What Happens During a Solar Eclipse?

Partial solar eclipse

Fig 3.1

A partial solar eclipse will occur outside the path of totality and within the area swept by the Moon’s penumbra. During a partial solar eclipse, the Moon appears to gradually cover the Sun, starting with a small bite out of it and progressing to increasing coverage. The proportion of the Sun’s diameter covered (called the magnitude of the eclipse) will increase to a maximum and then reduce again as the Moon moves across the Sun. Figure 3-1 shows the progression of a partial solar eclipse of maximum magnitude 73%.

Fig 3.2

The whole event can take up to about three hours. The maximum magnitude of a partial eclipse and the duration of the partial eclipse depend mainly on the distance of the observing location from the path of totality. As examples, for the maximum magnitude of the partial eclipse seen at various locations across Queensland see the map in the article on the 14 November 2012 eclipse  and the map in the article on the annular eclipse of 2013.

During these partial phases, images of the crescent shape of the Sun can be seen projected under trees on the ground or onto adjacent walls, as the gaps between leaves act as pinhole projectors. See figure 3-2. Interesting crescent shapes can be seen using items with one or more holes in them such as a kitchen colander or a loosely woven straw hat.

Fig 3.3

Shadows become unnatural as the crescent Sun becomes thin, being very sharp in one direction and blurry at right angles. This is easy to see by observing the shadow of both hands with the fingers on one hand orientated at right angles to the fingers on the other hand as demonstrated in figure 3-3.

During a partial solar eclipse the light level will drop to an extent dependent on the maximum magnitude of the eclipse. Because our eyes are very good at compensating for varying light levels, this will often go unnoticed until the Sun is reduced to a very thin crescent.

How much of the Sun is eclipsed during a partial eclipse can also be described by the term obscuration which is the fraction of the Sun’s area covered by the Moon.

It is not safe to view any part of a partial solar eclipse without proper eye protection.

Construct and use a pinhole projector or use binocular or telescope projection to see a partial eclipse or use other safe observing methods as described in the article How to Observe the Sun Safely.

Upcoming partial solar eclipses visible from Australia will be on 14 November 2012, 10 May 2013 and 29 April 2014.

Total solar eclipse

Fig 3.5

A total solar eclipse starts with initial partial phases as described in the article about partial solar eclipses, with the Moon progressively covering the Sun. After about an hour or so, the Moon completely obscures the Sun for a brief period in the total phase of the eclipse. The Moon then moves progressively off the Sun in the final partial phase, taking a further hour or so. See figure 3-5.

The initial partial phase progresses with very little change being noticeable in the light level. As the total part of the eclipse approaches, the light level drops conspicuously and an ominous darkening of the sky (the Moon’s shadow) approaches silently from the west. The Sun is reduced to a very thin crescent. The temperature can drop and the light quickly begins to fade. By this time animals have become confused at the fading light and often react as if it is nightfall with, for example, birds returning to roost.

For a minute or two before totality begins and for a similar time after it ends, narrow bands of shadow sometimes seem to race across the landscape. These shadow bands occur when regions of the Earth’s upper atmosphere bend the final sliver of light from the thin crescent Sun.

 In the final few seconds before totality the last brilliant parts of the Sun’s surface shine through valleys around the edge of the Moon in a shimmering display called Baily’s beads. Finally, the beads are reduced to a single point and the Sun for a few seconds looks like a dazzling diamond ring. The diamond ring and Baily’s beads are clearly defined because of the lack of an atmosphere on the Moon

Fig 3.6

Figure 3-6 is an image of the Diamond Ring at the total solar eclipse of 2005 in the South Pacific Ocean. The last portion of the Sun is shining through a valley on the edge of the Moon to create one dazzling point, and the corona becoming visible around the rest of the Sun completes the effect.

As the last bright point winks out, the Sun’s crimson pink upper surface, the chromosphere, can be seen in an arc around the edge of the Moon. Often evident are prominences, bright pink loops of plasma which extend above the chromosphere.

