Solar Eclipses Past, Present and Future

Myths or, How were eclipses were conceived in the past?

Total eclipses of the Sun are undoubtedly one of the most spectacular of natural events and leave a lasting impression on their viewers. Perhaps then it is not too surprising that these magnificent events are often recorded in the surviving writings of ancient civilizations. Reactions to solar eclipses vary widely according to cultural beliefs. The loss of the Sun, the bringer of life, was often considered a bad omen; the word eclipse is derived from the Greek for abandonment. Milton captures the sense of unease:

As when the Sun, new risen,Looks through the horizontal misty air,

Shorn of his beams, or from behind the Moon,In dim eclipse, disastrous twilight sheds

On half the nations, and with fear of changePerplexes monarchs

Milton, Paradise Lost

The legends of many civilizations speak of legendary celestial monsters, such as dragons, black squirrels, vampires, jaguars or giant frogs devouring the Sun. The ancient Chinese tried to frighten their Sun-eating dragon into dropping its prey by banging drums, gongs and pots to make a lot of noise. Oxen were sacrificed so that the dragon could have some alternative food and archers dispatched to shoot arrows into the sky. Of course this action was always successful but there was a genuine fear that the skies would darken permanently and life on Earth changed forever. In some parts of East and South-East Asia the traditional dragon explanation of solar eclipses still survives. In fact the Chinese word for a solar eclipse is rsh which means Sun-eat.

The Vikings have a similar tale, telling of two wolves, Hati and Skoll. Skoll runs after the Sun, whilst Hati, who runs ahead of the Sun, is chasing the Moon. If either wolf caught their prey an eclipse would occur; then the Vikings would make a lot of loud noise to try to scare off the wolves and so rescue the Sun.

An Indian tribal legend tells of a local money lender, Rahdumari, who loaned grains such as wheat, corn and jute to the Damar tribal community. The Sun and the Moon acted as guarantors for the loan, which is not completely repaid. Eclipses occur because the Sun goes into hiding whenever Rahdumari demands what he is owed.

In certain cultures, although these are fewer in number, eclipses were viewed in a positive light. In some Native American mythologies, such as those of the Inuit, an eclipse occurs when the Sun and Moon temporarily leave their positions in the sky to check that all is well on the Earth.

Eclipses throughout history: the progression of understanding eclipses

2134 BCE Early Chinese records of eclipses are vague and sporadic, however historians and astronomers believe some writings may refer to an eclipse which occurred on 22nd October 2134 BCE. An anecdote tells of two royal astronomers who were beheaded for failing to predict the total eclipse.

1233 BCEA clay tablet unearthed in the ancient city of Ugarit (Syria) contains what is probably the oldest verifiable record of an eclipse. It is likely the tablet refers to an eclipse of 5th March 1233 BCE (but maybe to 3rd May 1375 BCE).

c. 8th Century BCEBabylonian astronomers kept systematic records of eclipses over several centuries. Their clay cuneiform tablets also contain eclipse predictions which are astonishingly accurate, typically to about two hours. They are thought to have discovered the Saros cycle which has a complicated period of just over eighteen years and helps in eclipse prediction. Many of the solar eclipses of this cycle were not even visible in Babylon! Babylonian observations of solar eclipses are unparalleled as a tool for exploring the ancient history of eclipse observation.

Babylonain clay tablets listing eclipses between 518 BCE and 465 BCE (image credit: NASA)

450 BCE

Anaxagoras of Clazomenae (500 428 BCE)

The Greek philosopher Anaxagoras of Clazomenae (500-428 BCE) was the first astronomer to correctly explain both eclipses of the Sun and the Moon. He argued that the Sun illuminates the Moon, or, as he put it

the Sun puts the brightness in the MoonThis Sun is then eclipsed when the Moon gets in the way of the Sun and casts its shadow on the Earth. Anaxagoras later lived in exile as punishment for the supposed sinfulness of his astronomical ideas. In 431 BCE, when Anaxagoras was in exile, Pericles calmed the Athenian people by explaining to them Anaxagoras theory that, rather than being an adverse omen, the solar eclipse was really caused by the Moon obscuring the Sun in the sky.

129 BCEThe Greek astronomer Hipparchus made use of a solar eclipse to estimate the distance between the Sun and the Moon. He used his knowledge that the eclipse was total at Hellespont but not in Alexandria, combined with geometrical arguments, to reason that the distance between the Earth and the Moon was around 67 1/3 Earth radii. Today we know the average Earth Moon distance is about 60 Earth radii (with small variations because of the elliptic nature of the Moons orbit), remarkably close to Hipparchus estimate.

