Eclipse
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What will we see?

Scenario

Partial phases (between First and Second, and between Third and Fourth Contact)

The first sign of the Eclipse on Earth is when the Moon's disc first touches the Sun. This moment is called First Contact. The Moon takes a small bite out of the solar disc. This bite gradually becomes larger. If sunspots are visible they will disappear behind the lunar disc one by one. This is a good opportunity to notice that the sunspots are not as black as the Moon. They are in fact regions with an intensity 10 to 50 percent of the intensity of the solar photosphere.

This first phase of the eclipse develops gradually. For the 1999 solar eclipse it will last about an hour. As long as a third of the solar disc remains visible, there are no noticeable effects on the ambient light. This phase of the eclipse can easily pass unnoticed and is in fact the same as what we see during the much more common partial eclipses (the last of which in Belgium occurred in May 1994 and October 1996).

After Third Contact (i.e. after the end of totality), the partial phase proceeds symmetrically in reverse order. The Moon slowly pulls away from the solar disc, until Fourth Contact when the eclipse is over. Outside the path of totality the eclipse is only partial. All the same, the largest magnitude will still be between 95 and 100 percent in the whole of Belgium. So while the South of the Belgium experiences totality, the rest of the country will see the solar disc almost completely covered by the Moon, which is an extraordinary event in its own right.

During the last 20 to 30 minutes before totality more than two thirds of the solar disc is occulted, leaving only a crescent, gradually shrinking to a thin sliver. Nevertheless, the light is still blinding and extreme caution is still necessary. Gradually the light will diminish, slowly at first, then faster. At that moment human beings and other living creatures become aware of strange things happening. All sorts of tangible signs show us that an eclipse is the only astronomical phenomenon that also happens on Earth.

The observer is himself a part of the spectacle. The light becomes more and more subdued. If the weather is sunny there will still be a good deal of light, but sunglasses are no longer necessary as the light is no brighter than it would be on a cloudy day. The last minutes before totality the light takes on an almost lunar hue, casting jet-black shadows with unusually sharp edges.

The eclipse will cause the brightness of the sunlight to decrease substantially in the whole of Belgium and in large parts of Europe for about half an hour around totality. Whether the weather is sunny or overcast, artificial lighting (streets, cars, and houses) will need to be turned on. In some cases this will happen automatically. So if you are planning to enjoy the eclipse, be sure not to stand under a street light which could suddenly switch on.

While the solar crescent is shrinking it still shines bright. It is fun to see how every small opening projects an image of the crescent, through the "pinhole camera" effect. Under the trees we see thousand of images of the solar crescent, one for every hole between the leaves. If there are no trees where you are standing a pinhole is easily made by crossing one hand over the other, leaving a small opening between the ring finger and the middle finger. If the shadow of your hands is cast far enough away, then the shape of a crescent will appear in the centre of the shadow.

Meanwhile the surroundings themselves are influenced. The temperature drops. The coolest moment is usually 10 to 15 minutes after totality when the temperature can be 10 degrees Celsius lower than before. On a quiet day, the wind can suddenly pick up. On a windy day the wind can suddenly drop, or change direction. Some plants may close as they do at night; birds go home to roost.

During the last five minutes events proceed so rapidly that one needs all one's attention not to miss anything. The bright light of the solar crescent diminishes rapidly and is replaced by the subdued and diffuse light of the sky background. This has the same appearance as at dusk and is dark blue. On the Western horizon we see a grey patch, later turning into black and slowly filling the sky. This is the shadow of the Moon closing in, projected onto the Earth atmosphere (or the clouds...). In this dark part of the sky the first stars become visible, even before the advent of totality. The brightest planets and stars become visible two to three minutes before Second Contact.
During totality a large fraction of the sky goes dark blue, as at dusk, while indirect light from outside of the path of totality is scattered by the atmosphere. This scattered light is yellow to orange and reminds one of a sunset or a sunrise. It appears to spring from the horizon and spreads in all directions, often in an irregular fashion.