Fig 3.7

Figure 3-7 is a combination of four images showing the transition at second contact (the start of the total part of the eclipse) at the total solar eclipse of 2006 in Egypt. Baily’s beads are visible along with the crimson arc of the chromosphere. Several prominences are seen rising up from the chromosphere. As the Moon gradually moves during the sequence, Baily’s beads, the chromosphere and the prominences are progressively being covered.

As the Sun disappears, the Moon’s shadow arrives, enveloping observers and causing the whole sky to become a dark steel blue, in a surreal twilight almost a million times fainter than the daytime sky, or about the same as night-time with a full moon. During totality when the Sun’s bright surface (photosphere) is completely obscured, the Moon appears as a black hole in the sky outlined by the Sun’s softly glowing pearly white corona, the Sun’s outer atmosphere composed of ionised gas. The corona curves out from the Sun, usually in a pattern formed by the Sun’s magnetic field. Observers can look for coronal streamers and polar tufts noting the length and distribution of the streamers and features indicating the magnetic field and level of the Sun’s activity.

Fig 3.8

Figure 3-8 shows the corona during the eclipse in July 2010 in French Polynesia. Polar brushes are seen at the top and bottom coming from the Sun’s north and south poles. Several long streamers can be seen in the areas between the poles and the Sun’s magnetic field is apparent. There is a prominence rising above the Moon on the left hand side of the Sun.

In the darkened sky, planets and bright stars become visible. There is a glowing light around the horizon which has a sunset colour caused by the scattering of different wavelengths of light in the atmosphere (see the explanation in the article on light and colours in the atmosphere).

Fig 3.9

Figure 3-9 shows the eclipsed Sun in the sky over the Great Wall of China in 2008. The dark area of the Moon’s shadow in the sky can be clearly seen as well as the pinkish sunset glow of the light from outside the shadow over the distant mountains. Planets extend to the upper left of the Sun.

It is the dramatic change from daylight to night, the ominous darkening of the sky approaching silently from the west, the chill in the air and the sudden disappearance of the Sun in an otherwise normal day that so terrorised ancient people. Even today, when we understand what is happening, it is still a spine tingling and awe inspiring event.

At the end of totality the entire sequence is reversed, with prominences, chromosphere, diamond ring, Baily’s beads and shadow bands again being visible. The Moon’s shadow races away to the east. The Moon then gradually uncovers the Sun taking another hour or so until the final partial phase is over.

Some people see a partial solar eclipse and wonder why others talk so much about a total solar eclipse. There is a dramatic difference between the two.

Because the change in level of brightness from full sun to being in an eclipse is a factor of almost a million to one, even if you are close to the path of totality and the Sun is covered 99%, the light is still up to 10 000 times brighter than in the path of totality. It is far too bright to see the corona, the chromosphere and any prominences and you will not be enveloped by the dark shadow or see Baily’s beads or the diamond ring. Being just outside the path of totality is just like almost winning the lottery. There is no prize for a ticket number that is close to the winning one.

Shown on these pages are images illustrating some of the phenomena that can be observed during a total solar eclipse. Beautiful as the photos are, no image or video screen can make it become a million times darker at midday, arousing some primal emotion. Nor can it effectively capture the beautiful detailed structure in the Sun’s corona, something that only our eyes can see. You have to see and experience a total solar eclipse outdoors under an open sky to fully appreciate it.

See How to Observe the Sun Safely for safe methods of observing a total solar eclipse.

Annular solar eclipse

Fig 3.10

An annular solar eclipse occurs when the Moon passes directly in front of the Sun but because the Moon at that time appears to be smaller than the Sun (the Moon is further from the Earth and/or the Earth is closer to the Sun) it cannot completely cover the Sun and a ring or annulus of the Sun remains. Figure 3-10 shows the sequence of the annular eclipse of 2010. An annular eclipse is similar to a partial eclipse, with some of the bright surface of the Sun always being visible. The sky does not become very dark and the dramatic effects of a total eclipse do not occur. The same interesting shadow effects and projected crescent images seen at a partial eclipse can be experienced with the highlight being circular images seen at annularity (when the ring of sunlight is seen around the Moon). For more information see the article about annual solar eclipses for description of the mechanism of an annular eclipse and the article about the annular eclipse of 2013.

See How to Observe the Sun Safely for safe viewing methods of an annual solar eclipse.