Sketch showing the solar corona during and eclipse(from Fourteen Weeks in Descriptive Astronomy J. Dorman Steele, 1873)

968 CEThe Byzantine historian Diaconus was in Constantinople when he observed the total eclipse of 22nd December 968. His observations record the first clear description of the solar corona

Darkness fell upon the Earth and all the brighter stars revealed themselves. Everyone could see the disc of the Sun without brightness, deprived of light, and a certain dull and feeble glow, like a narrow band, shining round the extreme parts of the edge of the disc.

The solar corona is the hot outer atmosphere of the Sun but was believed to be part of the Moon until well into the 19th Century!

This photograph, taken during a total eclipse of 2002, clearly shows prominences on the limb of the Sun. (image: credit Steele Hill, NASA)

1185 CEThe first recorded description of solar prominences followed the eclipse of 1st May 1185. It appeared in the Russian Chronicle of Novgorod:

... there was an eclipse of the Sun The sun became similar in appearance to the moon and from its horns came out somewhat like live embers

Prominences are made of plasma which is cooler than the surrounding material and held up in the Suns atmosphere by the solar magnetic field. During total eclipses prominences are often described as tongues of fire shooting out from the Sun.

1687 CEThe Englishman Newton published his great work Mathematical Principles of Natural Philosophy which included his general law of gravitation and makes possible very precise long-term eclipse prediction.

The first eclipse map, drawn by Halley, shows the predicted path of totality across the UK. (image from NASA)

1715 CEThe Englishman Halley made the first strides into modern eclipse astronomy. For just one eclipse, that of 3rd May 1715, Halley managed to:

Produce the first eclipse map showing the predicted path of the eclipse across England.

Use observations from the eclipse to accurately measure the diameter of the Sun.

Report a phenomenon now known as Bailys beads, bright beads of light that appear close to totality as the Suns light streams through valleys on the Moon.

Halley also calculated paths and times of ancient eclipses based on his theoretical knowledge and found they did not agree completely with the historical records. He correctly concluded that the length of a day on Earth is increasing.

1724 CEThe Italian Maraldi recognized that the corona is part of the Sun and not the Moon. In 1806 de Ferrer gave the name corona to the glow which he saw around the Sun during the eclipse of 16th June 1806 and also proposed that, because of its great size, the corona must belong to the Sun.

The Fraunhoffer heliometer that was used to take the first photograph of a total solar eclipse (image: NASA website)

1851 CEThe first successful photograph of a total solar eclipse was taken on 28th July 1851 in Knigsberg using a technique developed by the Frenchman Daguerre. Photographs taken since then make up part of an incredible illustrated history of solar physics.

1860 CESecchi of Italy and De La Rue of the UK observed the same eclipse of 18th July 1860 from locations 250 miles apart. They used eclipse photography to demonstrate that prominences are part of the Sun and not an effect of the scattering of sunlight.

Image courtesy of the High Altitude Observatory, National Center for Atmospheric Research (NCAR), Boulder, Colorado, USA. NCAR is sponsored by the National Science Foundation.

1878 CEThe Frenchman Janssen observed eclipses of 1871 and 1878 and noticed that the shape of the corona changes with the sunspot cycle, an eleven year periodic variation in the number of sunspots observed on the face of the Sun. He noticed that the corona is rounder when the number of sunspots is high and more equatorial when the number of sunspots is low.

1931 CEObservations of the Sun during eclipses were the only way to observe the corona until the Frenchman Bernard Lyot designed the instrument now known as the coronagraph. Consisting of a telescope with an occulting disc designed to block out the solar disc, the coronagraph makes full daylight images of the Sun achievable (given good weather of course!)

The total solar eclipse of 1966, as seen from the Gemini 12 spacecraft during its 12th revolution of the Earth.

(Image: NASA Photo ID: S66-63415, File Name: 10074596.jpg)

1966 CEThe first pictures of a total eclipse from outside of the Earths atmosphere were taken by the astronauts aboard Gemini 12.

What we know so far: Eclipse theory, Different types of eclipses.