At that time shadow bands might be seen. These are narrow bands of shadow and light racing across the ground. They are multiple images caused by irregular refraction of the light from the remaining crescent by the Earth's upper atmosphere (comparable to the twinkling of stars). As the contrast between the dark and bright bands is quite small, the shadow bands are hard to photograph. They are seen most easily on a pale and smooth surface. Whether or not the bands will be visible is difficult to predict as it depends on the state of the Earth's upper atmosphere.

Second and third contact

The few seconds before and after totality are probably the most impressive. The sky has gone quite dark, but not as dark as night. The dark bow of the Moon's shadow races across the sky and reaches the Sun at Second Contact. A few tens of seconds before this Second Contact the sliver of Sun has become so thin that the solar corona becomes visible at the opposite side (i.e. the western edge of the solar disc). The crescent and the corona form the so-called diamond ring. The crescent of the Sun breaks up into distinct points of light, known as Baily's beads. These beads are caused by the Sun shining through the valleys around the visible face of the Moon. This spectacular sight only lasts a couple of seconds. The best show of beads is visible at the edge of the path of totality, but of course the duration of totality there is only very short. For a number of scientists however, observations from the edge of the path of totality are an excellent way to learn more about the surface of the Moon.

After the last glimmer of the photosphere vanishes, a bright red or pink beam (or bow) can be seen. This is the chromosphere, a thin layer of gas just above the photosphere, which is very bright in the hydrogen wavelength region (the so-called H alpha line in the red part of the spectrum). Hence its name: chromosphere means "colour sphere". At that moment the sky goes dark while the corona "unfolds". That, at least, is the beautiful impression one gets when observing with the naked eye. Totality will end after the third contact, when we run the same string of events backwards like a movie.

Totality

After this rapid succession of events outlined above there follows a period of apparent calm. The solar corona surrounded by the stars looks as if it will be there forever. Only the slight relative movement of the lunar disc with respect to the corona can be observed through a telescope or a pair of binoculars. This apparent permanence is just an illusion though, as it lasts only a few minutes and if you want to take home a souvenir, now is the time to take a picture.
A watch (or better still: an electronic timer) is a useful device to avoid being surprised by the sudden reappearance of the photosphere at Third Contact. Once again, extreme caution should be exercised, especially when looking through an optical instrument: eyes are easily damaged.

During totality we see configurations of stars that can usually only be seen during winter, as well as a number of planets. Mercury and Venus will be visible very near to the Sun. Maybe Jupiter and Saturn will be visible, low in the Western sky. The stars, however, pale next to the breathtaking spectacle of the solar corona around the black lunar disc.

The corona is made up of a number of light beams with an irregular and unpredictable shape. They are visible up to a distance of about 1.5 degrees of arc from the Sun, where they fade into the blue of the sky. At the edge of the Sun the corona is about as bright as the full Moon, but at its outer edges it is a thousand times weaker. Only the human eye (if necessary aided by binoculars) has the ability to adapt to such differences in brightness; it is a lot more difficult to capture the corona on film.

As a matter of fact, no picture has captured the marvellous visual impression of the corona against the blue sky background. In reality, the corona is not coloured at all, as it is caused by the scattering of photospheric light. The prominences that can sometimes be observed at the edge of the Sun have the same reddish colour as the chromosphere. The prominences are loops and tongues of gas suspended in the far hotter corona.

The solar corona.

During a total solar eclipse the Sun is completely covered by the lunar disc and we are shielded from all direct sunlight. What we see is the fainter image of light scattered by the outer solar atmosphere. This irregular crown around the solar disc is aptly called the corona. The corona is characterised by its extremely high temperature (a few million degrees Celsius). These temperatures are high enough to ionise atoms. Such a soup of completely or partially ionised atoms is called a plasma. The plasma state is actually the most common state of matter in the Universe.

