes Kepler provided a scientific basis for a total solar eclipse. A century later, Edmund Halley (of comet fame) was able to determine the timing and path of the May 3, 1715 solar eclipse within four minutes and 30 kilometres.
During the July 28, 1851, solar eclipse, a Prussian photographer named Berkowski captured the first photographic image of the corona. Seventeen years later, during an eclipse, light from the corona was analyzed using spectroscopy by Jules Jansen leading to the detection of a new element. Its name is based on the Greek word for the sun, helios, and helium is the only element which was discovered off-world before being isolated on Earth.
During the May 29, 1919, solar eclipse, Sir Arthur Eddington was able to take pictures of the stars behind the Sun and demonstrate their light was bent by its gravitational mass, thereby validating Einstein's Theory of Relativity.
In 1999, scientists used a solar eclipse to monitor the rate of depletion of hydroxyl radicals in the air. Hydroxyl radicals are continuously produced by sunlight and clean the air. During an eclipse, the concentration of the radicals rapidly decays.
Science has indeed a long history with eclipses whether they are solar or lunar but why do they occur?
The simple answer to the question is a solar eclipse occurs when the moon passes between the sun and Earth. Any number of books will have an illustration showing Earth in the shadow of the moon but it is not quite so simple.
Imagine if Earth's orbit was a circle around the sun and the moon's orbit was a circle around Earth in the same plane as Earth's orbit. Under these circumstances, we would have a solar eclipse every month and the magic would have disappeared long ago.
Instead, there are physical factors at play. The first is the orbits of Earth and the moon are not circular. Rather, they are elliptical. This means the distance between Earth and the sun and Earth and the moon are constantly changing. Not a lot but enough to affect things.
The second consideration is Earth is tilted by 23.5 degrees relative to the plane of the ecliptic, which is the plane containing its orbit around the sun. The noon's orbit is tilted by five degrees relative to the Earth.
The net result is there are only two points in the moon's orbit where the sun, moon, and Earth are all in the same plane. These are called nodes.
Combine the tilt of Earth and the tilt of the moon's orbital plane and imagine this tilted system then orbiting the sun. The consequence, as viewed from Earth, is the moon moves up and down in the sky relative to the sun, which moves up and down with the seasons. (This perspective is a consequence of Earth's movement within this cosmic ballet.)
The result of all this is the sun, the moon, and Earth only come into proximate alignment when the nodes line up. This happens approximately twice a year and consequently there are two ecliptic seasons each year.
But add in the relative motion of the moon, along with the precession of its orbit, and the net result is the moon does not always cover the sun precisely, nor do two eclipses in a row occur in the same locations. Sometimes the shadow of the eclipses misses Earth entirely. As a result, we most often get partial eclipses.
Further, when Earth and the moon are at the ends of their elliptical orbits, the apparent coverage of the sun can change resulting in either a total eclipse with little corona or an annular eclipse with a burning ring of fire surrounding the moon. It depends on the angular size of the sun and moon relative to one another.
The whole interplay is determined by the lengths of the Draconic month - the time it takes for the moon to orbit back to a node - and the Synodic month - the time it takes the moon to go from one new moon to the next.
The Draconic month is 27.212220 days while the Synodic month is 29.530589 days. The difference is a result of the combination of Earth's orbit around the sun and the moon's orbit around Earth, both of which are counter-clockwise as viewed from above the North Pole. It takes a little longer for the moon to get back to a new moon than it does to get back to the same position against the stars.
A total of 223 Synodic months results in 6,585.321 days while 242 Draconic months yields 6,585.357 days or a complete Saros cycle. This period is 18 years, 10.3 days and is the period between the appearance of similar eclipses.
Further, because of the 0.3 days, it takes roughly 54 years and 32 days before an eclipse will occur in roughly the same location. And due to the precession of the moon's orbit, each successive eclipse will be shifted either north or south depending on whether it is an ascending or descending eclipse.
There are currently 42 Saros cycles operating simultaneously, which ensures at least one solar eclipse per year and most often two but they are never in the same place.
Indeed, the eclipse on Monday won't be repeated for well over 6,000 years, which makes it a pretty magical, mystical event.