Are there different types of eclipse?Yes! A solar eclipse occurs when the Sun, Moon and Earth are in the same straight line, or nearly so, and so the Moon casts its shadow on the surface of the Earth. There are a number of ways this can be seen from Earth:

Sequence of images of the Sun taken during a total eclipse1994 Photo by Fred Espenak

A total solar eclipse is one of the most dramatic of all astronomical events. The Moon completely covers the disc of the Sun and the solar corona, the outer atmosphere of the Sun, can be seen in all its grandeur.

Sequence of images of the Sun taken during an annular eclipse.photo 1997 by Fred Espenak

Sometimes the Moon does not appear to cover the entire disc of the Sun but leaves a small ring-like rim of the Sun visible around the Moon. In these annular solar eclipses, the corona remains hidden from view.

Sequence of images of the Sun taken during a partial eclipse.Photos by John McConnell

When the Sun and the Moon only partially overlap we say a partial solar eclipse occurs. The sky may still dim a little depending on how much of the Sun is covered by the Moon.

The curvature of the Earth means that hybrid solar eclipses can occur. These are a combination type eclipse where the same eclipse is seen as annular from some parts along the path and total from others!

How do eclipses occur?By chance from here on Earth the Moon and the Sun both appear to be a very similar size in the sky. The Sun however, which contains more than 99.8% of the mass of the solar system, has a diameter of more than 400 times that of the Moon. They only appear to be the same size because the Sun is also around 400 times further away from the Earth. People didnt always realize this the Greek philosopher Heraclitus (c. 500 BCE) even thought that the Sun is no bigger than a human foot!

The Moon is therefore is just the right size and at just the right distance from the Earth that, when properly aligned, it can obscure the bright Sun. The correct alignment doesnt happen very often, which is why total eclipses are relatively rare. They also quite often occur in quite remote places of the Earth, over the ocean for example, so few people have ever seen a total eclipse.

The diagram above (which is not at all to scale!) illustrates the geometry of a total solar eclipse.

(from http://www.sems.und.edu/~sems/ECLIPSESSOLAR/SolarEclipses.html)

The type of eclipse you do see depends on the part of the Moons shadow that passes over you. If you see a total eclipse then you are in the umbra part of the Moons shadow, for partial eclipses you are in the penumbra part of the shadow.

The elliptical nature of the Moons orbit (and also that of the Suns) means that sometimes the Moon appears too small in the sky to block out light from the whole Sun. This is when annular eclipses occur. If you see an annular eclipse then you are standing in the antumbra part of the Moons shadow.

The diagram above (also not to scale) illustrates the geometry of an annular solar eclipse.

(from http://www.sems.und.edu/~sems/ECLIPSESSOLAR/SolarEclipses.html)

What are the stages of an eclipse?There are four stages in an eclipse, although in a partial eclipse only two of them occur.

First contact is the moment when the discs of the Sun and of the Moon first appear to touch. The term is a bit misleading, the Moon and the Sun are really still separated by 150,000,000 km!

Second contact marks the beginning of totality. Third contact marks the end of totality. It can occur up to 7 minutes 31

seconds after first contact. Fourth contact occurs when the two apparent discs separate.

TOTAL ECLIPSE

PARTIAL ECLIPSE

What can you see during a total eclipse of the Sun?

Photo taken from the MIR spacecraft showing the Moons shadow cast on the Earth during the total solar eclipse of 11th August 1999. (image from NASA website)

The phenomenon is one of the most terrible that man can witness, and no degree of partial eclipses gives any idea of its horror

G. Biddell, Astronomer Royal (UK), 1853

Imagine the wonder inspired by a solar eclipse if the leading astronomer in the UK describes it as terrible at a time when the cause of eclipses was well known!

A total eclipse begins by stealth as the disc of the Moon first appears to touch that of the Sun. The phase of partial eclipse lasts for nearly an hour. During the final few minutes before totality, as daylight fades, alternating wavy light and dark lines, called shadow bands, can sometimes be seen on plain light coloured surfaces such as walls.

Image of shadow bands seen before totality during the eclipseof 26th February 1998. (image: E. H. Strach)

The above sequence of images shows the Suns crescent breaking up into Bailys beads at the start of totality. In this image solar prominences are also visible.

(image: Fred Espekak)

Just as the last crescent of the Sun disappears behind the Moon a string of glittering specks of light appear around the dark disc of the Moon. These are Bailys beads, named after the amateur astronomer Francis Baily (1774-1844) who once took note of them, although the beads had also been recorded more than 50 years before Bailys birth! Bailys beads are visible for only a few seconds and are the result of the last of the Suns rays shining through the valleys around the Moons limb.