     
  The corona at optical wavelengths during the solar eclipse of 16 February 1980 in Kenya.   An extreme ultraviolet image of the solar corona by the EIT telescope on board of the SOHO spacecraft. What we see is coronal plasma at a temperature of two million degrees Celsius. SOHO is an international collaboration (ESA/NASA).  

The solar corona doesn't only reflect visible light; it also emits extreme ultraviolet (EUV) and X-ray radiation, due to its high temperature. The ozone layer around the Earth protects us from this harmful radiation. The down side of this protection is that we cannot study the Sun at these wavelengths from the ground. Satellites orbiting above the ozone layer can take EUV and X-ray images of the solar corona.

The plasma making up the corona interacts with the solar magnetic field. The magnetic field gives structure to plasma and in a certain sense the plasma gives mass to the magnetic field. Both should be therefore considered together. EUV and X-ray images show that the corona is made up of plasma-filled loops following the magnetic field. These loops come in bunches called active regions and the coronal loops often end in sunspots that can be seen on the surface of the Sun at optical wavelengths. When an active region appears at the edge of the solar disc during an eclipse, one sees a so-called helmet streamer: a dome-like structure that has been compared to the helmets of German soldiers during World War I.

The coronal magnetic field is probably the reason why the corona is so much hotter (a few million degrees) than the solar surface (600 degrees Celsius). The magnetic loops carry electric currents and sometimes a collision of two or more magnetic loops can lead to a "short" or a reconnection. Some theories attempt to explain the temperature of the corona by a multitude of small reconnections. Other theories try to explain the corona using magnetic waves that travel in a coronal loop, comparable to some degree with the heating mechanism in a microwave oven.

There is a constant outflow of matter from the corona called the solar wind. Sometimes this outflow is not steady but impulsive: by a sudden rearrangement of the magnetic field part of the corona is ripped from the solar surface and flows out with the solar wind. This is called a coronal mass ejection (CME). When a CME collides with the Earth's magnetic field this can cause problems for satellites (loss of altitude), astronauts (radiation) and electrical networks (perturbations). Hence, understanding the coronal processes leading to a CME is of practical significance. The solar corona is also a unique plasma laboratory where all sorts of physical processes can be studied. As such the solar corona is a model for the coronae of other stars, but also for plasma behaviour in e.g. nuclear fusion reactors.

The sky during totality

As totality in Belgium will happen around noon, the Sun will be at its highest point above the horizon (about 53 degrees), and not far from the South. Around the occulted Sun a number of planets and bright stars will become visible. The sky will not be dark enough however to see all the stars that would be visible during a (winter) night. It will be difficult therefore to recognise the familiar constellations.

To find one's way across the sky it's easiest to start from the Sun and measure angular distances from there by stretching one's arm. One can see for instance that the Moon has an angular diameter of only half a degree, as a little finger (more than a degree) is largely sufficient to cover the Moon. When verifying this during the partial phase one should protect one's eyes with a special filter.



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Looking at the corona during totality (and who on Earth would look in another direction?), one can see Venus to the left (East) of and slightly lower than the Sun, at a distance of about 15 degrees, which is slightly less than a small span (see below). Above that, at about the same height as the Sun, one can see Regulus, a lot fainter than Venus, in the constellation Leo. At the other side (West) of the Sun stands Mercury, at a distance of 18 degrees (which is roughly a small span).

Some other planets (such as Jupiter and Saturn) and stars will also be above the horizon at totality. The brightest stars are indicated on the sky chart. Jupiter and Saturn will be quite low above the Western horizon.

Angular distances can be measured by stretching one's arm. Of course the distances given in the table below vary slightly from person the person

part of hand number of degrees
1 finger 1.5 °
2 fingers 3 °
3 fingers 4.5 °
4 fingers 6 °
fist 9 °
small span
(distance between stretched thumb and index finger)		 18 °
large span
(distance between stretched thumb and little finger)		 22 °

ECLIPSE Webmaster Last updated on 09/06/1999 by JV