An instant before totality just a single point of sunlight on the edge of the disc remains and is seen as a diamond ring.

(Image: credit & copyright Anthony Ayiomamitis)

http://www.perseus.gr/http://www.perseus.gr/

The before picture shown above was taken just 10 seconds before totality during the eclipse of 29th March 2006 in the Goreme region of central Turkey.

As totality occurs, light intensity falls to around 1/100,000th of the normal daytime levels. Animals can be quite confused by the darkness; birds, for example, may stop singing return to roost. The reduction in daylight brings the Suns corona into view as a shimmering halo of pearly white light.

Image showing the Suns corona during a total eclipse (Image: NASA)

The corona is the tenuous solar outer atmosphere, a million times fainter than the Sun itself, and extending to distances several times the diameter of the Sun. The shape of the corona depends strongly on the magnetic activity of the Sun and so can appear quite different from eclipse to eclipse. Also visible in the sky are the planets Mercury and Venus as well as some of the brighter stars.

During totality solar prominences are seen proceeding from the darkened disc. They appear as red flames arching above the edge of the dark disc. Physicists have discovered that prominences are made up of plasma that is cooler than the surrounding atmosphere but suspended there by the Suns magnetic field.

Path of the 2002 total eclipse. The shadow of the Moon moves from West to East across the Earth, as the Moon passes from West to East across the face of the Sun. from http://

www.hartrao.ac.za/other/eclipse2002/fullpath.html

It is not obvious to the ground observer that the elliptical shadow of the Moon is moving across the Earth at 1.6 million meters per hour and is never more than 167 miles wide. To any one observer, the spectacle of totality lasts only a few minutes and ends as suddenly as it began except, that is, to a group of scientists on 30th June 1974 who used the supersonic Concorde jet to chase the eclipse and so extend its duration to 74 minutes!

What have we learnt from observing and studying eclipses?

Solar eclipses have been an important research tool for hundreds of years. Several important scientific discoveries came from observing eclipses. The following are just a few examples!

The above picture, taken by the TRACE satellite, shows magnetic solar coronal loops arching up into the Suns atmosphere from the solar surface. The bright regions indicate high temperature

plasma. (image: TRACE)

The Hot Corona.During the eclipse of 7th August 1869 Young and Harkness found an unexpected bright emission line in the light coming from the corona. It was in the green part of the visible spectrum but no element on Earth was known to produce such a line. Astronomers decided it was due to a mysterious new element which they gave the name coronium. Other coronium lines continued to be found and belief in this element hung around for many years. It was not until Grotrian and Edln, in around 1940, discovered that the emission lines are really from ordinary iron and calcium at very high temperature that coronium was banished to the history books. The corona is now known to be extremely hot, more than 1,000,000 K and can parts even reach 10,000,000 K! The surface of the Sun

however is a mere 5780K, so there must be a physical mechanism at work in the corona maintaining its high temperature. This is one of the great puzzles of solar physics that many scientists are still working on today.

The Solar Radius.Precisely measuring the diameter of the Sun turns out to be quite difficult! The turbulent motions of the Earths atmosphere mean that even in the best Earth based photography taken using very advanced equipment the Sun appears blurry. However by placing observers in the very limits of totality during an eclipse the exact edge of the shadow of the Moon can be found and then the solar diameter calculated to great precision. Investigating historical eclipse records may even allow scientists to deduce long term changes in the solar diameter.

General RelativityThe eclipse of 29th May 1919 was probably the most important in eclipse history. British astronomers mounted expeditions to observe the eclipse in Principe, West Africa and Brazil to test a new theory of Einstein. His General Theory of Relativity says that space is distorted around matter and that gravitational effects are caused by this distortion. The astronomers photographed the stars in the Hyades cluster both in the night sky and during the eclipse when the massive Sun is in the path of the starlight. Einsteins theory says that the stars seen during the eclipse will seem to be slightly shifted in position because their light is deflected by the gravitational field of the Sun. The measurements taken during the eclipse confirmed the theory of General Relativity making Einstein the most famous scientist in the world. In a speech announcing the results at the Royal Society in London, J.J. Thomson described Einsteins theory as one of the greatest achievements in the history of human thought.

The Discovery of Helium.During the eclipse of 18th August 1868 the position of a line in the yellow part of the solar spectrum was found in the Suns atmosphere. This line was found to be the result of an unknown element which was named Helium after the Greek god of the Sun, Heilos. Although Helium is the second most common element in the Universe it is very rare on Earth and a terrestrial source of Helium was not discovered until 1895.

What scientists are looking for in the future.

Eclipses of the Sun are so useful for studying the solar corona but occur so rarely that scientists use coronagraphs to create artificial eclipses so that they can observe the corona almost continuously! These work best in space where, outside the Earths atmosphere, very clear observations can be made. There are several coronagraphs onboard the SOlar and Heliospheric Observatory (or SOHO), which orbits the Sun at the Lagrangian point where the gravitational forces of the Earth and the Sun are equal.

Artificial eclipse image taken by the LASCO instrument onboard the SOHO satellite. A huge mass of plasma (a Coronal Mass Ejection, or CME) is seen heading away from the Sun. The Suns relative position and size is indicated by the white circle and the field of view extends to more then

2 million kilometers. (image: SOHO)

Coronagraph observations such as this one have shown frequent ejections of plasma and magnetic field heading into interplanetary space from the Suns corona. These Coronal Mass Ejections (or CMEs) can grow to be larger than the Sun itself in just a few hours and if they hit the Earth can disrupt communications, produce electrical power surges along transmission lines and permanently damage satellites. CMEs were discovered by satellite observations such as this one, although scientists have recently realized that the first CME was seen during the eclipse of 1860! It has become clear, through the use of artificial eclipse observations, that CMEs occur very frequently on the Sun.

Eclipses also affect the Earth's atmosphere and so provide scientists with a valuable opportunity to study the behaviour of layers such as the thermosphere and the ionosphere. Temperatures and winds in the atmosphere are seen to respond to the eclipse and physicists search for waves of pressure changes. Such waves are very common but generally caused by a mixture of different physical effects; linking waves to a specific cause, such as an eclipse, can help a lot in understanding the processes. Research like this could have important consequences for understanding weather patterns on Earth.

One interesting part of eclipse research has nothing to do with astronomy! Biologists study the effect of total solar eclipses on animal behaviour, observing insects, birds and mammals during various eclipses. Eclipses turn out to be an interesting way of determining exactly which types of animal behaviour the grazing of dairy cattle for example are controlled by daylight.

The latest total solar eclipse

Path of the eclipse of 29th March 2006(http://www.gilbertguerin.com/htm/gallery-total-solar-eclipse-2006-5-turkey.htm)

The latest total eclipse occurred on 29th March 2006. It began at 08:36UT in Brazil and travelled half way around the Earth, reaching a maximum duration of 4 minutes 7 seconds as it passed through Libya, leaving the Earth's surface at 11:48 UT in northern Mongolia. The eclipse was visible from Egypt (although only partial in Alexandria) where a combination of good weather prospects and great travelling opportunities drew hoards of observers.

http://www.gilbertguerin.com/htm/gallery-total-solar-eclipse-2006-5-turkey.htmhttp://www.gilbertguerin.com/htm/gallery-total-solar-eclipse-2006-5-turkey.htm

This digital mosaic of the latest eclipse was taken in Turkey and shows the Moon in several stages as it passes between the Earth and the Sun.

(Image credit & copyright: Stefan Seip)

http://www.astromeeting.de/http://www.astromeeting.de/

Physicists used their modelling techniques to predict in advance how the Sun's corona would look during totality. They took observed measurement of the solar magnetic field and carried out a huge computation to come up with the prediction. As you can see, it compared very well with the observed eclipse, a great triumph for the theoreticians. For more information see http://imhd.net/

Magnetic field lines predicted for the latest total eclipse. The lines extend from the solar surface through the Suns atmosphere and out into interplanetary space. Knowledge of the Suns

magnetic field can be used to predict the appearance of the corona in an eclipse, since the corona is dominated by its magnetic field.

Theoretical prediction of the appearance of the solar corona during the latest total eclipse

The Suns corona as seen during the latest total eclipse.(Image: credit & Copyright: Koen van Gorp)

http://imhd.net/http://imhd.net/

Predictions of solar eclipses until 4000 CE have already been published! The following map shows the path of some future total eclipses. The next total eclipse in Egypt will take place on 2nd August 2027 and will have a maximum duration of 6 minutes 22 seconds!

Finally, remember that it is never safe to look at the Sun, even in an eclipse, without the proper viewing equipment in techniques. Even when 99% of the Sun's surface is eclipsed the remaining part of the Sun can still damage your eyes. It is only during totality, i.e. between second and third contact, that you can safely look at the Sun!